Battery separator relays

A battery isolator relay connects the auxiliary battery to the starter battery whilst the engine is running and isolates them when the engine stops. This allows you to charge the second battery using the alternator without running the risk of being unable to start the engine because the auxiliary battery has run flat.

How a battery separator works

The isolator detects that the alternator is charging — usually by monitoring the voltage — and closes the circuit between the two batteries. When you switch off the engine and the voltage drops, it opens the circuit and leaves each battery separate. The starter battery remains fully charged even if you spend the night using the fridge and the lights.
It is the standard solution for vans, motorhomes and boats, and remains suitable when the auxiliary battery is a lead-acid, AGM or gel battery.

Relay, diode or DC-DC converter: which one is right for you?

• Isolating relay: Simple and cost-effective. Both batteries are connected in parallel whilst charging, so they must be of the same type and voltage. This is the right choice for a lead-acid or AGM auxiliary battery.
• Diode separator: It electrically isolates the circuits and allows several banks to be charged at the same time. It causes a slight drop in voltage, which must be compensated for by the alternator.
• DC-DC converter or booster: mandatory if the auxiliary battery is a lithium. A relay would connect the LiFePO4 battery directly to the alternator, which does not supply the voltage required by the lithium battery and could burn out due to the current drawn by an empty battery pack. Furthermore, in vehicles with a smart alternator — virtually all models since 2015 — the relay simply does not work properly.

Battery isolator on a boat

In boating, the approach is the same but with more circuits: starting, services and, sometimes, a windlass or bow thruster. Here, the isolating diode or the Cyrix relays from Victron They enable the alternator’s load to be distributed amongst several banks without them discharging one another.

How to work out the correct size

The relay must be able to handle the alternator’s maximum current with a margin to spare. For a 120 A alternator, use a 120 or 180 A relay. Install it near the batteries, with its fuse at each end and a cable of the correct size: please refer to the Cross-section table by power rating.
If you tell us what type of vehicle you have, what alternator it’s fitted with, and whether the auxiliary battery is lead-acid or lithium, we’ll let you know whether a relay will do the job or if you need a booster. Free advice.