Automotive electrical and electronic systems, batteries and charging
Domain focus
This domain covers automotive electrical and electronic fundamentals, batteries and charging. Expect Ohm’s law, series and parallel circuits, fuses and relays, lead-acid battery construction, alternator charging, starter circuits, voltage drop testing and safe use of a multimeter.
Core rules
- Ohm’s law: V = I × R. In a series circuit current is common and voltages add; in parallel circuits voltage is common and currents add.
- A 12 V lead-acid battery has six cells of about 2.1 V each; state of charge is estimated by open-circuit voltage after rest.
- The alternator generates AC, rectifies it to DC and regulates charging, typically around 13.8–14.4 V at the battery with the engine running.
- Fuses protect circuits; a blown fuse indicates a fault, so replace it only after finding the cause. Relays use a small control current to switch a larger load.
- Measure voltage drop under load; high resistance shows as excessive drop, not necessarily as a low battery reading.
Worked example
A 12 V headlamp bulb rated 24 W draws I = P ÷ V = 24 ÷ 12 = 2 A. Its hot resistance is R = V ÷ I = 12 ÷ 2 = 6 Ω. If a connector adds 0.5 Ω in series, total resistance becomes 6.5 Ω and current falls to about 1.85 A, so the lamp is dim. A voltage-drop test across the connector under load would confirm the bad connection.
Traps
Disconnecting the battery while the engine runs; reverse-polarity boosting; testing a battery immediately after charging without rest; replacing fuses repeatedly without finding the short; assuming a relay is good because it clicks.