Cable sizing and voltage drop

Running a compressor fridge from a 12v leisure battery is straightforward, but the cable connecting them matters more than most people expect. Use a cable that is too thin or a run that is too long, and the fridge receives less voltage than it needs, works harder, performs worse, and is at greater risk of damage.

What this means in the real world

Your battery might read a healthy 12.8v fully charged, but that is not necessarily what your fridge is seeing. If the cable is undersized, or a connection somewhere along the way is loose or corroded, the fridge could be receiving as little as 10.5v to 11v by the time the power reaches it. The battery is fine. The fridge is being starved.

What voltage drop means

Every cable resists the flow of electricity a little. The longer and thinner the cable, the more resistance it has. That resistance eats into the voltage, so less arrives at the fridge than what left the battery.

A 12v fridge running on 11v has to work harder, drawing more current, running hotter, and sometimes tripping its low-voltage protection or just performing badly. This shows up most when the fridge draws the most power, which is exactly what happens when a compressor kicks in and starts up.

How to get it right

The solution is correctly sized cable for the run length and current draw: longer runs and higher current need thicker cable to keep voltage drop within limits.

As a general guide, use at minimum 2.5mm² for runs up to 3 metres, 4mm² up to 5 metres, and 6mm² beyond that. Always check the fridge manufacturer's recommendations, and if unsure, over rate rather than under rate the cable.

Avoid cigarette lighter or 12v sockets. They're usually fused at 10 to 15 amps and not built for sustained high-current use. Connect directly to the leisure battery with correct cable and an appropriate fuse.