Double battery circuit diagram

Hello all,

I am trying to think what the best way to do it in my defender. I will probably go for 2 PC1500. I am thinking 2 on off switches but not sure about the best circuit. I have a winch, extra lights, arb, additional air compressor, water pump, water heater, inverter etc. Any suggestions?

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Mr.Mitso said:

Hello all,

I am trying to think what the best way to do it in my defender. I will probably go for 2 PC1500. I am thinking 2 on off switches but not sure about the best circuit. I have a winch, extra lights, arb, additional air compressor, water pump, water heater, inverter etc. Any suggestions?

Click to expand...

Hello and welcome.

It all depends on how you are planning on running them? Split charge, second alternator?

I have a second alternator on my Discovery http://projectrusty1992.blogspot.com/2018/06/fitting-gwyn-lewis-dual-alternator-on.html

You could have any number of means of split charging off your main alternator, many run a camper van type Voltage Sensing Relay which can be found readily on Amazon/eBay/your local caravan and motor home dealers, T-Max make a very good and very reliable split charge system https://www.jgs4x4.co.uk/t-max-12v-...charging-system-relay-digital-status-monitor/ I had one for several years before I upgraded to the second alternator.

Once you have chosen your means of charging you can then work out the best circuits and cable routing to suit your needs.

Isolators are a must on your aux battery but also a good idea on your primary starter battery, I liked the T-max for it's fit and forget nature and the second alternator maintained that tradition for me, unlike manually linked circuits with cab mounted isolators that need you to think not only about switching them on but also switching them off.

Let us know how you get on

Oh and we likes pictures, lots of pictures

I run 2 Numax batteries connected by a voltage sensitive relay. My alternator is upgraded to 100 amps and I have a simple voltage gauge to keep an eye on things. It’s a bit low tech these days but it works well.

MudHunter said:

I run 2 Numax batteries connected by a voltage sensitive relay. My alternator is upgraded to 100 amps and I have a simple voltage gauge to keep an eye on things. It’s a bit low tech these days but it works well.

Click to expand...

Low tech is our old timer fitters van, that has a red battery isolator key sticking out the dash

There are various options available. There are fairly simple ways such as just having a 200Amp relay linking between the two +ve poles and triggered by the alternator so that when the alternator is charging (the engine is running) the aux battery will be charged.

I personally have a T-Max smart split charger which again links the two batteries via a 200A relay but it monitors the voltage of both batteries and controls the switching better as well as providing the option to link the batteries to ‘self jump start’ if needed or add extra power for winching etc.

Here’s a wiring schematic;


Although I’d have the winch on the Aux battery to preserve starting ability....

It kinda depends on your planned use to a certain degree.

I’ll state the bleedin’ obvious... the main aims of an extra battery and split charge system are essentially;

1) maintain the charge of the chassis battery to ensure you can start the vehicle.

2) enable self rescue in case of a flat chassis battery.

3) provide power when the engine is not running whilst preserving point 1)

So essentially, any form of battery/charging system that provides all the above is sufficient. You can make it as simple/technical as you wish.

I use a yacht system. As thats more my experience. Fitted a large switch that goes between my heels on the battery box. Both batteries connect to it. and the engine and all engine stuff is on the other side. I use Battery 1 for starting, switch to BOTH when running and back to off when at rest. All aux stuff (radio, fridge, 12v sockets, etc) come off the Number 2 battery so there is NO chance i can drain the starting battery except leaving it on BOTH. But that hasn't happened yet.

This sort of thing.... https://www.gaelforcemarine.co.uk/e...MIg-GA9KTk3wIVg7DtCh1lbwyCEAkYCCABEgLtb_D_BwE