Why a second battery?
If you run a fridge, lights or a compressor while parked, you can flatten your starting battery. A second battery on a separate circuit protects it.
How it works
An isolator or DC-DC charger connects the second battery to the alternator while driving, and separates it when parked. Modern smart alternators usually need a DC-DC charger to charge correctly.
Sizing
Add up what you run and how long. A fridge is usually the main constant load; lights and chargers add to it.
Wiring safely
Safe systems need:
- Fuses close to each battery
- Cables sized for the current
- Secure, ventilated mounting
- Heat protection in hot climates
Published 2026-11-07. General advice only; every vehicle is different, so ask us about yours.