A battery isolator protects your starter battery from being drained by leisure loads, and lets the alternator charge both batteries while you drive. Get the rating and the voltage thresholds wrong and you either cook the relay or flatten the battery you were protecting.
What an isolator actually does
Three jobs, and they are easy to confuse:
- Separates the starter battery from the leisure bank when the engine is off, so a fridge cannot flatten the battery you need to start the van.
- Connects them when the alternator is charging, so the leisure bank fills as you drive.
- Protects the wiring from a dead-short by sitting behind a fuse rated to the cable, not to the load.
A switch does the first two manually. A voltage-sensitive relay (VSR) does them automatically by watching voltage. A DC-DC charger does all three and replaces the alternator's crude fixed-voltage charging with a proper multi-stage profile — which matters most on a modern van with a smart alternator.
The four approaches
| Approach | How it decides | Best for | Watch out for |
|---|---|---|---|
| Manual isolator switch | You turn it | Tiny setups, weekend vans, simplicity | You will forget, and a forgotten switch is a flat battery |
| Voltage-sensitive relay (VSR) | Cut-in around 13.3V, cut-out around 12.8V | Older vans with a conventional alternator | Hysteresis that is too narrow chatters on and off |
| Split-charge relay with ignition feed | Ignition signal | Vans where voltage sensing is unreliable | Needs a feed from the ignition circuit, so more wiring |
| DC-DC charger | Electronic, multi-stage | Smart alternators, lithium banks, long cable runs | Costs more and needs a higher-rated cable |
If your van was built after roughly 2010, check whether it has a smart (variable-voltage) alternator before buying a VSR. On those vans the alternator can drop to 12.5V when the ECU thinks the battery is full, which a VSR reads as "engine off".
What to check before buying
- Continuous current rating. Add up what the charger can deliver, then add headroom. A 60A relay on a 60A alternator feed runs at its limit every journey.
- Voltage thresholds. You want a cut-in the alternator actually reaches and a cut-out low enough that it does not drop out at idle with lights on.
- Cable and fuse. The fuse protects the cable, so size the cable first and the fuse to the cable. A 16mm² run wants a fuse well below its 100A-ish ceiling.
- Terminal type and mounting. Stud terminals with locking washers survive vibration better than spade connectors. Mount vertically if the manufacturer says so.
- Dual sensing. Dual-sense relays watch both batteries, which helps when the leisure bank is deeply discharged and drags the sensed voltage down.
Safety
Work with the negative terminal disconnected, fuse at the battery end of every positive run, and do not use the chassis as a return path for high-current charging — run a proper negative cable. If you are not confident sizing cable and fusing, have an auto electrician spec it; this is the one part of a build where a mistake starts a fire.
What this guide is not
It is not a test of any specific relay. Ratings and thresholds are manufacturer-published figures, and they change. Check the listing for the current rating, the wiring diagram and the warranty before you buy.
