Batteries and Charging Systems
Batteries and Charging Systems
This domain covers lead-acid battery construction, ratings, state of charge, charging principles, alternator operation and charging-system faults.
Core rules
- A 12 V lead-acid battery has six cells, each about 2.1 V at rest, giving about 12.6–12.8 V fully charged.
- Ratings: Ah (capacity at a stated rate, usually 20 h), CCA (cold cranking current), reserve capacity.
- Open-circuit voltage indicates state of charge: about 12.6 V = 100%, 12.2 V = 50%, 12.0 V = 25%. Allow surface charge to dissipate or apply a load before testing.
- Specific gravity: about 1.265 full, 1.150 at 50%, corrected for temperature.
- Alternator: rotor field, three-phase stator, six diodes rectify to DC; the regulator holds about 13.8–14.4 V, temperature-dependent. The warning lamp is excited through the field circuit.
- Charging current falls as the battery absorbs charge; gassing occurs near full charge.
Worked example
A 60 Ah battery is 50% discharged, so about 30 Ah must be replaced. A charger delivers 6 A. Ignoring losses, time = 30/6 = 5 h. With 80% charging efficiency, 30/(6×0.8) = 6.25 h, plus a finishing period. A 60 Ah battery tested at the 20 h rate delivers 3 A for 20 h; a 5 A load does not simply last 12 h because capacity falls at higher discharge rates.
Traps
Treating 12.0 V as fully charged; measuring voltage immediately after charging; testing specific gravity after topping up with water; assuming a spinning alternator is charging without checking output voltage and ripple; disconnecting the battery while the engine runs on vehicles with sensitive electronics; reversing polarity during boost starting.