---
title: "Battery and working principle"
subject: "Energy Environment and Society"
chapter: 5
key: "semester_7/energy_environment_and_society:battery-working-principle"
source: https://www.bctengineer.com/q/semester_7/energy_environment_and_society/battery-working-principle
site: BCT Engineering
---

# Battery and working principle

Energy Environment and Society · Chapter 5: Energy Storage

- Asked in: 2070 Asar, 4 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/energy_environment_and_society/battery-working-principle
- Whole chapter: https://www.bctengineer.com/md/semester_7/energy_environment_and_society/chapter-5

## Question

Write about battery along with the working principle of any one type.

## Short answer

A battery converts chemical to electrical energy via redox reactions. In a lead-acid battery, discharging reacts lead and lead dioxide with sulfuric acid to form lead sulfate and water, producing 2 V per cell; charging reverses the reaction, restoring acid density.

## Answer

A **battery** is an electrochemical device that stores chemical energy and converts it into electrical energy through redox reactions. It consists of one or more cells; each cell has a positive electrode (cathode), a negative electrode (anode), and an electrolyte, with a separator between the electrodes. Primary batteries cannot be recharged (alkaline), while secondary batteries can (lead-acid, Li-ion).

#### Working principle (lead-acid battery as example)
Electrodes: lead dioxide (PbO₂) positive plate, spongy lead (Pb) negative plate, electrolyte of dilute sulphuric acid (H₂SO₄). Each cell gives about 2 V.

**Discharging (supplying current to the load)**
- Negative plate: Pb + SO₄²⁻ → PbSO₄ + 2e⁻
- Positive plate: PbO₂ + 4H⁺ + SO₄²⁻ + 2e⁻ → PbSO₄ + 2H₂O
- Overall: Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O
Both plates become lead sulphate and the acid becomes weaker (the density falls).

**Charging (reverse reaction with an external source)**
- 2PbSO₄ + 2H₂O → Pb + PbO₂ + 2H₂SO₄
The acid density rises again (about 1.28 g/cm³ when full).

```text
   (-) Pb plate | H2SO4 | PbO2 plate (+)
        e- --> load --> e-
```
Uses: vehicles, UPS and inverters, solar home systems in Nepal. Advantages: cheap, reliable, high surge current. Drawbacks: heavy, low energy density (30–50 Wh/kg), limited cycles, and lead toxicity.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What are the parts of a cell?
   A. Only two terminals
   B. Rotor and stator
   C. Anode and turbine
   D. Positive electrode, negative electrode, electrolyte and separator
   Answer: D. Redox reactions convert chemical energy to electrical energy. (Key point: Battery parts)

2. What happens to both plates on discharge in a lead-acid battery?
   A. Both become lead sulphate and the acid becomes weaker
   B. Both become lead dioxide
   C. Both become pure lead and the acid becomes stronger
   D. Plates dissolve completely
   Answer: A. Pb + PbO2 + 2H2SO4 -> 2PbSO4 + 2H2O. (Key point: Lead-acid discharge)

3. What happens to acid density on charging?
   A. It falls to zero
   B. It rises to about 1.28 g/cm3 when full
   C. It stays the same
   D. It falls to 1.0 g/cm3
   Answer: B. Charging reverses the reaction. (Key point: Charging)

4. Which is a primary battery?
   A. Lead-acid
   B. Li-ion
   C. Alkaline
   D. NiMH
   Answer: C. Primary batteries cannot be recharged. (Key point: Primary vs secondary)

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