---
title: "Lead-acid cell characteristics and charging"
subject: "Industrial Power Distribution and Illumination"
chapter: 9
key: "semester_6/industrial_power_distribution_and_illumination:lead-acid-characteristics-charging"
source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/lead-acid-characteristics-charging
site: BCT Engineering
---

# Lead-acid cell characteristics and charging

Industrial Power Distribution and Illumination · Chapter 9: Emergency and Backup Supply Systems

- Asked in: 2077 Chaitra, 8 marks
- Video: [Lead Acid Battery (Construction, Working, Chemical Reactions) BE/BTECH 1st year (in हिन्दी)](https://www.youtube.com/watch?v=Y6S9IS3Wp3E) — Gautam Varde
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/lead-acid-characteristics-charging
- Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-9

## Question

What are the electrical characteristics of the lead-acid cell? Explain the methods of battery charging system.

## Short answer

Lead-acid cell gives about 2.0-2.1 V open circuit, must not discharge below 1.8 V, charges to 2.5-2.7 V, and has low internal resistance. Charging methods are constant current, constant voltage, CC-CV, trickle, float, boost and equalising.

## Answer

#### Electrical characteristics of a lead-acid cell
1. **EMF / voltage:** about **2.0–2.1 V** per cell when fully charged (open circuit); it depends on the specific gravity of the acid (EMF ≈ 0.84 + specific gravity).
2. **Discharge voltage:** falls quickly to about 2.0 V, stays nearly flat, and must not go below **1.8 V** (end-point); deeper discharge causes sulphation.
3. **Charge voltage:** rises from about 2.1 V to **2.5–2.7 V** at the end of charge; gassing starts at about 2.4 V/cell.
4. **Capacity (Ah):** product of discharge current and time to reach 1.8 V; it **decreases at higher discharge rates** and at low temperature, and is usually quoted at the 10-hour rate.
5. **Specific gravity:** 1.25–1.28 when fully charged, 1.10–1.15 when discharged – used to judge state of charge.
6. **Internal resistance:** very low (a few milliohms), so it can supply large currents (engine starting); it rises as the cell discharges.
7. **Efficiency:** ampere-hour efficiency 80–90%; watt-hour efficiency 70–80%.
8. **Self-discharge:** about 2–5% per month.

```text
V/cell
 2.6 ┤            ___ charge
 2.2 ┤  ______----
 2.0 ┤‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾\  discharge
 1.8 ┤                \
     └──────────────────── time
```

#### Methods of battery charging
1. **Constant-current charging:** current held constant (≈ C/10) while voltage rises; may be two-rate (high then low) to limit gassing. Simple but needs supervision.
2. **Constant-voltage (constant-potential) charging:** fixed voltage (≈ 2.3–2.4 V/cell); current falls automatically as EMF rises. Fast and self-limiting; used in vehicles.
3. **CC–CV (modified constant voltage):** constant current until a set voltage, then constant voltage; used in most modern chargers.
4. **Trickle charging:** small continuous current (1–2% of capacity) to balance self-discharge of a standby battery.
5. **Float charging:** charger, battery and load in parallel at ≈ 2.2 V/cell; charger feeds load and keeps battery full; battery takes over at mains failure without a break (substations, telecom, UPS).
6. **Boost charging:** high current for short time to quickly restore a discharged battery.
7. **Equalising charge:** periodic small overcharge to equalise all cells of a string.

## Check yourself

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

1. What is the open-circuit voltage of a charged lead-acid cell and the discharge end-point?
   A. About 2.0-2.1 V; do not go below 1.8 V
   B. About 1.2 V; 1.0 V
   C. About 3.7 V; 3.0 V
   D. About 2.0 V; 0 V
   Answer: A. Deeper discharge below 1.8 V causes sulphation. (Key point: Cell voltage)

2. What does the specific gravity indicate?
   A. Temperature of the cell
   B. State of charge: 1.25-1.28 charged, 1.10-1.15 discharged
   C. Internal resistance
   D. Cell age
   Answer: B. Hydrometer readings judge state of charge. (Key point: Specific gravity)

3. How does capacity change with discharge rate and temperature?
   A. Increases at higher rates
   B. Unaffected
   C. Decreases at higher discharge rates and low temperature
   D. Increases at low temperature
   Answer: C. Capacity is usually quoted at the 10-hour rate. (Key point: Capacity and rate)

4. What is CC-CV charging?
   A. Constant voltage first, then constant current
   B. Fixed small current only
   C. Periodic overcharge
   D. Constant current until a set voltage, then constant voltage
   Answer: D. Most modern chargers use it, ending with float. (Key point: CC-CV charging)

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