Lead-acid cell characteristics and charging

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.

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.