Your battery reads 12.8V but the fridge only sees 11.9V. Here is where the rest went.
Every cable has a little resistance. When current flows through it, some voltage is used up pushing the current along the wire. That loss is called voltage drop.
1. Ohm's law in a cable
Voltage drop = current × cable resistance. Double the current and you double the drop. Double the cable length and you double the drop. Use a cable with twice the copper area and you halve it.
2. Remember both wires
Current goes out on the positive and comes back on the negative. The drop happens in both, so the total length is the run there and back.
3. Why it matters
- 12V devices are fussy. A fridge or DC-DC charger may cut out or charge slowly if its input voltage sags.
- The lost voltage turns into heat in the cable.
- At 12V, a drop of 1V is about 8% of your supply. At 240V AC the same 1V is less than half a percent. That's why low-voltage systems need thicker cable.
4. How much is OK
Many people aim for under about 3% drop for important loads, and up to 5% for less critical ones like lights. Check the device maker's minimum input voltage.
5. Worked example
A 10A fridge on a 6m run (12m there and back) of 4mm² copper cable:
- Resistance is about 0.0172 × 12 ÷ 4 = 0.052 Ω.
- Drop = 10A × 0.052 Ω = 0.52V, about 4% of 12.8V.
- Going up to 6mm² brings it down to about 0.34V.
6. Quick summary
- Drop = current × resistance of the whole loop.
- Longer and thinner means more drop.
- Low-voltage systems need thicker cable.
- Work it out with the Cable Resistance Calculator.
240V AC mains can kill. In Australia, work on household mains wiring must be carried out by a licensed electrician. Low-voltage DC can still deliver very high current, so treat batteries with respect.
