What Is a Primary Cell?

A primary cell is an electrochemical cell designed to be used until its chemical reactants are largely consumed and then discarded or recycled.

Primary cells are normally:

non-rechargeable.

They convert stored:

chemical energy → electrical energy

through spontaneous redox reactions.

Common primary cells include:

• alkaline cells
• zinc-carbon cells
• primary lithium cells
• zinc-air cells
• silver oxide button cells

Primary cells are widely used because they are often:

• inexpensive
• portable
• convenient
• easy to store
• ready to use

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Cells and Batteries

The words cell and battery are often used interchangeably in everyday language, but they have slightly different scientific meanings.

A cell is a single electrochemical unit.

A battery can contain one or more electrochemical cells connected together.

For example, an AA alkaline "battery" is technically a single:

electrochemical cell.

A larger battery may contain several individual cells.

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How Does a Primary Cell Produce Electricity?

A primary cell contains two electrodes and an electrolyte.

At one electrode:

oxidation occurs.

At the other electrode:

reduction occurs.

These reactions transfer electrons from one substance to another.

If the electrons are forced to travel through an external circuit, they can provide:

electrical energy.

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The Basic Energy Conversion

The overall energy transformation is:

chemical energy → electrical energy

The chemical reactants have stored chemical energy.

As the spontaneous redox reaction occurs, electrons travel through the external circuit.

These moving electrons can power devices such as:

• flashlights
• clocks
• remote controls
• toys
• smoke detectors
• calculators

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The Anode

In a discharging primary cell, the anode is where:

oxidation occurs.

Oxidation involves:

loss of electrons.

The anode supplies electrons to the external circuit.

Remember:

AN OX

ANode = OXidation

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The Cathode

The cathode is where:

reduction occurs.

Reduction involves:

gain of electrons.

Electrons travel through the external circuit toward the cathode.

Remember:

RED CAT

REDuction = CAThode

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Electron Flow

During discharge, electrons move through the external circuit:

anode → device → cathode

For example, if the cell powers a flashlight:

anode → wire → lamp → wire → cathode

As electrons pass through the device, electrical energy can be converted into other forms.

In a flashlight:

electrical energy → light + thermal energy

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What Happens Inside the Cell?

Electrons travel through the external circuit, but ions move inside the cell.

The electrolyte contains mobile ions that help:

• carry charge
• maintain electrical neutrality
• complete the internal circuit

Therefore:

electrons move through the external circuit

while:

ions move through the electrolyte.

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Why Are Primary Cells Non-Rechargeable?

As a primary cell operates, its chemical reactants are converted into:

products.

Eventually:

• reactants become depleted
• products accumulate
• electrode materials may change
• the cell voltage decreases
• the cell can no longer provide useful electrical energy

The chemical system is designed primarily for:

discharge.

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Irreversible Chemical Changes

In a rechargeable cell, applying an external electrical current can drive the cell chemistry back toward its original state.

In a primary cell, the discharge reactions are generally not practically reversible under normal operating conditions.

During discharge, there may be changes in:

• electrode composition
• electrode structure
• electrolyte composition
• distribution of reaction products

Simply forcing electrons backward does not reliably restore the original:

chemical system.

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Why Can't We Just Reverse the Reaction?

It might seem that connecting a primary cell to a power supply should simply reverse the redox reaction.

In practice, unwanted reactions may occur instead.

Attempting to recharge a cell not designed for recharging can cause:

• gas production
• leakage
• overheating
• internal damage
• pressure buildup
• rupture or fire in some battery chemistries

Therefore primary cells should generally:

not be recharged unless the manufacturer explicitly states that they are rechargeable.

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Primary vs Secondary Cells

Electrochemical cells are commonly divided into two broad categories.

Primary cells

• designed mainly for one discharge cycle
• generally non-rechargeable
• discarded or recycled after use

Secondary cells

• designed for repeated charging and discharging
• rechargeable
• reactions can be driven back toward the charged state

Examples of secondary cells include:

• lithium-ion cells
• nickel-metal hydride cells
• lead-acid cells

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Primary Cell

During normal operation:

chemical energy → electrical energy

Once the useful reactants have been consumed:

replace or recycle the cell.

---

Secondary Cell

During discharge:

chemical energy → electrical energy

During charging:

electrical energy → stored chemical energy

This cycle can be repeated many times, although rechargeable cells also eventually:

degrade.

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Common Primary Cells

Several different chemical systems are used in primary cells.

Important examples include:

Zinc-carbon cells

Often used in inexpensive, low-drain devices.

Alkaline cells

Common in household AA, AAA, C, D, and 9 V batteries.

Primary lithium cells

Used where long shelf life, low mass, or reliable performance is important.

Silver oxide cells

Common in small button-cell applications.

Zinc-air cells

Used in applications including many hearing aids.

Each uses a different:

electrochemical system.

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Zinc-Carbon Cells

The zinc-carbon cell is one of the older common types of dry cell.

It typically uses:

• zinc as an important anode material
• manganese dioxide in the cathode mixture
• an electrolyte paste

Zinc-carbon cells are inexpensive and are often suitable for:

low-drain devices.

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Why Is It Called a Dry Cell?

Early electrochemical cells often contained large amounts of liquid electrolyte.

A dry cell uses an electrolyte that is immobilized as a paste or similar material rather than a freely flowing bulk liquid.

This makes the cell:

• portable
• easier to handle
• less likely to spill

However, the term "dry" does not mean that there is:

no electrolyte present.

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Structure of a Zinc-Carbon Cell

A typical zinc-carbon cell contains:

• zinc container
• electrolyte paste
• manganese dioxide-containing cathode mixture
• carbon rod used as a current collector
• separator
• terminals

The carbon rod helps collect current but is not simply the chemical reactant responsible for the cathode reaction.

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Alkaline Cells

One of the most familiar primary cells is the:

alkaline cell.

Alkaline cells are commonly sold as:

• AA
• AAA
• C
• D
• 9 V batteries

They are used in many everyday devices.

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Alkaline Cell Chemistry

A typical alkaline cell uses:

zinc

as an anode material and:

manganese dioxide

as a cathode material.

The electrolyte is usually based on:

potassium hydroxide (KOH).

Potassium hydroxide is alkaline, which gives the battery its name.

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Energy Conversion in an Alkaline Cell

As the alkaline cell discharges:

zinc is oxidized

and:

manganese-containing species are reduced.

Electrons leave the anode and travel through the external circuit.

The overall process converts:

chemical energy → electrical energy.

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Alkaline Cells vs Zinc-Carbon Cells

Both systems commonly use zinc and manganese dioxide chemistry, but their designs and electrolytes differ.

Compared with traditional zinc-carbon cells, alkaline cells generally provide:

• longer useful life
• better performance under moderate or higher drain
• longer shelf life
• greater capacity for many applications

However, alkaline cells are usually:

more expensive than basic zinc-carbon cells.

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Primary Lithium Cells

Some primary cells use:

lithium metal.

These are different from the rechargeable lithium-ion cells commonly used in phones and laptops.

This distinction is important.

Primary lithium cell = normally non-rechargeable

Lithium-ion cell = rechargeable

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Why Use Lithium?

Lithium has useful electrochemical properties.

Primary lithium cells can provide advantages such as:

• high energy density
• relatively low mass
• long shelf life
• useful performance across a range of temperatures

Different primary lithium cells use different cathode materials and therefore have different:

voltages and characteristics.

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Coin and Button Cells

Small electrochemical cells are often manufactured in:

coin or button shapes.

They are useful where devices require compact power sources.

Examples include:

• watches
• calculators
• car key fobs
• small sensors
• medical devices

However, not all button or coin cells use the same chemistry.

Some are:

primary cells

while others may be:

rechargeable cells.

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Silver Oxide Cells

Silver oxide primary cells are often used in small devices requiring relatively stable voltage.

Applications can include:

• watches
• calculators
• small instruments

They typically use:

zinc as an anode material

and:

silver oxide as a cathode material.

Their relatively high cost limits their use in larger everyday batteries.

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Zinc-Air Cells

Zinc-air cells are unusual because one of the reactants comes from:

the surrounding air.

Oxygen enters the cell and participates in the cathode reaction.

Because the cell does not need to store all of this oxygen internally, more space can be used for other active materials.

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Zinc-Air Applications

Zinc-air cells are commonly associated with:

hearing aids.

Many zinc-air batteries are activated when a protective tab is removed.

Removing the tab allows:

oxygen from the air

to enter the cell.

Once activated, the cell gradually begins functioning and should generally be used within its intended period.

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Why Do Primary Cells Eventually Go Flat?

A cell does not contain an unlimited supply of chemical energy.

As it operates:

reactants → products

Eventually, one or more reactants become sufficiently depleted that the cell can no longer maintain a useful:

voltage and current.

We commonly say that the battery is:

dead or flat.

The matter inside the battery has not disappeared.

Instead, its:

chemical composition has changed.

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Voltage During Discharge

A primary cell's voltage may change during use.

As discharge proceeds:

• reactant concentrations change
• products accumulate
• internal resistance may increase
• electrode surfaces may change

Eventually the voltage becomes too low to operate the device effectively.

Different battery chemistries have different:

discharge curves.

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Capacity

The amount of electrical charge a battery can deliver is called its:

capacity.

Battery capacity is often expressed in:

ampere-hours (Ah)

or:

milliampere-hours (mAh).

For example:

2000 mAh = 2.0 Ah

Capacity is not the same thing as:

voltage.

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Voltage vs Capacity

Imagine two cells:

Cell A:

1.5 V, 1000 mAh

Cell B:

1.5 V, 2500 mAh

They have approximately the same nominal voltage.

However, Cell B has a greater rated:

capacity.

Under suitable conditions, it can deliver more total charge before becoming discharged.

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Energy Stored in a Battery

Battery energy depends on both:

voltage and capacity.

A useful approximate relationship is:

Energy (Wh) = Voltage (V) × Capacity (Ah)

For example, a 1.5 V cell with a capacity of 2.0 Ah has an approximate nominal energy of:

1.5 × 2.0 = 3.0 Wh

Actual usable energy depends on operating conditions.

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Advantages of Primary Cells

Primary cells have several important advantages.

They are often:

• inexpensive initially
• simple to use
• widely available
• portable
• compact
• reliable after long periods of storage

Some primary chemistries also have:

very low self-discharge.

This makes them useful for devices that may remain unused for long periods.

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Long Shelf Life

Shelf life describes how long a battery can be stored while retaining useful capacity.

Some primary cells have excellent shelf lives because their chemistry changes relatively slowly when the circuit is:

open.

This can make them useful for:

• emergency equipment
• smoke detectors
• backup devices
• rarely used electronics

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Ready to Use

Primary cells normally arrive:

already charged and ready to operate.

They do not require:

• a charger
• a charging period
• charging electronics

For simple applications, this makes them very:

convenient.

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Low Initial Cost

Many primary cells are inexpensive to purchase.

For a device used only occasionally, buying a primary cell may initially cost less than buying:

• rechargeable batteries
• a compatible charger

However, this changes when batteries must be replaced:

frequently.

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Limitations of Primary Cells

Primary cells also have significant limitations.

They are generally:

• single-use
• not rechargeable
• replaced after discharge
• capable of producing substantial waste
• potentially more expensive over repeated use

Their suitability therefore depends on the:

application.

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Long-Term Cost

Imagine a device that requires new batteries every week.

Using disposable primary cells repeatedly can become:

expensive.

A rechargeable battery may cost more initially but can potentially be used for many:

charge-discharge cycles.

Therefore primary cells are often less economical for:

high-use devices.

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Environmental Impact

Used primary cells contain:

• metals
• electrolytes
• plastics
• other chemical materials

Manufacturing replacements requires additional:

• raw materials
• energy
• transportation
• packaging

Proper battery collection and recycling can help reduce some environmental impacts.

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Battery Disposal

Battery disposal rules vary between locations and battery chemistries.

Some batteries should be taken to designated:

battery collection or recycling facilities.

This can:

• recover useful materials
• reduce waste
• prevent inappropriate disposal

Users should follow local regulations and manufacturer instructions.

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Leakage

Some primary batteries can leak after:

• long storage
• deep discharge
• physical damage
• exposure to unsuitable conditions

Leaked electrolyte can:

• damage electronics
• corrode contacts
• irritate skin

Old batteries should therefore be removed from devices that will not be used for:

long periods.

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Mixing Batteries

Devices commonly use several cells together.

It is generally poor practice to mix:

• old and new cells
• different battery chemistries
• different brands or capacities where manufacturers advise against it

Cells at different states of discharge may behave differently and can increase the risk of:

leakage or poor performance.

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Choosing a Primary Cell

The best battery depends on the device.

Important factors include:

• required voltage
• required current
• capacity
• size
• mass
• shelf life
• operating temperature
• cost
• expected frequency of use

There is no single battery chemistry that is best for:

every application.

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Example: TV Remote

A television remote control typically uses relatively little electrical energy.

It may operate for months using one set of batteries.

Primary alkaline cells can be suitable because they offer:

• convenience
• long storage life
• reasonable cost
• adequate capacity

The batteries do not need frequent replacement.

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Example: Digital Camera

Some devices draw considerably more current.

If a device is used frequently and consumes batteries quickly, disposable primary cells may become:

expensive and wasteful.

Rechargeable secondary cells may be more appropriate.

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Example: Smoke Detector

Some smoke detectors must operate continuously for long periods while consuming relatively little power.

A battery with a long:

shelf and service life

can be valuable.

Certain primary lithium batteries are used for applications where long operating life is particularly important.

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Example: Emergency Flashlight

An emergency flashlight may sit unused for months.

Important battery properties include:

• long shelf life
• reliability
• low self-discharge

A suitable primary battery can therefore be advantageous even though it is:

non-rechargeable.

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Example: Hearing Aid

A hearing aid needs a small, lightweight power source with useful energy capacity.

Zinc-air cells are widely used because oxygen comes from:

outside the battery.

This helps provide useful energy capacity in a compact package.

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Evaluating Primary Cells

When deciding whether a primary cell is suitable, consider both:

advantages and limitations.

For occasional use:

Primary cells may offer excellent convenience.

For frequent use:

Rechargeable cells may reduce long-term cost and waste.

For long-term storage:

Some primary cells may offer excellent shelf life.

For high-energy repeated use:

Secondary cells may be more suitable.

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Primary vs Secondary Cells

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Worked Example 1

A battery converts stored chemical energy into electrical energy.

What type of reaction makes this possible?

A:

redox reaction.

Oxidation occurs at the anode while reduction occurs at the cathode.

Electrons travel through the external circuit.

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Worked Example 2

Why is an alkaline AA cell considered a primary cell?

Because it is designed primarily for:

one discharge cycle

and is normally:

non-rechargeable.

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Worked Example 3

A remote control uses one set of batteries for a year.

Would primary cells be reasonable?

Yes.

The device has relatively low energy demand, so the advantages of:

• convenience
• low initial cost
• long shelf life

may outweigh the disadvantage of being non-rechargeable.

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Worked Example 4

A child's electronic toy requires a new set of batteries every few days.

What limitation of primary cells becomes important?

Repeated replacement causes:

• greater long-term cost
• more battery waste

A rechargeable secondary battery may be more practical for such frequent use.

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Worked Example 5

Why does a primary cell eventually stop producing useful electricity?

Its chemical reactants are gradually:

consumed or transformed.

As the chemical system changes, the cell can no longer maintain sufficient:

voltage and current.

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Worked Example 6

A cell has:

1.5 V

and:

2.4 Ah

Estimate its nominal energy.

Use:

Energy = Voltage × Capacity

Energy = 1.5 × 2.4

Energy = 3.6 Wh

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Worked Example 7

Two batteries both provide 1.5 V.

One has a capacity of 1000 mAh and the other 2500 mAh.

Do they have the same capacity?

No.

They have similar nominal voltage, but the 2500 mAh cell can deliver more total:

charge under specified conditions.

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Worked Example 8

Why might a primary lithium battery be useful in an emergency device?

Primary lithium cells can offer properties such as:

• long shelf life
• high energy density
• relatively low mass

These can be useful when a device must remain ready after long periods of:

storage.

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Common Mistake: Primary Means First Battery

The word primary does not mean that it was the first type of battery invented.

It describes a cell intended mainly for:

non-rechargeable use.

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Common Mistake: A Dead Battery Contains No Energy or Chemicals

A discharged battery still contains:

matter and chemical substances.

However, its chemical system has changed so that it can no longer provide useful electrical energy under normal operating conditions.

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Common Mistake: Primary Cells Have No Electrolyte

All functioning electrochemical cells require ionic conduction.

Primary cells contain an:

electrolyte.

In many dry cells, the electrolyte is immobilized rather than existing as a large volume of free liquid.

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Common Mistake: Electrons Move Through the Electrolyte

Electrons primarily move through the:

external circuit.

Inside the electrolyte, charge is transported mainly by:

ions.

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Common Mistake: All Lithium Batteries Are Rechargeable

They are not.

Primary lithium batteries

are normally non-rechargeable.

Lithium-ion batteries

are designed to be rechargeable.

The words "lithium" and "lithium-ion" do not describe the same battery chemistry.

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Common Mistake: All Primary Cells Are Identical

Primary cells can use many different chemical systems.

They can differ in:

• voltage
• capacity
• size
• shelf life
• cost
• energy density
• operating temperature

The term primary cell describes their general use category rather than one particular chemistry.

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Common Mistake: Rechargeable Is Always Better

Rechargeable cells have important advantages, but primary cells can be more suitable for some applications.

For example, a primary cell may be preferred when:

• the device is rarely used
• long shelf life is important
• charging is inconvenient
• very low self-discharge is desirable

Battery choice depends on the:

intended application.

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Check Your Understanding

1. Define a primary cell.

2. What energy transformation occurs in a primary cell?

3. Why are primary cells normally non-rechargeable?

4. What type of chemical reaction produces electricity in a primary cell?

5. Where does oxidation occur?

6. Where does reduction occur?

7. In which direction do electrons move through the external circuit?

8. What is the role of the electrolyte?

9. Why do primary cells eventually stop producing useful electricity?

10. Name four common types of primary cells.

11. What is a zinc-carbon cell?

12. Why is it called a dry cell?

13. What are two important materials used in an alkaline cell?

14. Why is an alkaline cell called alkaline?

15. Give three common uses of alkaline cells.

16. What is the difference between a primary lithium cell and a lithium-ion cell?

17. Give two advantages of primary lithium cells.

18. What is a silver oxide cell commonly used for?

19. How does a zinc-air cell obtain one of its reactants?

20. Why are zinc-air cells useful in hearing aids?

21. Explain the difference between voltage and capacity.

22. What unit is commonly used for battery capacity?

23. A 1.5 V cell has a capacity of 2.0 Ah. Estimate its nominal energy in Wh.

24. Give four advantages of primary cells.

25. Give four limitations of primary cells.

26. Why might primary cells be suitable for a television remote?

27. Why might rechargeable cells be more suitable for a frequently used toy?

28. Why is shelf life important for emergency equipment?

29. What is self-discharge?

30. Why should primary cells generally not be recharged?

31. What problems could occur if a non-rechargeable battery is incorrectly charged?

32. Why should old batteries be removed from devices being stored?

33. Explain why battery leakage can damage electronics.

34. Why is recycling batteries useful?

35. Explain why repeated use of primary cells can create more waste than rechargeable cells.

36. Distinguish between a cell and a battery.

37. Explain why increasing battery capacity does not necessarily increase its voltage.

38. Compare primary and secondary cells.

39. A device is used only once every few months. Explain why a primary cell might be appropriate.

40. Evaluate whether primary or secondary cells would be more suitable for a frequently used electronic device. Support your answer using cost, convenience, lifetime, and environmental impact.

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Key Terms

• Primary cell: Electrochemical cell designed mainly for one discharge cycle and normally not rechargeable.
• Secondary cell: Electrochemical cell designed for repeated charging and discharging.
• Battery: One or more electrochemical cells used as a source of electrical energy.
• Anode: Electrode where oxidation occurs.
• Cathode: Electrode where reduction occurs.
• Oxidation: Loss of electrons.
• Reduction: Gain of electrons.
• Redox reaction: Reaction involving both oxidation and reduction.
• Electrolyte: Material containing mobile ions that conducts charge inside an electrochemical cell.
• Dry cell: Cell in which the electrolyte is immobilized rather than present as a large amount of free-flowing liquid.
• Alkaline cell: Common primary cell using an alkaline electrolyte, typically potassium hydroxide.
• Zinc-carbon cell: Inexpensive primary dry-cell chemistry using zinc and manganese dioxide-based materials.
• Primary lithium cell: Non-rechargeable cell using lithium metal as part of its chemistry.
• Zinc-air cell: Cell that uses oxygen from the surrounding air as a reactant.
• Silver oxide cell: Primary cell commonly used in small button-cell applications.
• Capacity: Amount of electrical charge a battery can deliver under specified conditions.
• Ampere-hour: Unit commonly used to express battery capacity.
• Shelf life: Length of time a battery can be stored while retaining useful capacity.
• Self-discharge: Gradual loss of stored chemical energy while a battery is not being used.
• Discharge: Process in which a cell supplies electrical energy through a spontaneous chemical reaction.

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Key Takeaways

• A primary cell is designed mainly for one discharge cycle and is normally non-rechargeable.
• Primary cells convert chemical energy into electrical energy.
• Electricity is produced through spontaneous redox reactions.
• Oxidation occurs at the anode and reduction occurs at the cathode.
• Electrons travel through the external circuit from anode to cathode.
• Ions move through the electrolyte to maintain charge balance.
• Primary cells become discharged because their chemical reactants are progressively transformed into products.
• Their chemistry and physical structure are generally not designed to be restored safely by electrical charging.
• Common primary cells include alkaline, zinc-carbon, primary lithium, silver oxide, and zinc-air cells.
• Alkaline cells are among the most common household primary batteries.
• Primary lithium cells should not be confused with rechargeable lithium-ion cells.
• Zinc-air cells use oxygen from the surrounding air as a reactant.
• Important advantages of primary cells include convenience, portability, availability, low initial cost, and often long shelf life.
• Important limitations include single-use operation, repeated replacement costs, resource consumption, and waste generation.
• Voltage and battery capacity are different quantities.
• A battery with greater capacity does not automatically have a greater voltage.
• Primary cells can be especially useful in low-drain, infrequently used, or long-storage applications.
• Rechargeable secondary cells are often more suitable when batteries are used and replaced frequently.
• Battery choice should consider voltage, capacity, shelf life, cost, size, operating conditions, frequency of use, and environmental impact.

What Is a Rechargeable Battery?

A rechargeable battery is an electrochemical energy-storage device that can be charged, discharged, and then charged again many times.

Rechargeable batteries contain one or more:

secondary cells.

During discharge, stored chemical energy is converted into electrical energy:

chemical energy → electrical energy

During charging, an external source supplies electrical energy that drives chemical changes inside the battery:

electrical energy → chemical energy

This allows the battery to store energy for later use.

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How Does a Battery Store Energy?

A battery does not simply "store electricity."

Instead, energy is stored in the:

chemical state of the materials inside the battery.

During charging, an external power source drives chemical changes that increase the chemical potential energy of the system.

The battery can then release this stored energy during:

discharge.

A useful way to think about the process is:

Charging: electrical energy → stored chemical energy

Discharging: stored chemical energy → electrical energy

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What Happens During Discharge?

When a rechargeable battery powers a device, spontaneous:

redox reactions

occur inside its cells.

At the anode during discharge:

oxidation occurs.

Electrons are released.

At the cathode:

reduction occurs.

Electrons are accepted.

Electrons travel through the external circuit:

anode → device → cathode

As they pass through the device, they transfer:

electrical energy.

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What Happens Inside the Battery?

Electrons travel through the external circuit, but they cannot simply travel through the electrolyte.

Inside the battery, charge is transported mainly by:

ions.

Therefore:

electrons move through the external circuit

while:

ions move through the electrolyte.

Both movements are necessary for the battery to continue operating.

---

What Happens During Charging?

When a rechargeable battery is connected to an appropriate charger, electrical energy is supplied to the cell.

The charger drives chemical reactions in the opposite direction from normal discharge.

In simplified form:

Discharge:

Charged materials → discharged materials + electrical energy

Charging:

Discharged materials + electrical energy → charged materials

Charging therefore restores the battery toward a:

higher-energy chemical state.

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Charging Is Not Perfectly Reversible

The reactions in rechargeable batteries are designed to be:

substantially reversible.

However, they are never perfectly reversible.

During repeated charging and discharging:

• side reactions can occur
• electrode structures can change
• electrolyte can degrade
• active materials can be lost
• internal resistance can increase

This is why rechargeable batteries eventually:

lose capacity and performance.

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Lithium-Ion Batteries

One of the most important rechargeable battery technologies is the:

lithium-ion battery.

Lithium-ion batteries are used in:

• smartphones
• laptops
• tablets
• wireless headphones
• cameras
• power tools
• electric bicycles
• electric vehicles
• energy-storage systems

They are popular because they can store a relatively large amount of energy while remaining:

compact and lightweight.

---

Structure of a Lithium-Ion Battery

A simplified lithium-ion cell contains:

• a negative electrode
• a positive electrode
• an electrolyte
• a separator
• current collectors

The negative electrode commonly contains:

graphite.

The positive electrode contains a:

lithium-containing material.

Many different positive-electrode chemistries exist.

---

The Electrolyte

The electrolyte allows:

lithium ions

to move between the electrodes.

The electrolyte conducts ions but is not intended to provide a direct electronic pathway between the electrodes.

Electrons instead travel through the:

external circuit.

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The Separator

The separator is positioned between the positive and negative electrodes.

Its job is extremely important.

It:

• prevents direct electrical contact between the electrodes
• allows ions to move through the cell
• helps prevent an internal short circuit

If the electrodes make direct electrical contact, a large current may flow internally and produce:

dangerous heating.

---

Lithium-Ion Discharge

During discharge, lithium ions move through the electrolyte from the negative-electrode side toward the positive-electrode side.

At the same time, electrons travel through the external circuit.

The electrons can power a device before reaching the positive electrode.

Therefore two movements occur simultaneously:

Lithium ions → through electrolyte

Electrons → through external circuit

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Lithium-Ion Charging

During charging, an external power source forces the electrochemical processes in the opposite direction.

Lithium ions move back toward the negative-electrode material.

Electrons are also driven through the external circuit in the corresponding direction.

The battery returns toward its:

charged chemical state.

Energy is stored as:

chemical potential energy.

---

Charging and Discharging

A simplified cycle is:

CHARGING

Electrical energy enters battery

↓

Chemical changes occur

↓

Energy stored chemically

↓

DISCHARGING

Chemical reactions occur

↓

Electrons flow through external circuit

↓

Electrical energy powers device

↓

Battery returns toward discharged state

The cycle can then begin again.

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Lithium Ions vs Lithium Metal

The name lithium-ion battery is important.

Lithium-ion cells operate mainly through the movement of:

Li⁺ ions

between electrode materials.

This is different from many primary lithium batteries, which can contain lithium metal.

Therefore:

primary lithium battery ≠ lithium-ion battery

Primary lithium batteries are generally non-rechargeable.

Lithium-ion batteries are:

rechargeable.

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Why Is Lithium Useful?

Lithium is useful in battery technology because it is:

• a very light element
• highly electropositive
• capable of contributing to high cell voltages
• suitable for battery systems with high energy per unit mass

These characteristics help lithium-ion batteries achieve high:

energy density.

---

Energy Density

Energy density describes how much energy can be stored relative to the battery's mass or volume.

A battery with high energy density can store a large amount of energy without becoming excessively:

large or heavy.

This is especially important for:

• smartphones
• laptops
• drones
• electric vehicles

where mass and space are important design considerations.

---

Specific Energy

When energy storage is compared with mass, we often use:

specific energy.

It can be expressed in:

watt-hours per kilogram (Wh/kg).

For example, if a battery stores 200 Wh and has a mass of 1 kg:

specific energy = 200 Wh/kg

A larger value means more energy is stored for each kilogram of battery.

---

Battery Capacity

Battery capacity describes how much electrical charge a battery can deliver under specified conditions.

It is commonly measured in:

ampere-hours (Ah)

or:

milliampere-hours (mAh).

For example:

5000 mAh = 5.0 Ah

Capacity and voltage are different quantities.

---

Estimating Battery Energy

Battery energy can be estimated using:

Energy (Wh) = Voltage (V) × Capacity (Ah)

Suppose a battery is rated:

3.7 V and 5.0 Ah

Then:

Energy = 3.7 × 5.0

Energy = 18.5 Wh

The nominal stored energy is approximately:

18.5 Wh.

---

Voltage Is Not Capacity

Consider two batteries.

Battery A:

3.7 V, 2.0 Ah

Battery B:

3.7 V, 5.0 Ah

Both have the same nominal voltage.

Battery B has a greater:

capacity.

Its approximate energy is also greater:

Battery A:

3.7 × 2.0 = 7.4 Wh

Battery B:

3.7 × 5.0 = 18.5 Wh

---

Different Rechargeable Battery Technologies

Lithium-ion is not the only rechargeable battery technology.

Other important types include:

• lead-acid
• nickel-metal hydride
• nickel-cadmium
• sodium-ion
• several specialized rechargeable systems

Each battery chemistry has different advantages and limitations.

---

Lead-Acid Batteries

Lead-acid batteries have been used for more than a century.

They contain:

• lead
• lead dioxide
• sulfuric acid electrolyte

They are widely used in:

• cars
• motorcycles
• backup power systems
• uninterruptible power supplies

---

Advantages of Lead-Acid Batteries

Lead-acid batteries can:

• provide very large currents
• be manufactured relatively inexpensively
• operate reliably
• be recycled through established systems

This makes them particularly useful for:

starting vehicle engines.

---

Limitations of Lead-Acid Batteries

Lead-acid batteries are:

• heavy
• relatively low in energy density
• made using toxic lead
• based on corrosive sulfuric acid

They are therefore less suitable than lithium-ion batteries for many lightweight portable devices.

---

Nickel-Metal Hydride Batteries

Nickel-metal hydride (NiMH) batteries are rechargeable cells commonly available in:

AA and AAA sizes.

They are used in:

• toys
• cameras
• flashlights
• game controllers
• household electronics
• some hybrid vehicles

---

Advantages of NiMH Batteries

NiMH batteries can offer:

• good capacity
• repeated rechargeability
• common household battery sizes
• useful performance in moderately high-drain devices

They can replace many disposable alkaline cells in compatible devices.

---

Limitations of NiMH Batteries

Compared with lithium-ion cells, NiMH batteries generally have:

• lower energy density
• lower cell voltage
• greater mass for a given amount of stored energy

Some types also experience noticeable:

self-discharge.

---

Nickel-Cadmium Batteries

Nickel-cadmium (NiCd) batteries are another rechargeable technology.

They can provide:

• high currents
• good cycle performance
• reliable operation

However, cadmium is:

toxic.

Environmental concerns and regulations have reduced their use in many consumer applications.

---

Sodium-Ion Batteries

Sodium-ion batteries operate using principles similar in some ways to lithium-ion batteries, but charge is transported using:

sodium ions (Na⁺).

Sodium is relatively abundant.

Sodium-ion technology is being developed and commercialized for applications where factors such as:

• material availability
• cost
• safety
• stationary storage

may be important.

Its characteristics differ from lithium-ion, so it is not simply a direct replacement in every application.

---

Comparing Rechargeable Technologies

The exact performance varies considerably between individual battery designs.

---

What Determines Battery Performance?

Battery performance is affected by many factors.

Important examples include:

• temperature
• charge rate
• discharge rate
• age
• number of cycles
• depth of discharge
• state of charge
• battery chemistry
• internal resistance

These factors can affect:

• capacity
• power
• efficiency
• lifetime
• safety

---

Temperature

Temperature can strongly affect rechargeable batteries.

At low temperatures:

• chemical reactions may proceed more slowly
• internal resistance can increase
• available power may decrease
• usable capacity may temporarily decrease

At high temperatures:

• reactions may proceed faster
• degradation may accelerate
• unwanted side reactions may increase

Very high temperatures can create:

serious safety problems.

---

Why Does Your Phone Battery Perform Differently in the Cold?

At low temperatures, electrochemical processes inside the battery become less effective.

The battery may temporarily:

• provide less current
• show reduced available capacity
• experience greater voltage drop under load

This does not necessarily mean that all of the stored chemical energy has permanently disappeared.

Performance can improve when the battery returns to an appropriate:

operating temperature.

---

Charge Rate

Charging a battery quickly can be convenient.

However, very rapid charging can increase:

• heating
• electrochemical stress
• unwanted side reactions

Modern charging systems therefore carefully control:

current and voltage.

The safe charging rate depends on the battery chemistry and design.

---

Discharge Rate

How quickly energy is removed also affects battery performance.

A high-power device demands a large:

current.

High discharge rates can cause:

• greater internal heating
• larger voltage drops
• reduced usable capacity in some systems

Battery designers therefore distinguish between:

energy requirements and power requirements.

---

Energy vs Power

These terms are related but different.

Energy

describes how much work can ultimately be done.

Power

describes how quickly energy is transferred.

A battery may store a large amount of energy but be unable to release it safely at an extremely high:

power.

---

Internal Resistance

Real batteries have:

internal resistance.

When current flows, some energy is converted into heat inside the battery.

The power lost internally can be described by:

P = I²R

where:

P = power converted to heat

I = current

R = internal resistance

Because current is squared, high currents can cause substantial:

heating.

---

Battery Age

Rechargeable batteries gradually change even if they are not used heavily.

This is called:

calendar aging.

Factors contributing to aging can include:

• time
• temperature
• state of charge
• chemical side reactions

Therefore a rechargeable battery can lose capacity simply through:

aging.

---

Cycle Aging

Using and recharging a battery also causes degradation.

This is called:

cycle aging.

Repeated cycling can cause:

• loss of active material
• electrode structural changes
• electrolyte degradation
• increased internal resistance

Eventually the battery can no longer store its original amount of:

energy.

---

Cycle Life

Cycle life describes how many charge-discharge cycles a battery can complete before its capacity falls to a specified level.

A battery's cycle life depends on:

• chemistry
• temperature
• charging conditions
• discharge conditions
• depth of discharge

Therefore there is no single cycle-life value that applies to all rechargeable batteries.

---

Depth of Discharge

Depth of discharge (DoD) describes the percentage of usable battery capacity that has been removed.

For example:

A battery begins at 100%.

It is used until 40% remains.

The depth of discharge is approximately:

60%.

For some battery systems, repeated deep discharge contributes to faster degradation.

---

State of Charge

State of charge (SoC) describes how much usable charge remains.

For example:

100% SoC = fully charged

50% SoC = approximately half charged

0% SoC = discharged to its defined lower operating limit

State of charge and depth of discharge are related but opposite ideas.

---

Battery Management Systems

Large or sophisticated rechargeable batteries often use a:

battery management system (BMS).

A BMS can monitor:

• voltage
• current
• temperature
• state of charge
• individual cells in a battery pack

It can help protect the battery from:

• overcharging
• excessive discharge
• overheating
• excessive current

---

Why Battery Packs Need Management

A large battery pack may contain hundreds or thousands of individual cells.

Small differences can develop between cells.

One cell might become:

• more charged
• less charged
• hotter
• more degraded

than neighboring cells.

A BMS helps keep the battery pack operating within appropriate:

limits.

---

Electric Vehicle Batteries

Electric vehicles require batteries capable of storing large amounts of energy while also providing high power.

Important EV battery characteristics include:

• energy density
• power density
• charging speed
• cycle life
• temperature performance
• mass
• cost
• safety

Lithium-ion technology is widely used because it provides a useful balance of these properties.

---

Regenerative Braking

Electric and hybrid vehicles can recover some energy while slowing down.

Normally, braking converts kinetic energy into:

thermal energy.

With regenerative braking, the electric motor can operate as a generator.

The energy transformations are approximately:

kinetic energy → electrical energy → chemical energy

Some of this recovered energy is stored in the battery.

---

Renewable Energy Storage

Rechargeable batteries can also store energy generated by:

• solar panels
• wind turbines
• other electricity sources

For example:

During sunny conditions:

solar energy → electrical energy → chemical energy

Later:

chemical energy → electrical energy

This allows energy generated at one time to be used:

later.

---

Environmental Benefits of Rechargeable Batteries

Rechargeable batteries can reduce the number of batteries that must be manufactured and discarded.

One rechargeable battery may replace many single-use batteries over its useful life.

Potential benefits include:

• less repeated battery waste
• reduced packaging
• fewer replacement batteries
• support for renewable-energy storage
• electrification of transportation

However, rechargeable batteries still have:

environmental impacts.

---

Raw Materials

Battery manufacturing requires raw materials.

Depending on battery chemistry, these can include:

• lithium
• nickel
• cobalt
• manganese
• graphite
• copper
• aluminum
• lead

Mining and processing these materials can require:

• energy
• water
• land
• transportation

and can create environmental and social impacts.

---

Mining Impacts

Mining can potentially affect:

• habitats
• water resources
• soil
• local ecosystems
• surrounding communities

The severity depends on factors such as:

• material being mined
• mining method
• location
• environmental controls
• energy sources
• waste management

Therefore battery sustainability must consider the entire:

life cycle.

---

Manufacturing

Battery manufacturing also requires energy.

Important processes can include:

• refining materials
• producing electrode materials
• manufacturing cells
• assembling battery packs
• transporting components

If manufacturing energy comes from fossil fuels, this can increase the battery's:

carbon footprint.

---

Battery Use

The environmental impact during use depends partly on where the charging electricity comes from.

For example, electricity could be generated from:

• solar
• wind
• hydroelectric power
• nuclear power
• natural gas
• coal

Therefore the environmental impact of operating a rechargeable battery depends partly on the:

electricity system supplying it.

---

End of Life

Eventually rechargeable batteries lose enough performance that they are no longer suitable for their original application.

At this stage they may be:

• reused
• repurposed
• recycled
• disposed of according to local regulations

Proper management is important because batteries contain:

valuable and potentially hazardous materials.

---

Battery Recycling

Battery recycling can recover materials for future use.

Depending on the process and battery chemistry, recoverable materials can include:

• lithium
• nickel
• cobalt
• copper
• aluminum
• lead

Recycling can reduce:

• demand for newly mined material
• waste sent for disposal
• loss of valuable resources

---

Lead-Acid Battery Recycling

Lead-acid batteries have well-established recycling systems in many regions.

Materials that can be recovered include:

• lead
• plastics
• other components

Because lead is toxic, responsible collection and recycling are particularly:

important.

---

Lithium-Ion Recycling

Lithium-ion battery recycling is increasingly important as the number of:

• electric vehicles
• portable electronics
• energy-storage systems

increases.

Recycling technologies aim to recover valuable battery materials so that they can potentially be returned to:

manufacturing supply chains.

---

Second-Life Batteries

An electric vehicle battery may eventually lose enough capacity that it is no longer ideal for transportation.

However, it may still have useful capacity for less demanding applications.

Possible second-life applications can include:

stationary energy storage.

This can extend the useful life of the battery before final:

recycling.

---

A Battery Life Cycle

The life cycle of a rechargeable battery can be represented as:

Raw material extraction

↓

Material processing

↓

Battery manufacturing

↓

Transportation

↓

Battery use

↓

Reuse or second life

↓

Recycling

↓

Recovered materials

↓

New products

A more circular system attempts to keep useful materials in circulation rather than treating used batteries simply as:

waste.

---

Comparing Environmental Impacts

Rechargeable batteries are not automatically environmentally harmless.

A complete comparison should consider:

• raw material extraction
• manufacturing
• transportation
• number of uses
• charging electricity
• lifespan
• recycling
• disposal

A rechargeable battery used hundreds of times may have advantages over repeatedly replacing primary batteries, particularly when it is:

properly recycled.

---

Safety and Lithium-Ion Batteries

Lithium-ion batteries store a large amount of energy in a relatively small space.

If a battery is:

• physically damaged
• internally short-circuited
• severely overheated
• improperly charged

rapid heating can occur.

In severe cases, this can lead to:

thermal runaway.

---

Thermal Runaway

Thermal runaway occurs when heat-producing reactions cause the battery temperature to rise, which then causes further heat-producing reactions.

This creates a dangerous positive feedback cycle:

temperature rises → reactions accelerate → more heat → temperature rises further

Battery design includes multiple safety systems intended to reduce this risk.

---

Battery Safety Systems

Safety measures can include:

• separators
• protective circuits
• temperature sensors
• current limits
• voltage limits
• battery management systems
• thermal management

These systems are especially important in:

large lithium-ion battery packs.

---

Comparing Battery Technologies

When evaluating a battery, there is rarely one technology that is "best" in every category.

A useful comparison should consider:

Energy density

How much energy can it store for its mass or volume?

Power

How quickly can it provide energy?

Cycle life

How many times can it be charged and discharged?

Cost

How expensive is the battery?

Safety

How does it behave under abnormal conditions?

Mass

How heavy is it?

Temperature performance

How well does it operate in hot or cold environments?

Environmental impact

What materials and processes are required?

---

Choosing a Battery for a Smartphone

Important characteristics include:

• low mass
• small size
• high energy density
• rechargeable operation
• useful cycle life

Lithium-ion technology is well suited to these requirements.

A lead-acid battery would be impractical because it would be:

far too heavy and bulky for comparable portable use.

---

Choosing a Battery for Starting a Car

A starter battery must provide a very large:

current

for a short period.

Lead-acid batteries can provide high currents relatively inexpensively.

Therefore they remain widely used for:

vehicle starting systems.

---

Choosing Rechargeable AA Batteries

For frequently used household devices, NiMH batteries can be useful.

For example:

• game controllers
• cameras
• toys
• flashlights

Instead of repeatedly purchasing disposable alkaline batteries, the same NiMH cells can be:

recharged and reused.

---

Choosing a Battery for Grid Storage

For large stationary energy-storage systems, battery mass may be less important than it is for smartphones or electric vehicles.

Important considerations can include:

• cost
• cycle life
• safety
• material availability
• efficiency
• lifetime

This means technologies that are not ideal for portable electronics may still be useful for:

stationary storage.

---

Worked Example 1

A phone battery is rated:

3.8 V, 4000 mAh

Convert capacity:

4000 mAh = 4.0 Ah

Calculate approximate nominal energy:

Energy = V × Ah

Energy = 3.8 × 4.0

Energy = 15.2 Wh

---

Worked Example 2

A battery stores 60 Wh and has a mass of 0.30 kg.

Calculate its specific energy.

Specific energy = energy ÷ mass

= 60 ÷ 0.30

= 200 Wh/kg

---

Worked Example 3

A battery begins at 100% state of charge and is used until 25% remains.

Depth of discharge:

100% − 25% = 75%

Therefore:

DoD = 75%.

---

Worked Example 4

A rechargeable battery originally has a capacity of:

5000 mAh

After several years it can store only:

4000 mAh

Percentage of original capacity remaining:

4000 ÷ 5000 × 100

= 80%

The battery retains approximately:

80% of its original capacity.

---

Worked Example 5

Why might a battery become warmer when supplying a large current?

Some electrical energy is converted into thermal energy because of:

internal resistance.

Since:

P = I²R

increasing current can greatly increase internal heating.

---

Worked Example 6

Why are lithium-ion batteries useful in electric vehicles?

They provide a useful combination of:

• high energy density
• high power capability
• rechargeability
• relatively low mass
• useful cycle life

Vehicle designers must still manage cost, degradation, temperature, and safety.

---

Worked Example 7

Why might lead-acid be preferred over lithium-ion for some applications?

Lead-acid batteries can offer:

• low initial cost
• high current capability
• mature technology
• established recycling

If mass and energy density are less important, these advantages can make lead-acid suitable.

---

Worked Example 8

A student says:

"Rechargeable batteries are environmentally friendly because they create no waste."

This statement is:

incomplete.

Rechargeable batteries can reduce waste through repeated use, but they still require:

• raw materials
• manufacturing
• electricity
• eventual recycling or disposal

Their full environmental impact must be evaluated across the battery's:

life cycle.

---

Common Mistake: Batteries Store Electricity

Batteries primarily store:

chemical energy.

During discharge, this chemical energy is converted into electrical energy.

---

Common Mistake: Lithium Ions Travel Through the Wire

Lithium ions move mainly through the:

electrolyte.

Electrons move through the:

external circuit.

Keeping these two charge movements separate is essential for understanding lithium-ion batteries.

---

Common Mistake: Rechargeable Means Unlimited Use

Rechargeable batteries have a finite:

cycle life.

Chemical and physical degradation gradually reduces their performance.

---

Common Mistake: Higher Capacity Means Higher Voltage

Capacity and voltage are different.

A battery can have:

greater capacity but the same voltage.

Capacity describes available charge.

Voltage describes electrical potential difference.

---

Common Mistake: Fast Charging Has No Effect

Fast charging can be convenient, but it may increase:

• heat
• chemical stress
• degradation

Battery and charger design determine how rapidly charging can occur safely.

---

Common Mistake: Cold Permanently Destroys All Capacity

Cold temperatures can temporarily reduce battery performance because electrochemical processes slow and internal resistance increases.

Some performance may return when the battery warms to an appropriate operating temperature.

Extreme temperatures, however, can contribute to:

permanent damage.

---

Common Mistake: Rechargeable Batteries Have No Environmental Impact

Rechargeable batteries require:

• mining
• processing
• manufacturing
• transportation
• electricity
• end-of-life management

Their environmental benefits and costs must be evaluated across their entire:

life cycle.

---

Common Mistake: All Rechargeable Batteries Are the Same

Different battery technologies have very different characteristics.

The best battery depends on the required balance of:

energy, power, cost, mass, safety, lifetime, and environmental impact.

---

Check Your Understanding

1. What type of energy is primarily stored in a rechargeable battery?
2. What energy transformation occurs during discharge?
3. What energy transformation occurs during charging?
4. What type of chemical reactions occur during battery operation?
5. Why can secondary batteries be recharged?
6. Where do electrons travel during discharge?
7. What carries charge through the electrolyte?
8. Name the major components of a lithium-ion cell.
9. What is the purpose of the separator?
10. What happens to lithium ions during discharge?
11. What happens to lithium ions during charging?
12. Why is a lithium-ion battery different from a primary lithium battery?
13. Define battery capacity.
14. What units are commonly used for capacity?
15. Define energy density.
16. Why is high energy density important for smartphones?
17. Calculate the energy of a 3.7 V, 2.5 Ah battery.
18. Name three rechargeable battery technologies.
19. Give two common applications of lead-acid batteries.
20. Why are lead-acid batteries suitable for starting vehicles?
21. Give two applications of NiMH batteries.
22. Give two advantages of lithium-ion batteries.
23. Give two limitations of lithium-ion batteries.
24. What is sodium-ion battery technology?
25. Define self-discharge.
26. Define cycle life.
27. Explain the difference between calendar aging and cycle aging.
28. What is depth of discharge?
29. What is state of charge?
30. Explain how temperature can affect battery performance.
31. Why can high current cause battery heating?
32. What is internal resistance?
33. What does a battery management system do?
34. Why are battery management systems particularly important in large battery packs?
35. Explain how regenerative braking can recharge an electric vehicle battery.
36. Explain how rechargeable batteries can support renewable-energy systems.
37. Identify three environmental impacts associated with battery production.
38. Explain why battery recycling is important.
39. What is meant by a second-life battery?
40. What is thermal runaway?
41. Why can damaged lithium-ion batteries present a safety hazard?
42. A 12 V battery has a capacity of 8 Ah. Calculate its approximate energy in Wh.
43. A 100 Wh battery has a mass of 0.5 kg. Calculate its specific energy.
44. A battery originally stored 3000 mAh but now stores 2400 mAh. What percentage of its original capacity remains?
45. Compare lithium-ion, lead-acid, and NiMH batteries in terms of energy density, mass, applications, and environmental considerations.
46. Explain why no single rechargeable battery technology is ideal for every application.
47. Evaluate which battery characteristics are most important for a smartphone.
48. Evaluate which characteristics are most important for an electric vehicle.
49. Explain why environmental comparisons should consider the entire battery life cycle.
50. Describe the complete energy-storage cycle of a rechargeable battery from charging through discharge and recharging.

---

Key Terms

Rechargeable battery: Battery containing secondary cells designed for repeated charging and discharging.

Chemical energy: Energy stored in the chemical state and arrangement of substances.

Discharge: Process in which stored chemical energy is converted into electrical energy.

Charging: Process in which electrical energy drives chemical changes that restore stored chemical energy.

Lithium-ion battery: Rechargeable battery in which lithium ions move between electrode materials.

Electrolyte: Material through which ions move inside an electrochemical cell.

Separator: Material that prevents direct electronic contact between electrodes while allowing ionic movement.

Capacity: Amount of electrical charge a battery can deliver under specified conditions.

Energy density: Amount of energy stored per unit volume or, more broadly in common usage, relative to battery size.

Specific energy: Energy stored per unit mass, commonly measured in Wh/kg.

Cycle life: Number of charge-discharge cycles a battery can undergo before reaching a specified performance limit.

State of charge: Measure of how much usable charge remains in a battery.

Depth of discharge: Percentage of usable battery capacity that has been removed.

Self-discharge: Gradual loss of stored charge while a battery is not being used.

Internal resistance: Resistance within a battery that contributes to voltage loss and heating when current flows.

Battery management system: Electronic system used to monitor and control battery operation.

Regenerative braking: Recovery of some kinetic energy during vehicle braking for storage in the battery.

Thermal runaway: Self-accelerating heating process that can occur in a severely failing battery.

Battery recycling: Recovery and processing of materials from used batteries.

---

Key Takeaways

• Rechargeable batteries store energy mainly as chemical energy.
• During discharge, chemical energy is converted into electrical energy.
• During charging, electrical energy is converted into stored chemical energy.
• Rechargeable batteries use electrochemical reactions that are sufficiently reversible for repeated operation.
• Lithium-ion batteries move Li⁺ ions through the electrolyte while electrons move through the external circuit.
• The separator prevents direct electrical contact between the electrodes while allowing ions to move.
• Lithium-ion batteries offer high energy density and are widely used in portable electronics and electric vehicles.
• Lead-acid batteries are heavier but inexpensive, capable of high currents, and widely recycled.
• NiMH batteries are useful in rechargeable household cells and some vehicle applications.
• Battery performance depends on temperature, charge rate, discharge rate, age, internal resistance, state of charge, and cycling.
• Capacity, voltage, energy, and power describe different aspects of battery performance.
• Rechargeable batteries gradually lose capacity because their chemistry is not perfectly reversible.
• Battery management systems help control voltage, current, temperature, and other operating conditions.
• Rechargeable batteries can support renewable energy and electric transportation.
• Battery production requires raw materials and energy and therefore has environmental impacts.
• Environmental evaluation should consider the entire battery life cycle, not only its use.
• Reuse, second-life applications, and recycling can reduce waste and recover valuable materials.
• Different battery technologies involve trade-offs between energy density, power, cost, mass, safety, lifespan, and environmental impact.

What Is Energy Storage?

Energy storage is the process of capturing energy so that it can be used:

at a later time.

Energy cannot simply disappear into a storage device. Instead, it is converted from one form into another.

For example, a rechargeable battery converts:

electrical energy → chemical energy

during charging.

When the battery is used:

chemical energy → electrical energy

Different storage technologies store energy in different forms.

These include:

• chemical energy
• gravitational potential energy
• kinetic energy
• elastic potential energy
• thermal energy
• electrical energy

---

Why Do We Need Energy Storage?

Electricity production and electricity demand do not always occur at the same time.

For example, solar panels may generate large amounts of electricity:

during the day.

However, people may require large amounts of electricity:

after sunset.

Energy storage allows some of the daytime energy to be saved and used later.

---

The Basic Storage Cycle

Most energy-storage systems follow the same general process:

Energy input

↓

Energy converted into a storable form

↓

Energy stored

↓

Stored energy converted back

↓

Useful energy output

Every conversion involves some energy loss.

Therefore no real storage system is:

100% efficient.

---

Forms of Energy Storage

Energy can be stored in many different forms.

Chemical

• batteries
• hydrogen

Gravitational

• pumped hydroelectric storage
• gravity-storage systems

Kinetic

• flywheels

Thermal

• hot water
• molten salts
• other thermal-storage materials

Electrical

• capacitors
• supercapacitors

Mechanical

• compressed-air systems

Different technologies are suitable for different:

applications and timescales.

---

Rechargeable Batteries

Rechargeable batteries are one of the most familiar energy-storage technologies.

During charging:

electrical energy → chemical energy

During discharge:

chemical energy → electrical energy

Examples include:

• lithium-ion batteries
• lead-acid batteries
• nickel-metal hydride batteries
• sodium-ion batteries

---

Lithium-Ion Battery Storage

Lithium-ion batteries are widely used because they offer:

• high energy density
• high efficiency
• relatively fast response
• rechargeable operation
• useful cycle life

They are used in:

• smartphones
• laptops
• electric vehicles
• homes
• businesses
• large electricity grids

Large battery installations are often called:

battery energy storage systems, or BESS.

---

How Grid Batteries Work

Imagine a solar farm producing more electricity than the grid currently needs.

The excess electricity can charge batteries:

electrical → chemical

Later, when electricity demand increases:

chemical → electrical

The stored electricity can then be supplied back to the:

grid.

---

Advantages of Battery Storage

Rechargeable batteries can offer:

• rapid response
• high round-trip efficiency
• modular construction
• installation at many different scales
• no moving mechanical parts in the cells themselves
• useful storage from seconds to several hours or longer depending on design

Battery systems can respond quickly to changes in electricity:

supply and demand.

---

Limitations of Battery Storage

Battery systems also have limitations.

These can include:

• cost
• gradual degradation
• limited cycle life
• raw-material requirements
• fire and safety considerations
• environmental impacts from manufacturing
• eventual recycling requirements

Different battery chemistries have different:

advantages and disadvantages.

---

Fuel Cells and Hydrogen

Hydrogen provides another method of storing energy.

Strictly speaking, the fuel cell itself is an energy-conversion device, rather than the main storage device.

The energy is stored in:

hydrogen fuel.

A fuel cell converts the chemical energy of hydrogen into:

electrical energy.

---

Hydrogen Energy Storage

Electricity can first be used to produce hydrogen through:

electrolysis.

The sequence can be represented as:

electricity

↓

electrolysis

↓

hydrogen

↓

storage

↓

fuel cell

↓

electricity

Hydrogen therefore acts as an:

energy carrier.

---

Advantages of Hydrogen Storage

Hydrogen may be useful for:

• long-duration storage
• storing large quantities of energy
• transportation
• industrial applications
• storing excess renewable energy
• situations where batteries may be less practical

Hydrogen can potentially be stored for:

long periods.

This makes it interesting for seasonal energy storage.

---

Limitations of Hydrogen Storage

The electricity-to-hydrogen-to-electricity pathway involves several conversions.

Energy can be lost during:

• electrolysis
• compression
• storage
• transportation
• fuel-cell operation

Therefore its overall round-trip efficiency is generally lower than that of:

battery storage.

Hydrogen storage also requires specialized infrastructure.

---

Pumped Hydroelectric Storage

One of the most established forms of large-scale energy storage is:

pumped hydroelectric storage.

It uses two reservoirs at different elevations.

When excess electricity is available, water is pumped from the lower reservoir to the:

upper reservoir.

This stores energy as:

gravitational potential energy.

---

How Pumped Hydro Works

During charging:

electrical energy → kinetic energy of pumps → gravitational potential energy

Water is moved uphill.

During discharge:

Water flows downhill through turbines.

Energy conversion:

gravitational potential energy → kinetic energy → electrical energy

The same water can be cycled between the reservoirs many times.

---

Advantages of Pumped Hydro

Pumped hydro can provide:

• very large storage capacity
• long operating life
• large power output
• storage for many hours
• well-established technology

It is especially useful for:

large electricity grids.

---

Limitations of Pumped Hydro

Pumped hydro requires suitable:

geography.

A site generally needs:

• significant elevation difference
• appropriate reservoirs
• access to water
• suitable geology
• substantial construction

Projects can also affect:

• ecosystems
• land use
• waterways
• local communities

Therefore pumped hydro cannot simply be built:

anywhere.

---

Gravitational Potential Energy

The gravitational energy stored can be estimated using:

E = mgh

where:

E = gravitational potential energy in joules

m = mass in kilograms

g = gravitational field strength

h = height difference in metres

Increasing either the mass of water or the height difference increases the amount of:

stored energy.

---

Worked Example: Pumped Hydro

Suppose:

m = 10,000 kg

g = 9.8 N/kg

h = 50 m

Then:

E = mgh

E = 10,000 × 9.8 × 50

E = 4,900,000 J

Therefore:

E = 4.9 MJ

This assumes an ideal system. The useful energy recovered would be less because of:

energy losses.

---

Flywheel Energy Storage

A flywheel stores energy in a rapidly rotating object.

During charging, an electric motor accelerates the flywheel.

Energy conversion:

electrical energy → rotational kinetic energy

When energy is required, the rotating flywheel drives a generator.

Energy conversion:

rotational kinetic energy → electrical energy.

---

Advantages of Flywheels

Flywheels can:

• respond extremely quickly
• provide high power
• undergo many cycles
• operate without the same electrochemical degradation mechanisms as batteries

They are particularly useful for:

short-duration energy storage and power stabilization.

---

Limitations of Flywheels

Flywheels generally store energy for shorter periods than some other technologies.

Energy is gradually lost through:

• friction
• bearing losses
• air resistance
• electrical losses

Advanced flywheels may operate in vacuum chambers and use specialized bearings to reduce these:

losses.

---

Compressed-Air Energy Storage

Another method is:

compressed-air energy storage, or CAES.

Excess electricity powers compressors.

Air is compressed and stored under pressure.

Energy is therefore stored partly through the:

compressed state of the gas.

Later, the compressed air can be expanded through machinery to help generate electricity.

---

Compressed-Air Storage

The basic process is:

electrical energy

↓

compressor

↓

compressed air

↓

storage

↓

expansion

↓

generator

↓

electrical energy

Large systems may use underground:

caverns.

---

Advantages of Compressed-Air Storage

Potential advantages include:

• large-scale energy storage
• relatively long storage duration
• potentially long equipment lifetime

However, performance depends strongly on how the system manages:

heat.

---

Heat and Compression

When a gas is compressed, its temperature tends to:

increase.

When compressed gas expands, its temperature tends to:

decrease.

Managing this thermal energy is an important part of designing efficient compressed-air storage systems.

---

Thermal Energy Storage

Energy can also be stored as:

thermal energy.

Examples include:

• hot water tanks
• molten salt
• heated rocks
• chilled water
• ice storage

Thermal storage is particularly useful when the desired final form of energy is:

heating or cooling.

---

Molten Salt Storage

Some solar thermal power plants use:

molten salt.

Solar energy heats the salt.

The thermal energy can be stored and later used to:

• heat water
• produce steam
• drive a turbine
• generate electricity

This allows solar energy collected during daylight hours to be used:

after sunset.

---

Advantages of Thermal Storage

Thermal energy storage can be:

• relatively simple
• inexpensive in some applications
• useful over several hours
• valuable for heating and cooling
• compatible with solar thermal systems

It can reduce the need to convert energy repeatedly when the final requirement is:

heat.

---

Limitations of Thermal Storage

Thermal energy gradually moves from warmer objects to cooler surroundings.

This causes:

heat loss.

Good insulation reduces heat transfer but cannot eliminate it completely.

If stored heat must later be converted back into electricity, additional energy losses occur.

---

Capacitors

A capacitor stores energy using separated electrical charges.

Unlike a battery, it does not primarily store energy through bulk chemical reactions.

Capacitors can:

• charge rapidly
• discharge rapidly
• provide high power

However, conventional capacitors store relatively:

small amounts of energy.

---

Supercapacitors

Supercapacitors are designed to store much more energy than conventional capacitors while retaining rapid charging and discharging.

They are useful where energy must be:

absorbed or released very quickly.

---

Advantages of Supercapacitors

Supercapacitors can provide:

• extremely fast charging
• extremely fast discharging
• high power
• very large numbers of cycles

They can be useful in:

• regenerative braking
• short-term backup
• power stabilization
• rapid energy recovery

---

Limitations of Supercapacitors

Compared with batteries, supercapacitors generally have much lower:

energy density.

This means a large supercapacitor system may be required to store the same amount of energy as a much smaller battery.

Therefore they are better suited to:

short bursts of power

than long-duration energy storage.

---

Gravity Storage

Pumped hydro is not the only way to store gravitational potential energy.

Other proposed and developing systems lift:

• heavy blocks
• large masses
• weights in shafts

When excess electricity is available:

electrical energy → gravitational potential energy

When electricity is required:

gravitational potential energy → electrical energy

---

Energy Storage Is About Trade-Offs

No storage technology is ideal for every purpose.

A system that is excellent for:

one second

may be poor for:

one month.

A system suitable for a:

smartphone

may be unsuitable for an:

electricity grid.

Energy storage must therefore be selected according to the:

application.

---

Storage Timescales

Different technologies are useful over different timescales.

Very short duration:

• capacitors
• supercapacitors
• flywheels

Short to medium duration:

• batteries

Long duration:

• pumped hydro
• compressed air
• some battery systems

Very long or potentially seasonal storage:

• hydrogen
• some thermal-storage approaches

These categories overlap depending on system design.

---

Power vs Energy

An important distinction is between:

power

and:

energy.

Energy describes:

how much can be stored.

Power describes:

how quickly that energy can be delivered.

A storage system may have high power but relatively little total energy.

For example, a supercapacitor can release energy:

very quickly

but may not store enough energy to operate a home overnight.

---

Energy Capacity

Energy-storage capacity is commonly measured in:

• joules (J)
• kilojoules (kJ)
• megajoules (MJ)
• watt-hours (Wh)
• kilowatt-hours (kWh)
• megawatt-hours (MWh)

Remember:

1 Wh = 3600 J

Therefore:

1 kWh = 3.6 MJ

---

Worked Example: Energy Conversion

A storage system contains:

20 kWh

Convert this to megajoules.

Since:

1 kWh = 3.6 MJ

Then:

20 × 3.6 = 72 MJ

Therefore:

20 kWh = 72 MJ.

---

Round-Trip Efficiency

An important way to evaluate storage is:

round-trip efficiency.

This compares the useful energy recovered with the energy originally supplied.

The equation is:

Efficiency (%) = useful energy output ÷ energy input × 100

A storage system can never return more energy than was originally supplied.

---

Worked Example: Storage Efficiency

A battery receives:

100 kWh

during charging.

Later it returns:

90 kWh.

Efficiency:

90 ÷ 100 × 100 = 90%

Therefore the round-trip efficiency is:

90%.

The remaining energy has been converted into less useful forms, mainly:

thermal energy.

---

Worked Example: Comparing Systems

System A receives 500 kWh and returns 450 kWh.

Efficiency = 450 ÷ 500 × 100

= 90%

System B receives 500 kWh and returns 300 kWh.

Efficiency = 300 ÷ 500 × 100

= 60%

System A has the higher:

round-trip efficiency.

However, efficiency alone does not determine which system is better for a particular application.

---

Where Does the Lost Energy Go?

Energy is not destroyed.

Instead, some energy is transformed into forms that are less useful for the intended purpose.

Losses can occur through:

• electrical resistance
• friction
• heating
• chemical side reactions
• pumps
• compressors
• generators
• power electronics

Eventually much of the lost energy becomes:

thermal energy.

---

Efficiency Is Not the Only Factor

Imagine two storage systems.

System A has very high efficiency but can store energy for only:

a few minutes.

System B has lower efficiency but can store enormous quantities of energy for:

several months.

The best system depends on what the energy storage is required to:

do.

---

Other Important Factors

When comparing storage technologies, consider:

• round-trip efficiency
• storage capacity
• power output
• storage duration
• response time
• cycle life
• cost
• size
• mass
• location requirements
• safety
• environmental impact
• material availability

A complete evaluation considers several factors rather than:

only efficiency.

---

Renewable Energy and Storage

Solar and wind power are:

variable energy sources.

Their output depends on environmental conditions.

Solar generation changes with:

• time of day
• season
• cloud cover

Wind generation changes with:

wind conditions.

Electricity demand does not automatically change in the same way.

---

The Solar Energy Problem

Solar panels may produce their greatest power around:

the middle of the day.

However, electricity demand may remain high during the:

evening.

Without storage, excess midday electricity might not be available after:

sunset.

Storage allows:

daytime generation → stored energy → evening electricity.

---

The Wind Energy Problem

Wind turbines produce electricity when:

wind is available.

Wind speed can change from:

• hour to hour
• day to day
• season to season

Energy storage can absorb electricity during periods of:

high wind generation

and return electricity during periods of:

lower generation.

---

Balancing the Electricity Grid

Electricity supply and demand must be balanced continuously.

If generation suddenly exceeds demand, storage systems can:

absorb energy.

If demand suddenly exceeds generation, storage systems can:

release energy.

Fast-response technologies such as batteries, flywheels, and supercapacitors can help stabilize:

electricity grids.

---

Peak Demand

Electricity demand is not constant.

There are periods of:

peak demand

when many consumers require electricity simultaneously.

Storage systems can charge during periods of lower demand and discharge during:

peak demand.

This process can reduce stress on electricity generation and transmission infrastructure.

---

Renewable Energy Curtailment

Sometimes renewable generators could produce electricity, but the grid cannot use or transport all of it.

The generator may therefore have to reduce its output.

This is called:

curtailment.

Energy storage can sometimes reduce curtailment by absorbing electricity that would otherwise be:

unused.

---

Short-Term vs Long-Term Storage

Different renewable-energy challenges require different storage durations.

For fluctuations lasting seconds:

flywheels or batteries may be useful.

For shifting solar energy from afternoon to evening:

batteries or pumped hydro may be suitable.

For periods lasting several days:

long-duration storage becomes more important.

For seasonal differences:

technologies such as hydrogen may potentially play a role.

---

Comparing Major Storage Technologies

---

Batteries vs Hydrogen

Batteries generally offer:

• higher round-trip efficiency
• rapid response
• compact systems
• direct electrical charging

Hydrogen can offer:

• potentially very long storage duration
• large-scale storage
• transportable fuel
• use outside the electricity sector

Therefore batteries and hydrogen may serve:

different roles.

---

Batteries vs Pumped Hydro

Batteries can be installed in many locations and expanded in:

modular units.

Pumped hydro can store very large quantities of energy but requires:

appropriate geography.

Pumped hydro facilities may operate for many decades.

Batteries are generally easier to deploy in smaller:

distributed systems.

---

Batteries vs Supercapacitors

Batteries have much greater:

energy density.

Supercapacitors have much greater ability to:

charge and discharge rapidly.

Therefore:

battery → better for storing more energy

supercapacitor → better for delivering short bursts of power

Hybrid systems can use:

both.

---

Hybrid Energy-Storage Systems

Sometimes the best solution is to combine technologies.

For example:

Battery + supercapacitor

The battery stores substantial energy.

The supercapacitor handles sudden power demands.

Another example:

Solar + battery + hydrogen

The battery can manage daily variations.

Hydrogen could potentially store surplus energy for:

longer periods.

---

Future Battery Technologies

Researchers are developing new battery technologies to improve:

• energy density
• safety
• cycle life
• charging speed
• cost
• material availability
• recycling

Examples under development or expanding commercialization include:

• solid-state batteries
• sodium-ion batteries
• flow batteries
• improved lithium-ion chemistries

---

Solid-State Batteries

Many conventional lithium-ion batteries use a liquid or gel electrolyte.

A solid-state battery uses a:

solid electrolyte.

Potential advantages may include improvements in:

• safety
• energy density
• battery design

However, challenges remain in:

• manufacturing
• interfaces between materials
• durability
• cost

---

Sodium-Ion Batteries

Sodium-ion batteries use:

Na⁺ ions

instead of lithium ions as the main charge carrier.

Potential advantages include the widespread availability of:

sodium-containing resources.

They may become particularly useful where:

• low cost
• material availability
• stationary storage

are more important than maximum energy density.

---

Flow Batteries

A flow battery stores chemical energy in liquid electrolytes held in external tanks.

The liquids are pumped through an electrochemical cell during operation.

A major advantage is that storage capacity can be increased by using:

larger electrolyte tanks.

---

Why Flow Batteries Are Interesting

Flow batteries can be attractive for stationary storage because:

• energy capacity can be scaled using tank size
• they can be designed for many cycles
• long-duration storage may be possible

However, they generally have lower energy density than lithium-ion batteries.

This matters less when the battery does not need to:

move.

---

Future Hydrogen Technology

Future developments may improve:

• electrolyzer efficiency
• fuel-cell efficiency
• hydrogen storage
• hydrogen transport
• catalyst cost
• system durability

Hydrogen may become particularly useful in sectors where direct battery electrification is:

difficult.

---

Future Thermal Storage

New thermal-storage systems are exploring materials such as:

• molten salts
• rocks
• ceramics
• phase-change materials

These systems may allow inexpensive materials to store large quantities of:

thermal energy.

---

Phase-Change Materials

A phase-change material stores or releases energy when it changes state.

For example:

solid → liquid

can absorb thermal energy.

Later:

liquid → solid

can release thermal energy.

These materials can be useful for:

• buildings
• heating systems
• cooling systems
• industrial thermal storage

---

Gravity-Based Storage

Some emerging systems use heavy masses rather than water.

When excess electricity is available:

motors lift the mass.

This stores:

gravitational potential energy.

Later, the mass is lowered while driving a generator.

The principle is the same as:

pumped hydro.

---

Improving Recycling

Future energy storage also depends on better:

recycling.

Battery materials can be valuable.

Recycling can potentially recover materials such as:

• lithium
• nickel
• cobalt
• copper
• lead

Improved recycling could:

• reduce waste
• reduce demand for new mining
• recover valuable materials
• improve resource security

---

Second-Life Batteries

A battery that is no longer suitable for an electric vehicle may still have useful:

storage capacity.

It could potentially be used in less demanding stationary applications before final recycling.

This is called:

second-life use.

---

Evaluating Future Technologies

New technology should not be evaluated only by asking:

"Does it store more energy?"

We should also ask:

• How efficient is it?
• How expensive is it?
• How long does it last?
• Is it safe?
• Can it be manufactured at large scale?
• Are its materials abundant?
• Can it be recycled?
• What is its environmental impact?
• How quickly can it respond?
• How long can it store energy?

The best technology depends on the:

problem being solved.

---

Worked Example 1

A storage system receives:

800 kWh

and later returns:

680 kWh.

Calculate its round-trip efficiency.

Efficiency = output ÷ input × 100

= 680 ÷ 800 × 100

= 85%

---

Worked Example 2

A system is 75% efficient and receives:

200 kWh.

Useful output:

0.75 × 200 = 150 kWh

Therefore:

150 kWh

can be recovered as useful energy.

---

Worked Example 3

A solar farm produces excess electricity during the afternoon but the community needs more electricity after sunset.

A suitable storage system could:

store afternoon energy and release it in the evening.

A battery or pumped-hydro system might be suitable depending on the:

scale and location.

---

Worked Example 4

A system must provide a very large burst of power for only a few seconds.

A:

supercapacitor or flywheel

may be more appropriate than a storage technology designed mainly for long-duration energy supply.

---

Worked Example 5

A system needs to store a very large amount of energy for several months.

A conventional short-duration battery system may not be the most practical choice.

A technology such as:

hydrogen or another long-duration storage system

may be considered.

The final choice would depend on cost, efficiency, infrastructure, and location.

---

Worked Example 6

Why isn't the storage technology with the highest efficiency automatically the best?

Because other factors matter, including:

• cost
• storage duration
• capacity
• power
• location
• lifetime
• materials
• safety

A lower-efficiency system may be more useful if it can meet a requirement that the higher-efficiency system:

cannot.

---

Worked Example 7

A pumped-hydro system stores 1000 MWh and returns 800 MWh.

Its efficiency is:

800 ÷ 1000 × 100 = 80%

Energy not recovered:

1000 − 800 = 200 MWh

The energy was transformed mainly into less useful:

thermal energy.

---

Worked Example 8

Why might a grid use both batteries and pumped hydro?

Batteries can provide:

fast response and flexible installation.

Pumped hydro can provide:

large capacity and long operating life.

Combining technologies allows each to perform tasks suited to its:

strengths.

---

Common Mistake: Energy Storage Creates Energy

Storage systems do not create energy.

They:

store and transform energy.

Because of energy losses, they always return less useful energy than was originally supplied.

---

Common Mistake: Electricity Is Always Stored as Electricity

Most storage technologies convert electrical energy into another form.

For example:

Battery:

electrical → chemical

Pumped hydro:

electrical → gravitational

Flywheel:

electrical → kinetic

Hydrogen:

electrical → chemical

---

Common Mistake: A Fuel Cell Stores Hydrogen

A fuel cell converts chemical energy into electrical energy.

The hydrogen itself must be stored in a separate:

storage system.

The fuel cell is mainly an:

energy-conversion device.

---

Common Mistake: Efficiency Is the Only Important Factor

A storage system must also be evaluated by:

• cost
• capacity
• power
• duration
• lifespan
• location
• safety
• environmental impact

A highly efficient system may still be unsuitable for a particular application.

---

Common Mistake: Batteries Are Always the Best Storage Technology

Batteries are extremely useful, but they are not ideal for every situation.

Large-scale or long-duration storage may sometimes be better served by:

• pumped hydro
• hydrogen
• compressed air
• thermal storage
• other technologies

---

Common Mistake: Renewable Energy Cannot Work Without Constant Sun or Wind

Renewable generation is variable, but electricity systems can use combinations of:

• energy storage
• transmission
• different generation sources
• demand management
• geographic diversity

Storage is one important tool for matching:

energy supply with demand.

---

Check Your Understanding

1. Define energy storage.
2. Why is energy storage useful?
3. Name six forms in which energy can be stored.
4. What energy conversion occurs when a battery charges?
5. What energy conversion occurs when a battery discharges?
6. Give two advantages of lithium-ion battery storage.
7. Give two limitations of battery storage.
8. Explain how hydrogen can be used to store energy.
9. Why is a fuel cell not technically the main energy-storage component of a hydrogen system?
10. What is pumped hydroelectric storage?
11. What form of energy is stored in pumped hydro?
12. Write the equation for gravitational potential energy.
13. Explain how a flywheel stores energy.
14. Give one advantage and one limitation of flywheels.
15. Describe compressed-air energy storage.
16. Give three examples of thermal energy storage.
17. How does molten salt store energy?
18. What is a supercapacitor?
19. Why are supercapacitors useful for short bursts of power?
20. Why are supercapacitors less suitable for long-duration storage?
21. Distinguish between energy and power.
22. Convert 5 kWh into MJ.
23. Define round-trip efficiency.
24. A storage system receives 500 kWh and returns 425 kWh. Calculate its efficiency.
25. Why can no storage system be 100% efficient?
26. Identify four sources of energy loss in storage systems.
27. Why is efficiency not the only factor used to compare storage technologies?
28. Why is energy storage important for solar power?
29. Why is energy storage important for wind power?
30. What is peak demand?
31. What is renewable-energy curtailment?
32. Explain how storage can reduce curtailment.
33. Compare batteries and hydrogen storage.
34. Compare batteries and pumped hydro.
35. Compare batteries and supercapacitors.
36. Why might an electricity grid use several different storage technologies?
37. What is a hybrid energy-storage system?
38. What is a solid-state battery?
39. What is a sodium-ion battery?
40. Describe how a flow battery works.
41. Why can flow batteries be useful for stationary storage?
42. What is a phase-change material?
43. Explain the basic principle behind gravity-storage systems.
44. What is meant by second-life battery use?
45. Why is battery recycling important?
46. A storage system is 70% efficient and receives 600 kWh. Calculate the useful energy output.
47. Explain why long-duration storage may become increasingly important as renewable electricity generation increases.
48. Compare at least four energy-storage technologies in terms of energy form, efficiency, duration, cost, and limitations.
49. Evaluate which storage technology might be suitable for storing solar energy from afternoon until evening.
50. Explain why the future electricity grid may use a mixture of different energy-storage technologies rather than relying on only one.

---

Key Terms

Energy storage: Capturing energy for use at a later time.

Battery energy storage system (BESS): System using rechargeable batteries to store electrical energy as chemical energy.

Hydrogen storage: Storage of energy in the chemical form of hydrogen.

Pumped hydroelectric storage: Storage of energy by pumping water to a higher elevation.

Gravitational potential energy: Energy stored because of an object's position in a gravitational field.

Flywheel: Device that stores energy as rotational kinetic energy.

Compressed-air energy storage: Storage of energy using compressed gas.

Thermal energy storage: Storage of energy as heat or cold.

Supercapacitor: Device capable of rapidly storing and releasing electrical energy.

Round-trip efficiency: Percentage of input energy recovered as useful output after storage.

Energy capacity: Total amount of energy a system can store.

Power: Rate at which energy is transferred.

Curtailment: Reduction of electricity generation because the available electricity cannot currently be used or transported.

Long-duration energy storage: Storage designed to supply energy over extended periods.

Flow battery: Rechargeable electrochemical system using liquid electrolytes stored in external tanks.

Solid-state battery: Battery using a solid electrolyte rather than a conventional liquid or gel electrolyte.

Sodium-ion battery: Rechargeable battery using sodium ions as charge carriers.

Phase-change material: Material that stores or releases thermal energy during a change of state.

Second-life battery: Used battery repurposed for a less demanding application before recycling.

---

Key Takeaways

• Energy storage captures energy so that it can be used later.
• Storage systems usually convert energy into another form rather than simply storing electricity directly.
• Batteries store energy as chemical energy.
• Hydrogen stores chemical energy and can later supply a fuel cell.
• Pumped hydro stores gravitational potential energy.
• Flywheels store rotational kinetic energy.
• Compressed-air systems store energy using compressed gas.
• Thermal systems store energy as heat or cold.
• Supercapacitors can absorb and release energy extremely quickly.
• Different technologies are suitable for different timescales and applications.
• Round-trip efficiency compares useful energy recovered with energy originally supplied.
• No real storage system is 100% efficient.
• Efficiency is important, but so are cost, capacity, power, duration, lifetime, safety, location, and environmental impact.
• Energy storage is particularly important for variable renewable sources such as solar and wind.
• Storage can shift renewable electricity from times of high generation to times of high demand.
• Storage can help stabilize electricity grids and reduce renewable-energy curtailment.
• Batteries are useful for fast response and short-to-medium-duration storage.
• Pumped hydro can provide very large-scale storage where suitable geography exists.
• Hydrogen may have advantages for some forms of long-duration or seasonal storage, although its round-trip efficiency is generally lower.
• Future technologies include solid-state batteries, sodium-ion batteries, flow batteries, advanced thermal storage, hydrogen systems, and gravity storage.
• Future energy systems will likely use multiple storage technologies, because no single technology is ideal for every energy-storage problem.