Battery Isolators

What are battery isolators?

Battery isolators are devices that isolate one battery from another. Why do we use them in our vehicles? Their main purpose is to isolate the main starter battery from the aux battery at certain times.

The main starter battery as the name suggests is used to start the vehicle. The aux battery is used to power all the extras in your vehicle eg refrigerator, lights, water pumps etc.

A battery isolating system has to enable both batteries to be charged properly, but also isolate them when the motor is not running. The purpose being that you can flatten the aux battery, but the starter battery is still fully charged to start the vehicle.

The simplest and cheapest way I have found to do this is to fit a switch between the starter and aux battery, turn switch on when the motor is running and off when it is not. Remembering to, this can be a problem. The way I overcome this is to replace the switch with a relay controlled by the ignition system, so when the ignition is turned on the relay is energised and the batteries are joined together.

There are a number of other systems available that use electronics to isolate the batteries.

There are some systems on the market that only use one battery, but monitor the state of charge of the battery and disconnect the load when the battery voltage drops below a certain voltage. The problems I have with this system are what happens if it does not work properly, if the battery is getting sick, cold weather, etc and what happens when the power is suddenly turned off. I feel it is safer to have two batteries.

Auxiliary Batteries

Why do we need aux batteries?

We need aux batteries so that we can use power to run things like refrigerators, lights, water and air pumps, etc off one battery and still have power to start our vehicles off the other.

Where do we fit this extra battery?

Usually there is room under the bonnet somewhere depending on the size of the aux battery and what else is under the bonnet. Sometimes it is just a matter of rearranging things under the bonnet, turn the existing battery sideways and fitting the aux beside it. Some of us fit the aux battery under or in the back of our vehicles.

What type of battery should I use?

As a general rule it should be a deep cycle battery. These types of batteries are designed to be heavily discharged and then recharged, not like a starter battery which is not.

Come to the next maintenance day at The Subaru Shop and see some of the different battery locations and isolator systems.

Ron Penfold SC002