Electrical and Electronic Systems, Battery and Charging
Electrical and Electronic Systems, Battery and Charging
This domain covers the 12 V system, lead-acid battery construction, starting, charging, lighting, sensors and electronic control. The exam tests battery cells, capacity, alternator rectification and regulation, and basic ECU input-output logic.
Core rules
- A 12 V lead-acid battery has six cells in series, each about 2.1 V, giving about 12.6 V at rest. Capacity is rated in ampere-hours (Ah); cold cranking amps (CCA) indicate starting ability at low temperature.
- The alternator generates AC, which a rectifier converts to DC; the voltage regulator holds charging voltage near 14.2 V. A drive belt fault stops charging.
- The starter motor draws high current through a solenoid and pinion. Electronic control units read sensors, process signals and actuate outputs; CAN bus reduces wiring.
- Battery sulphation occurs when a battery is left discharged; overcharging causes gassing and water loss.
Worked example
A battery has six lead-acid cells, each with an open-circuit voltage of 2.1 V. Expected battery voltage = 6 × 2.1 = 12.6 V. If a voltmeter reads 10.5 V under no load, one cell may be shorted, because 10.5 ÷ 2.1 = 5 effective cells. Reasoning: series voltages add; a low reading points to cell failure rather than a charging fault.
Traps
- Connecting jumper leads in reverse polarity; this can damage the alternator and ECU.
- Confusing Ah with A: Ah is charge capacity, A is current.
- Assuming a battery at 12.0 V is fully charged; it is significantly discharged.
- Testing the alternator only at idle without checking belt tension and wiring.