What Is a Secondary Cell?

A secondary cell is an electrochemical cell designed to be:

recharged and used repeatedly.

Like a primary cell, a secondary cell converts stored chemical energy into electrical energy when it is supplying power.

During discharge:

chemical energy → electrical energy

However, a secondary cell can also use electrical energy from an external source to restore much of its original chemical state.

During charging:

electrical energy → chemical energy

This ability to undergo repeated charging and discharging is the defining characteristic of a:

secondary cell.

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Rechargeable Cells

Secondary cells are commonly called:

rechargeable cells.

They are used in devices that require electrical energy repeatedly.

Examples include:

• smartphones
• laptops
• tablets
• electric vehicles
• cordless tools
• rechargeable flashlights
• cameras
• backup power systems
• vehicles

The ability to recharge the battery reduces the need to replace it after every:

discharge cycle.

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How Does a Secondary Cell Produce Electricity?

During discharge, a secondary cell behaves as a:

galvanic cell.

A spontaneous redox reaction occurs.

At the anode:

oxidation occurs.

At the cathode:

reduction occurs.

Electrons travel through the external circuit:

anode → device → cathode

The movement of electrons provides electrical energy to the device.

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What Happens During Discharge?

During discharge:

• oxidation releases electrons
• reduction consumes electrons
• electrons flow through the external circuit
• ions move within the battery
• reactants are converted into products
• stored chemical energy decreases

The cell therefore converts:

chemical energy → electrical energy.

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What Happens During Charging?

Charging changes the direction of the electrochemical process.

An external power source supplies:

electrical energy.

This energy drives chemical reactions in the opposite direction from the spontaneous discharge reaction.

In simplified form:

During discharge:

Reactants → Products + electrical energy

During charging:

Products + electrical energy → Reactants

The cell stores energy again as:

chemical energy.

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Charging Requires Energy

The reverse reaction does not occur spontaneously during charging.

A charger must provide sufficient electrical potential to drive the reaction in the:

nonspontaneous direction.

The cell temporarily operates as an:

electrolytic system.

Electrical energy forces chemical changes that restore the battery toward its charged state.

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Reversible Chemical Reactions

The reactions in secondary cells are designed to be sufficiently:

reversible.

This does not mean they reverse perfectly.

During every charge-discharge cycle, small unwanted changes may occur.

Over many cycles:

• electrode materials may change
• side reactions may occur
• electrolyte may degrade
• internal resistance may increase
• usable capacity may decrease

Therefore rechargeable batteries do not last:

forever.

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The Charge-Discharge Cycle

One complete sequence of:

discharge → recharge

is commonly called a:

charge-discharge cycle.

A secondary battery may undergo hundreds or even thousands of cycles depending on:

• battery chemistry
• operating conditions
• temperature
• depth of discharge
• charging method
• battery design

Eventually its performance decreases.

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Primary vs Secondary Cells

The major difference is rechargeability.

Primary cell

Designed mainly for:

one discharge cycle.

Once its useful chemical reactants have been consumed or transformed, the cell is replaced or recycled.

Secondary cell

Designed for:

repeated charging and discharging.

Electrical energy can drive the chemical reactions back toward the charged state.

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Comparing Energy Transformations

For a primary cell:

chemical energy → electrical energy

For a secondary cell during discharge:

chemical energy → electrical energy

For a secondary cell during charging:

electrical energy → chemical energy

This last transformation makes secondary cells:

rechargeable.

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Common Types of Secondary Cells

Important rechargeable battery systems include:

• lithium-ion
• lead-acid
• nickel-metal hydride
• nickel-cadmium
• rechargeable alkaline systems in some specialized applications

The most important modern examples include:

lithium-ion, lead-acid, and nickel-metal hydride batteries.

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Lithium-Ion Batteries

Lithium-ion batteries are among the most widely used rechargeable batteries.

They are commonly found in:

• smartphones
• laptops
• tablets
• cameras
• electric vehicles
• cordless power tools
• energy-storage systems

Their high energy density makes them particularly useful in:

portable electronics and transportation.

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Structure of a Lithium-Ion Cell

A simplified lithium-ion cell contains:

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

During operation, lithium ions move between the two electrodes through the:

electrolyte.

Electrons travel through the:

external circuit.

The separator prevents direct electrical contact between the electrodes while allowing:

ionic movement.

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Lithium-Ion Discharge

During discharge:

• oxidation occurs at the negative electrode
• electrons enter the external circuit
• lithium ions move through the electrolyte
• electrons power the external device
• reduction occurs at the positive electrode

Energy conversion:

chemical energy → electrical energy.

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Lithium-Ion Charging

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

Electrical energy causes lithium ions and electrons to move so that the battery returns toward its:

charged state.

Energy conversion:

electrical energy → chemical energy.

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Why Lithium-Ion Batteries Are Popular

Lithium-ion batteries offer several useful characteristics:

• high energy density
• relatively low mass
• rechargeable operation
• relatively low self-discharge
• useful cycle life
• high cell voltage

These characteristics make them especially suitable for:

portable electronics.

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Limitations of Lithium-Ion Batteries

Lithium-ion batteries also have limitations.

They can:

• degrade with time
• lose capacity through repeated cycling
• be damaged by excessive heat
• require electronic protection systems
• present fire risks if severely damaged, defective, improperly charged, or overheated

Battery management systems are therefore important in many lithium-ion applications.

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Electric Vehicles

Electric vehicles commonly use large rechargeable battery packs based on:

lithium-ion technology.

A battery pack contains many individual cells arranged to provide the required:

• voltage
• capacity
• power

During driving:

chemical energy → electrical energy → kinetic energy

During charging:

electrical energy → chemical energy.

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Regenerative Braking

Some electric and hybrid vehicles can recover part of their kinetic energy during braking.

Instead of converting all the vehicle's kinetic energy into heat through friction, the motor can act as a:

generator.

Energy can be transferred:

kinetic energy → electrical energy → chemical energy

and stored in the battery.

This process is called:

regenerative braking.

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Lead-Acid Batteries

The lead-acid battery is one of the oldest widely used rechargeable battery systems.

It is commonly used in:

• cars
• motorcycles
• backup power systems
• uninterruptible power supplies
• some energy-storage applications

A typical car battery contains multiple lead-acid cells.

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Lead-Acid Cell Chemistry

A lead-acid cell uses:

• lead
• lead dioxide
• sulfuric acid electrolyte

During discharge, both electrode materials are converted toward:

lead sulfate.

During charging, an external electrical source drives the reactions in the opposite direction, restoring the electrode materials toward their charged forms.

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Why Use Lead-Acid Batteries?

Lead-acid batteries have several advantages:

• relatively low cost
• reliable technology
• ability to provide large currents
• established recycling systems
• useful performance for vehicle starting

However, they also have disadvantages:

• relatively high mass
• lower energy density than lithium-ion batteries
• toxic lead
• corrosive electrolyte

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Car Starting Batteries

Starting a car engine requires a very large electrical current for a short period.

Lead-acid batteries are well suited to this because they can provide:

high current.

Once the engine is running, the vehicle's charging system restores energy to the battery.

The battery can therefore be used:

repeatedly.

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Nickel-Metal Hydride Batteries

Nickel-metal hydride, or NiMH, batteries are another common type of secondary cell.

They are used in:

• rechargeable AA and AAA cells
• cameras
• toys
• household electronics
• some hybrid vehicles

NiMH batteries can often replace disposable alkaline cells in compatible devices.

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Advantages of NiMH Batteries

NiMH cells offer:

• rechargeability
• useful capacity
• common AA and AAA sizes
• reduced disposable-battery waste when repeatedly used

They can be useful for devices such as:

• cameras
• game controllers
• toys
• flashlights

especially when the device is used:

frequently.

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Nickel-Cadmium Batteries

Nickel-cadmium, or NiCd, batteries are rechargeable cells that were once widely used.

They have useful characteristics such as:

• high current capability
• good cycle performance
• operation across a range of conditions

However, cadmium is:

toxic.

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

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Rechargeable Batteries Are Not All the Same

Different secondary battery chemistries have different:

• voltages
• capacities
• energy densities
• charging requirements
• cycle lives
• operating temperatures
• safety characteristics

A charger designed for one chemistry may not be appropriate for:

another chemistry.

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Why Does Charging Need to Be Controlled?

Charging a battery is not simply a matter of connecting it to any electrical source.

The charger must control factors such as:

• voltage
• current
• charging time
• sometimes temperature

Incorrect charging can cause:

• overheating
• unwanted chemical reactions
• battery damage
• reduced lifespan
• safety hazards

Rechargeable batteries should therefore be used with:

appropriate charging systems.

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Battery Management Systems

Large lithium-ion battery packs commonly use a:

battery management system (BMS).

A BMS can monitor factors such as:

• cell voltage
• current
• temperature
• state of charge

It can help prevent:

• overcharging
• excessive discharge
• overheating
• unsafe operating conditions

This is especially important in:

electric vehicles and large battery packs.

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Why Do Rechargeable Batteries Lose Capacity?

Rechargeable batteries gradually degrade.

Over time:

• electrode structures may change
• side reactions may consume active material
• electrolyte may degrade
• internal resistance may increase

As a result, the battery stores less usable energy.

A battery that originally operated a device for 10 hours might eventually operate it for only:

7 hours, then 5 hours, and so on.

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Capacity

Battery capacity measures the amount of electrical charge a battery can deliver under specified conditions.

It is commonly measured in:

ampere-hours (Ah)

or:

milliampere-hours (mAh).

For example:

3000 mAh = 3.0 Ah

Greater capacity generally means that a battery can deliver more total charge before needing to be:

recharged.

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Battery Energy

An approximate measure of stored electrical energy is:

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

Suppose a rechargeable battery is rated:

3.7 V, 4.0 Ah

Then:

Energy = 3.7 × 4.0

Energy = 14.8 Wh

The battery stores approximately:

14.8 watt-hours under its nominal rating.

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Energy Is Lost During Charging

Charging is not perfectly efficient.

Some electrical energy is converted into:

thermal energy.

Therefore:

electrical energy supplied > chemical energy successfully stored

Likewise, not all stored chemical energy can be converted into useful electrical work.

Real batteries always involve some:

energy loss.

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Charging Efficiency

The efficiency of charging depends on:

• battery chemistry
• charger design
• charging rate
• temperature
• battery condition

A battery becoming slightly warm during charging can indicate that some energy is being converted into:

thermal energy.

Excessive heating, however, may indicate a problem.

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Self-Discharge

Even when a rechargeable battery is not connected to a device, it may gradually lose stored charge.

This is called:

self-discharge.

Different battery chemistries have different self-discharge rates.

This can be important when choosing batteries for devices that are:

rarely used.

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Cycle Life

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

Cycle life depends on:

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

A battery's cycle life is therefore not always a single fixed number.

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Depth of Discharge

Depth of discharge describes how much of a battery's stored capacity has been used.

For example, if a battery begins fully charged and 30% of its usable capacity is consumed, its depth of discharge is approximately:

30%.

For some battery chemistries, repeated very deep discharge can contribute to faster:

degradation.

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Primary vs Secondary: Initial Cost

Primary cells often have a:

lower initial purchase cost.

Secondary cells may cost more initially because they require:

• rechargeable chemistry
• more complex construction
• sometimes a charger

However, the long-term picture can be different.

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Primary vs Secondary: Long-Term Cost

Suppose a device requires new batteries every week.

Over several years, repeatedly buying primary batteries can become expensive.

A rechargeable battery can potentially be used for:

many cycles.

Therefore secondary cells may provide lower long-term cost in:

frequently used devices.

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Primary vs Secondary: Waste

A primary battery is replaced after discharge.

A secondary battery can be recharged many times before replacement.

Therefore secondary cells can reduce the number of batteries:

discarded over time.

However, rechargeable batteries still require responsible recycling at the end of their useful lives.

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Primary vs Secondary: Shelf Life

Primary batteries often have excellent:

shelf life.

This can make them suitable for:

• emergency equipment
• rarely used devices
• long-term storage

Some rechargeable batteries gradually lose charge during storage and may need:

periodic recharging.

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Primary vs Secondary: High-Use Devices

Secondary cells are often advantageous for devices used frequently.

Examples include:

• smartphones
• laptops
• electric vehicles
• power tools
• game controllers
• cameras

Replacing primary cells every time these devices became discharged would be:

impractical and expensive.

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Primary vs Secondary: Low-Use Devices

Primary cells may be practical in devices that use very little energy or are rarely operated.

Examples can include:

• clocks
• remote controls
• emergency equipment
• some sensors

The best choice depends on:

the application and battery chemistry.

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Comparison of Primary and Secondary Cells

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Choosing Between Primary and Secondary Cells

There is no single battery type that is ideal for every situation.

Important questions include:

• How often will the device be used?
• How much energy does it require?
• Is recharging convenient?
• How long must the battery remain in storage?
• Is low mass important?
• What voltage is required?
• How much current is required?
• What is the long-term cost?
• What environmental impacts should be considered?

Battery choice requires balancing:

performance, cost, convenience, safety, and sustainability.

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Example: Television Remote

A television remote consumes relatively little energy.

A set of alkaline primary cells may last for:

months or years.

In this situation, primary cells can be convenient because frequent charging is unnecessary.

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Example: Smartphone

A smartphone consumes significant electrical energy every day.

Using disposable primary cells would require constant:

replacement.

A rechargeable lithium-ion battery is much more practical because it can be:

recharged repeatedly.

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Example: Digital Camera

A frequently used camera may consume batteries relatively quickly.

Rechargeable NiMH batteries can be useful because they can be:

recharged many times.

For a camera used only occasionally, long-shelf-life primary batteries might sometimes be more convenient.

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Example: Electric Vehicle

An electric vehicle requires a large amount of stored electrical energy and must be used repeatedly.

A rechargeable battery is essential.

During driving:

chemical → electrical → kinetic

During charging:

electrical → chemical

A primary battery would be economically and practically unsuitable for normal repeated vehicle use.

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Example: Emergency Flashlight

An emergency flashlight may remain unused for a long time.

A primary battery with:

long shelf life and low self-discharge

may be advantageous.

A rechargeable battery could also work, but it might require periodic checking and charging.

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Example: Solar Energy Storage

Solar panels generate electricity when sufficient sunlight is available.

However, electricity may be needed:

at night.

Secondary batteries can store some of the daytime electrical energy as:

chemical energy.

Later:

chemical energy → electrical energy

when the stored energy is required.

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Secondary Cells and Renewable Energy

Rechargeable batteries can help balance differences between:

energy production and energy demand.

For example:

During periods of high renewable electricity generation:

electrical energy → stored chemical energy

Later:

stored chemical energy → electrical energy

This makes secondary cells important in some modern:

energy-storage systems.

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Environmental Considerations

Secondary batteries can reduce waste because a single battery may replace many primary batteries over its lifetime.

However, secondary batteries still require materials such as:

• lithium
• nickel
• cobalt in some chemistries
• lead in lead-acid batteries
• graphite
• copper
• aluminum

Mining, processing, manufacturing, and transportation all have environmental impacts.

Rechargeability does not mean:

zero environmental impact.

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Battery Recycling

At the end of their useful life, secondary batteries should be appropriately:

collected and recycled.

Recycling can help recover materials such as:

• lead
• nickel
• cobalt
• copper
• lithium-containing materials

The exact recycling process depends on the:

battery chemistry.

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Safety

Secondary batteries store substantial amounts of energy.

Safe use includes:

• using appropriate chargers
• avoiding physical damage
• avoiding excessive temperatures
• following manufacturer instructions
• properly storing and transporting batteries
• recycling damaged or worn-out batteries appropriately

A swollen, damaged, leaking, or overheating rechargeable battery should not be:

used normally.

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Worked Example 1

A battery powers a laptop while disconnected from its charger.

What energy transformation occurs?

Chemical energy → electrical energy

The battery is:

discharging.

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Worked Example 2

A laptop is connected to its charger.

Energy from the power supply restores the battery toward its charged state.

The main energy transformation is:

electrical energy → chemical energy.

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Worked Example 3

Why can a secondary cell be recharged?

Its electrochemical system is designed so that an external electrical source can drive the discharge reactions substantially in the:

reverse direction.

This restores the chemical system toward its original charged state.

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Worked Example 4

Why can't a rechargeable battery be recharged indefinitely?

Each cycle causes some:

irreversible chemical and physical changes.

Over time these reduce the battery's capacity and performance.

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Worked Example 5

A battery is rated:

12 V, 5 Ah

Estimate its nominal stored energy.

Use:

Energy = Voltage × Capacity

Energy = 12 × 5

Energy = 60 Wh

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Worked Example 6

A student says:

"Rechargeable batteries do not use chemical reactions because electricity is stored inside them."

Is this correct?

No.

Rechargeable batteries store energy mainly through:

chemical changes.

During discharge, chemical reactions produce electrical energy.

During charging, electrical energy drives chemical changes that restore the battery.

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Worked Example 7

Why is a rechargeable battery usually more appropriate for a smartphone than a primary battery?

A smartphone:

• is used frequently
• requires substantial energy
• is charged repeatedly

Constantly replacing primary cells would be:

impractical, expensive, and wasteful.

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Worked Example 8

Why might a primary cell still be preferable in an emergency device?

Some primary cells offer:

• excellent shelf life
• low self-discharge
• immediate readiness

These properties can be useful when a device is stored for:

long periods.

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Worked Example 9

A rechargeable battery originally has a capacity of 3000 mAh.

After several years, its maximum capacity is only 2100 mAh.

Percentage of original capacity remaining:

2100 ÷ 3000 × 100 = 70%

The battery now stores approximately:

70% of its original rated capacity.

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Worked Example 10

Why can a lead-acid car battery be used repeatedly?

The chemical changes occurring during discharge can be substantially reversed when the vehicle's charging system supplies:

electrical energy.

This restores the battery toward its charged state.

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Common Mistake: Rechargeable Means Perfectly Reversible

Secondary-cell reactions are sufficiently reversible for repeated use, but they are not:

perfectly reversible.

Side reactions and structural changes gradually cause battery degradation.

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Common Mistake: Charging Creates Energy

Charging does not create energy.

The charger transfers:

electrical energy into the battery.

Some becomes stored chemical energy and some is lost, mainly as:

thermal energy.

Energy is transformed, not created.

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Common Mistake: A Charger Supplies Electrons That Stay in the Battery

A rechargeable battery does not simply store a container full of extra electrons.

Charging drives:

chemical and ionic changes.

Energy is stored mainly in the altered chemical state of the battery.

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Common Mistake: Secondary Cells Never Need Replacing

All rechargeable batteries eventually:

degrade.

Their useful capacity and performance decrease with age and cycling.

Eventually they must be:

replaced and appropriately recycled.

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Common Mistake: All Rechargeable Batteries Are Lithium-Ion

Lithium-ion is extremely common, but other rechargeable chemistries include:

• lead-acid
• nickel-metal hydride
• nickel-cadmium

Different chemistries are suitable for different:

applications.

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Common Mistake: Primary Cells Are Always Worse

Primary cells can be better suited to some applications.

For example, they may offer:

• long shelf life
• low initial cost
• low self-discharge
• convenience for rarely used devices

The best choice depends on:

how the battery will be used.

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Common Mistake: Secondary Cells Produce Electricity Differently

Both primary and secondary cells produce electricity during discharge through:

spontaneous redox reactions.

The important difference is that secondary cells are designed so their chemical changes can be substantially reversed through:

charging.

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Common Mistake: Bigger Capacity Means Bigger Voltage

Capacity and voltage measure different things.

Voltage describes electrical potential difference.

Capacity describes how much charge a battery can deliver.

A battery can have a large capacity without having a large:

voltage.

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Check Your Understanding

1. Define a secondary cell.

2. What is another common name for a secondary cell?

3. What energy transformation occurs during discharge?

4. What energy transformation occurs during charging?

5. What type of reaction produces electricity during discharge?

6. Where does oxidation occur during galvanic discharge?

7. Where does reduction occur during galvanic discharge?

8. In which direction do electrons travel through the external circuit during discharge?

9. Why can secondary cells be recharged?

10. Why does charging require an external energy source?

11. What is a charge-discharge cycle?

12. Why do secondary batteries eventually degrade?

13. Name three common types of secondary batteries.

14. Give four common applications of lithium-ion batteries.

15. What moves through the electrolyte in a lithium-ion cell?

16. What travels through the external circuit?

17. Give three advantages of lithium-ion batteries.

18. Give two limitations of lithium-ion batteries.

19. What is a battery management system?

20. Why are battery management systems important?

21. Name three applications of lead-acid batteries.

22. Why are lead-acid batteries useful for starting cars?

23. What is a NiMH battery?

24. Where might NiMH batteries be used?

25. Why has the use of nickel-cadmium batteries decreased in many consumer applications?

26. Define battery capacity.

27. What units are commonly used for capacity?

28. A 3.7 V battery has a capacity of 5.0 Ah. Calculate its approximate nominal energy.

29. What is self-discharge?

30. What is cycle life?

31. Explain why charging is not 100% efficient.

32. Compare the initial costs of primary and secondary cells.

33. Compare their long-term costs for frequently used devices.

34. Why can secondary cells reduce battery waste?

35. Why might primary cells still be preferable for some emergency devices?

36. Explain why a smartphone normally uses a secondary battery.

37. Explain why an electric vehicle requires rechargeable batteries.

38. Describe how secondary batteries can support renewable-energy systems.

39. Compare primary and secondary cells in terms of rechargeability, cost, applications, shelf life, and environmental impact.

40. A school must choose batteries for frequently used digital equipment and rarely used emergency equipment. Recommend an appropriate type for each situation and justify your choices.

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Key Terms

• Secondary cell: Electrochemical cell designed for repeated charging and discharging.
• Rechargeable battery: Common name for a secondary cell or battery.
• Discharge: Process in which stored chemical energy is converted into electrical energy.
• Charging: Process in which external electrical energy drives chemical changes that restore the battery toward its charged state.
• Reversible reaction: Reaction that can proceed significantly in both forward and reverse directions under suitable conditions.
• Charge-discharge cycle: One sequence of using stored energy and then recharging the battery.
• Cycle life: Number of charge-discharge cycles a battery can undergo before its performance falls below a specified level.
• Capacity: Amount of electrical charge a battery can deliver under specified conditions.
• Self-discharge: Gradual loss of stored charge while a battery is not powering a device.
• Lithium-ion battery: Rechargeable battery in which lithium ions move between electrode materials.
• Lead-acid battery: Rechargeable battery using lead-based electrodes and sulfuric acid electrolyte.
• Nickel-metal hydride battery: Rechargeable battery commonly used in AA and AAA formats and some vehicles.
• Battery management system: Electronic system that monitors and controls battery operation.
• Depth of discharge: Fraction or percentage of battery capacity removed during use.
• Regenerative braking: Process that recovers some vehicle kinetic energy and transfers it back toward battery storage.
• Energy density: Amount of energy stored per unit mass or volume.
• Electrolyte: Material containing mobile ions that conducts charge within a cell.
• Separator: Material that prevents direct electrical contact between electrodes while permitting ionic movement.

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Key Takeaways

• Secondary cells are rechargeable electrochemical cells.
• During discharge, they convert chemical energy into electrical energy.
• During charging, they convert supplied electrical energy into stored chemical energy.
• During galvanic discharge, oxidation occurs at the anode and reduction occurs at the cathode.
• Charging uses an external power source to drive the cell chemistry substantially in the reverse direction.
• Secondary cells differ from primary cells mainly because they are designed for repeated charge-discharge cycles.
• Rechargeable reactions are not perfectly reversible, so secondary cells gradually degrade.
• Common secondary batteries include lithium-ion, lead-acid, and nickel-metal hydride batteries.
• Lithium-ion batteries are widely used in smartphones, laptops, electric vehicles, and power tools.
• Lead-acid batteries are widely used in vehicles and backup power systems.
• NiMH cells are commonly available as rechargeable AA and AAA batteries.
• Rechargeable batteries usually cost more initially but can be more economical for frequently used devices.
• Primary cells can still be useful for devices requiring long shelf life or very infrequent use.
• Secondary cells can reduce the number of batteries discarded because one battery can be reused many times.
• Rechargeable batteries still require raw materials, energy for manufacturing and charging, and appropriate end-of-life recycling.
• Capacity and voltage are different battery characteristics.
• Charging is not perfectly efficient, so some energy is lost as heat.
• Battery management and appropriate charging are important for safety and battery lifespan.
• Secondary cells are important for portable electronics, transportation, backup power, and renewable-energy storage.
• Choosing between primary and secondary cells depends on frequency of use, energy requirements, cost, shelf life, convenience, safety, and environmental considerations.