Ohm's law
Voltage
Volts equals amps multiplied by ohms.
Current
12 V across a 4 Ω load: 12 ÷ 4 = 3 A
Resistance
A 12 V circuit drawing 2 A: 12 ÷ 2 = 6 Ω
Power
Power from volts and amps
A 12 V fan drawing 10 A: 12 × 10 = 120 W
Current from watts
A 55 W headlamp bulb at 12 V: 55 ÷ 12 ≈ 4.6 A
Power from current and resistance
Useful for heat lost in a cable or connector.
Power from volts and resistance
12 V across 2 Ω: 144 ÷ 2 = 72 W
Resistors in series and parallel
Series
2 Ω + 3 Ω in series = 5 Ω
Parallel
The total is always lower than the smallest resistor.
Two in parallel
4 Ω and 4 Ω in parallel: 16 ÷ 8 = 2 Ω
Cables and batteries
Voltage drop in a cable
L is the total cable length in metres, including any return cable (use the feed length alone where the circuit returns through the body). I is the current in amps, A the cable size in mm², and ρ for copper is about 0.0172 Ω·mm²/m.
10 m total (5 m feed + 5 m return), 10 A, 1.5 mm²: (10 × 10 × 0.0172) ÷ 1.5 ≈ 1.15 V
Battery run time (approximate)
A rough guide only. Real run time is shorter, especially at high loads or low temperatures, and a lead-acid battery shouldn't be run flat.
70 Ah battery, 2 A load: 70 ÷ 2 ≈ 35 hours
Fuse sizing
Work out the circuit's normal current, then choose a fuse a little above it that the cable can safely carry. The fuse protects the cable.
12 V battery state of charge
Approximate resting voltages for a 12 V battery by type. These are a guide; battery makers' own figures vary slightly.
| State of charge | Flooded lead-acid | EFB | AGM |
|---|---|---|---|
| 100% | 12.6–12.7 V | 12.6–12.7 V | 12.8–12.9 V |
| 75% | about 12.4 V | about 12.4 V | about 12.6 V |
| 50% | about 12.2 V | about 12.2 V | about 12.3 V |
| 25% | about 12.0 V | about 12.0 V | about 12.0 V |
| Discharged | 11.9 V or below | 11.9 V or below | 11.8 V or below |
- Let the battery rest with no load or charge for at least 4 hours before measuring. A reading taken straight after driving or charging will be too high.
- The figures assume about 25 °C. In freezing conditions a good battery reads roughly 0.1–0.2 V lower.
- Below 12.4 V the battery needs recharging. Below 12.0 V it is deeply discharged and needs a proper charger, not just a drive.
- An EFB is a strengthened flooded battery, so it reads much the same as a standard flooded one.
- On EFB and AGM stop-start batteries, voltage alone can mislead. A full test also checks remaining capacity and how well the battery accepts charge.
With the engine running, a healthy charging system typically shows around 13.8 to 14.7 V at the battery. Readings well outside that range are worth getting checked.