Electrical, Battery and Electronic Systems
What this domain tests
This domain covers Ohm's law, series and parallel circuits, lead-acid and lithium batteries, charging systems, stator and regulator-rectifier, starting, lighting, CDI/TCI, ECU, sensors, fuses and wiring faults. Expect voltage-drop and current calculations.
Core rules
- Ohm's law: V = I × R. Power: P = V × I. In series, current is common and voltages add; in parallel, voltage is common and currents add.
- A 12 V lead-acid battery at rest should read about 12.6–12.8 V fully charged. Specific gravity around 1.260–1.280 indicates full charge.
- Charging voltage on a running motorcycle is typically about 13.5–14.8 V, depending on system. Too low causes discharge; too high boils electrolyte.
- The stator produces AC; the regulator-rectifier converts it to controlled DC and protects the battery.
- Fuses protect wiring, not components. Replace with the same rating; a higher rating defeats protection.
- Electronic systems need clean grounds and stable voltage. A voltage drop of more than about 0.5 V on a high-current circuit indicates resistance.
Worked example
A 55 W headlamp on a 12 V system draws I = P ÷ V = 55 ÷ 12 = 4.58 A. If the feed cable has 0.05 Ω resistance, voltage drop = I × R = 4.58 × 0.05 = 0.23 V, so the lamp receives about 11.77 V. A corroded connector raising resistance to 0.5 Ω would drop 2.29 V and noticeably dim the lamp.
Traps that cost marks
Testing a battery only by open-circuit voltage; reverse-connecting a charger; fitting a larger fuse to stop blowing; assuming a charged battery is serviceable without a load test; probing ECU pins with a low-impedance test lamp.
Key points
- V = IR and P = VI.
- Resting 12 V battery reads about 12.6–12.8 V when full.
- Charging voltage is roughly 13.5–14.8 V.
- Fuses protect wiring; never uprate them.
- Voltage drop reveals high-resistance connections.