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Split Charge System

Upgrade Guide Split Charge System

This article documents my installation of a simple split charge system into my series III Land Rover. It has been installed as I use the vehicle for camping and need to run lights and radios for extended periods. This system will prevent the starting battery being drained.

Items I bought
  • Cable 126/0.40 - 1 Metre (Linking batteries to Relay)
  • Cable 28/0.30 - 5 Metres (Relay Trigger and Earth)
  • Heavy Duty Relay 12v 100Amp
  • Surface Mount Fuse Box - 6 way
  • Sundries (terminals and solder)
This was all Purchased from Vehicle Wiring Products Ltd for around £30 delivered.

The Goodies

Before I started the installation I drew up a simple wiring diagram so I had something to work off. This should be pretty generic and will work with most setups :)

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Wiring Diagram


The Relay I used is only rated to 100amps, but seeing as my alternator is only 65 amp its not really a big problem, although I may up rate this to 100amp at a later date to help charge the batteries easier.

The relay is wired up like this
  • Pin 30 - Main Battery Positive
  • Pin 87 - Aux Battery Positive
  • Pin 85 - Ignition Live
  • Pin 85 - Earth
Ideally you only want to activate the relay and link the batteries when the engine is running. I managed to find a spare terminal on the back of my ignition switch that is only live when the ignition is on or the engine is running. It is not live when cranking, meaning I only use the main battery for cranking, and prevent the aux battery draining the main when trying to start the engine.

Spare Terminal

I used two cables for the fuse box lives, using each cable to supply three separate auxiliary feeds. The cable is rated at 30amps which means I can split 30amp over both feeds. For example I can have two 5amp fuses and a 20amp, and then on the other side I can have three 10amp fuses without over loading the wiring.

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Aux fuse box and heavy duty 100amp Relay

After some quick testing I found I needed to tighten the fan belt as the extra load charging both batteries placed on the alternator made it squeal a little. After a tweak all was well and both batteries charge at 14V at tick over.

I intend to put a switch between the ignition feed and the relay so i can manually disconnect the aux battery in case there is a fault or the alternator is not charging correctly.

This is the completed setup with a single feed being used from the fuse box for my radio, I will be adding more accessories in the near future. The batteries and relay are housed in a locked MOD jerry can locker behind the passenger door.

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Finished battery box
First release
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