Batteries

What is a battery?

In plain words: a battery contains one or more cells and supplies electricity galvanically during discharge. Cells may be vented or sealed and connected in series, parallel or both. A primary torch cell can power many sessions until discharged; it is then replaced. A phone or car battery can be recharged: the charger drives the same reaction backwards, and meanwhile the battery works as an electrolytic cell. Cells in series add their voltages.

Primary batteries

In NCERT’s words:

In the primary batteries, the reaction occurs only once and after use over a period of time battery becomes dead and cannot be reused again.

NCERT Chemistry Part I (Class 12), Section 2.6.1, page 54

The two NCERT examples are the dry cell and the mercury cell.

Dry cell and mercury cell

• Dry cell (Leclanché cell), named after its discoverer and used in transistor radios and clocks (NCERT page 54):
  • anode: the zinc container;
  • cathode: a carbon (graphite) rod surrounded by powdered manganese dioxide and carbon;
  • electrolyte: a moist paste of ammonium chloride (NH₄Cl) and zinc chloride (ZnCl₂) between the electrodes.
• The reactions are complex, but approximately (NCERT page 54):
  • anode: Zn(s) → Zn²⁺ + 2e⁻
  • cathode: MnO₂ + NH₄⁺ + e⁻ → MnO(OH) + NH₃
• At the cathode manganese is reduced from the oxidation state +4 to +3. The ammonia forms a complex with Zn²⁺, [Zn(NH₃)₄]²⁺. The cell gives nearly 1.5 V (NCERT page 54). The graphite rod mainly collects the current; the MnO₂ around it is what is reduced (reasoned from the cathode reaction).
• Mercury cell, suitable for low-current devices such as hearing aids and watches (NCERT pages 54–55). Beyond the NCERT text: because mercury is toxic, the sale of mercury batteries has since been banned in many countries (Wikipedia, “Mercury battery”).
  • anode: zinc–mercury amalgam, Zn(Hg); cathode: a paste of HgO and carbon; electrolyte: a paste of KOH and ZnO;
  • anode: Zn(Hg) + 2OH⁻ → ZnO(s) + H₂O + 2e⁻
  • cathode: HgO + H₂O + 2e⁻ → Hg(l) + 2OH⁻
  • overall: Zn(Hg) + HgO(s) → ZnO(s) + Hg(l)
• The mercury cell gives about 1.35 V, and this remains constant during its life, because the overall reaction involves no ion in solution whose concentration could change (NCERT page 55). Strictly, it is nearly rather than exactly constant: zinc in an amalgam is part of a mixture, not a pure solid, and its activity falls as the zinc is used up, so E drifts slightly; load and temperature also affect the terminal voltage (reasoned from the Nernst equation). In an exam, give NCERT’s reason.
• The caption of NCERT Figure 2.9 (page 55) names zinc as the reducing agent and mercury(II) oxide as the oxidising agent: zinc goes from 0 to +2, mercury from +2 to 0.

Secondary batteries

In NCERT’s words:

A secondary cell after use can be recharged by passing current through it in the opposite direction so that it can be used again.

NCERT Chemistry Part I (Class 12), Section 2.6.2, page 55

• A good secondary cell can go through a large number of discharging and charging cycles (NCERT page 55).
• In use it is a galvanic cell; while charging it is an electrolytic cell, driven by the charger (reasoned from NCERT pages 32 and 55).

Lead storage battery

• NCERT calls it the most important secondary cell (page 55); it is used in automobiles and inverters. It has a lead anode and a grid of lead packed with lead dioxide (PbO₂) as cathode, in a 38% solution of sulphuric acid (NCERT page 55).
• In use (discharge) (NCERT page 55):
  • anode: Pb(s) + SO₄²⁻(aq) → PbSO₄(s) + 2e⁻
  • cathode: PbO₂(s) + SO₄²⁻(aq) + 4H⁺(aq) + 2e⁻ → PbSO₄(s) + 2H₂O(l)
  • overall: Pb(s) + PbO₂(s) + 2H₂SO₄(aq) → 2PbSO₄(s) + 2H₂O(l)
• Lead ends at +2 in PbSO₄ on both plates, up from 0 at the anode and down from +4 at the cathode (reasoned from NCERT page 55).
• Charging reverses the reaction: the PbSO₄ on the discharge anode and discharge cathode is converted back into Pb and PbO₂, respectively (NCERT page 55).
• Naming on charge (NCERT page 55): NCERT keeps the discharge names while charging, saying the PbSO₄ “on anode and cathode” is converted back. That treats the names as labels for the plates (reasoned). If anode and cathode mean the sites of the current reactions, the names swap on charge. Oxidation happens at the anode (NCERT pages 33 and 51). So on charging, the lead plate, where PbSO₄ is reduced, is the cathode, and the PbO₂ plate is the anode. The signs stay: lead negative, PbO₂ positive (NCERT Figure 2.10, page 55). In an exam, quote NCERT’s sentence for what charging does; if asked which plate is the anode or the cathode during charging, name each by its reaction.
• Discharge uses up sulphuric acid and makes water, so the acid gets weaker: 1 mol of H₂SO₄ for every mole of electrons (reasoned from the overall equation). Sulphuric acid is denser than water, so the electrolyte’s density falls on discharge and rises on charging (reasoned). Beyond the NCERT text: this makes the density a guide to a lead–acid battery’s charge. In the nickel-cadmium cell the potassium hydroxide is not used up, so its density is no such guide (Wikipedia, “Nickel–cadmium battery”).

Nickel-cadmium cell

• Another important secondary cell. It has a longer life than the lead storage cell but costs more to make (NCERT page 56); actual cost and cycle life depend on design and operating conditions (reasoned). Its plates are rolled up in a jelly-roll arrangement, separated by a layer soaked in moist sodium or potassium hydroxide (NCERT Figure 2.11, page 56).

• NCERT gives the overall reaction during discharge as (NCERT page 56):

  Cd(s) + 2Ni(OH)₃(s) → CdO(s) + 2Ni(OH)₂(s) + H₂O(l)

• NCERT slip (page 56): NCERT’s equation balances, but its Ni(OH)₃ and CdO are an older, simplified way of writing the materials. Beyond the NCERT text: the charged positive material is nickel oxyhydroxide, NiO(OH), and cadmium and nickel form hydroxides on discharge; infrared spectroscopy showed that the discharge products are cadmium hydroxide and nickel hydroxide. The modern net reaction is Cd + 2NiO(OH) + 2H₂O → Cd(OH)₂ + 2Ni(OH)₂, with a nearly steady 1.2 V (Wikipedia, “Nickel–cadmium battery”; the net equation is summed from its half reactions). For an NCERT-based question, write NCERT’s equation.

Solved examples

Common mistakes to avoid

Quick revision summary

• A battery contains one or more cells, which supply electricity galvanically during discharge; it should be light, compact and of steady voltage.
• Primary: the reaction occurs only once (dry cell, about 1.5 V; mercury cell, about 1.35 V and steady). Secondary: rechargeable (lead storage, nickel-cadmium).
• Dry cell: zinc can (anode), carbon rod with MnO_₂ (cathode), moist NH_₄Cl and ZnCl_₂ paste; Mn goes from +4 to +3.
• Lead storage: Pb anode, PbO_₂ cathode, 38% H_₂SO_₄; both plates turn to PbSO_₄ on discharge; charging reverses it.
• Nickel-cadmium: rechargeable; longer life than lead storage but more expensive (NCERT); nominally about 1.2 V.

Questions students ask

Is a lead accumulator the same as a lead storage battery?

Yes, the lead accumulator of the JEE Main and NEET syllabuses is NCERT’s lead storage battery, the most important secondary cell, used in automobiles and inverters (NCERT page 55). Unit 7 of the JEE Main syllabus lists “dry cell and lead accumulator, fuel cells” and Unit 7 of the NEET (UG) 2026 syllabus “Dry cell and lead accumulator; Fuel cells”, while NCERT Section 2.6.2 (page 55) says lead storage battery. Beyond the NCERT text, it is also called the lead–acid battery, and it is the oldest rechargeable battery, invented by Gaston Planté in 1859 (Wikipedia, “Lead–acid battery”).

What is the emf of a lead storage cell, and why is a car battery 12 V?

Beyond the NCERT text, one lead storage cell has a standard emf of 2.05 V and reads about 2.1 V on open circuit when fully charged, with a discharge cutoff around 1.8 V per cell under specified load conditions (the cutoff depends on the battery and discharge rate) (Wikipedia, “Lead–acid battery”). A battery may be several cells in series (NCERT page 54), and a 12 V car battery has six, as cells in series add their emfs: six cells of nominally 2 V give a nominal 12 V battery; fully charged open-circuit voltage is typically nearer 6 × 2.1 = 12.6 V (reasoned). Its voltage sags a little on discharge, because sulphuric acid is a reactant (NCERT page 55) and its falling concentration lowers the emf by the Nernst equation (reasoned).

What is the difference between a dry cell and a wet cell?

A dry cell holds its electrolyte as a moist paste, NH₄Cl and ZnCl₂ in NCERT’s Leclanché cell (NCERT page 54), while, beyond the NCERT text, a wet cell has a free liquid electrolyte, like the sulphuric acid of an ordinary lead storage battery (NCERT page 55; Wikipedia, “Lead–acid battery”). The Leclanché cell itself began in 1866 as a wet cell, its zinc and manganese dioxide electrodes standing in ammonium chloride solution, and Carl Gassner patented a dry version with a zinc case in 1886 (Wikipedia, “Leclanché cell” and “Zinc–carbon battery”). “Dry” says nothing about recharging: the dry cell is primary and the wet lead storage battery secondary (NCERT pages 54–55).

What is the difference between a dry cell and an alkaline battery?

Beyond the NCERT text, an alkaline battery is a dry cell with a different electrolyte: it keeps the zinc and manganese dioxide of NCERT’s dry cell but uses potassium hydroxide instead of the NH₄Cl–ZnCl₂ paste, and powdered zinc in a gel instead of a zinc can (Wikipedia, “Alkaline battery”). Both give about 1.5 V (NCERT page 54 for the dry cell), but the alkaline cell holds more energy for its size and keeps longer, as ammonium chloride slowly attacks the zinc can of a dry cell (Wikipedia, “Zinc–carbon battery”). Its reaction, Zn + 2MnO₂ + H₂O → ZnO + 2MnO(OH), uses up no hydroxide, and it too is a primary cell (Wikipedia, “Alkaline battery”).

Is a lithium-ion battery a primary or a secondary cell?

Beyond the NCERT text, a lithium-ion battery is rechargeable: lithium ions move between host electrode materials during discharge and return during charging (Wikipedia, “Lithium-ion battery”). Primary lithium-metal coin cells are a different category and are generally not rechargeable. Lithium-ion voltage depends on the particular electrode pair and composition; the aqueous Li⁺/Li standard potential in NCERT Table 2.1 cannot by itself predict the voltage of a nonaqueous intercalation cell.

Why are mercury cells no longer used?

Beyond the NCERT text, mercury batteries have largely been replaced in consumer devices because mercury creates environmental problems when released from waste. The US rule distinguishes battery types: the US Code provision on button mercuric-oxide cells bans sale of button-cell mercuric-oxide batteries, while the provision on other mercuric-oxide cells permits other mercuric-oxide batteries only with a collection system. The 25 mg exception in the provision on alkaline-manganese cells concerns alkaline-manganese button cells, not mercury-oxide button cells (US Code, Chapter 137; US EPA, “Mercury in Batteries”). NCERT’s mercury-cell chemistry remains useful to learn, but it is a historical textbook example, not a recommendation for buying a hearing-aid battery.

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. What is the difference between a primary and a secondary battery? Give one example of each.

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   In a primary battery the reaction occurs only once, and the battery cannot be reused, for example the dry cell. A secondary battery is recharged by passing current through it in the opposite direction, for example the lead storage battery (NCERT pages 54–55).

2. Q2. Name the anode, cathode and electrolyte of the mercury cell, and say what it is used for.

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   Anode: zinc–mercury amalgam; cathode: a paste of HgO and carbon; electrolyte: a paste of KOH and ZnO. It is suitable for low-current devices such as hearing aids and watches (NCERT pages 54–55).

3. Q3. What concentration of sulphuric acid does the lead storage battery use, and what is the cathode made of?

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   A 38% solution of sulphuric acid; the cathode is a grid of lead packed with lead dioxide, PbO₂ (NCERT page 55).