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Fit Second (Leisure) Battery & Split Charging Relay

Fitting the Battery | Wiring the Split Charging Relay | Wiring Diagram

When I go away on off-road trips, I am always charging up radios or lamps and I also wanted to fit some rear work lights, so I wanted to fit a second leisure battery that could handle being drained without causing any damage and also allow me to leave things on without risk of draining the starter battery. Leisure battery's are used in caravans or boats and can handle being drained and recharged unlike normal car starter battery's that can be damaged by discharging.

I also installed a split-charge relay system, so when the engine is running both battery's are charged. You can get specific relays for split charging as well as special "boxes of tricks" that also provide extra features but one of the easiest and cheapest ways to do it is to use an ordinary automotive relay with some wire and fuses - this is what I used.

I purchased most of my electrical items from Vehicle Wiring Products.

CAUTION: If you don't have a basic knowledge of vehicle wiring then I would not attempt this job. You can easily melt wires and could cause a fire if you accidentally short out a 12v car battery.

Tools/Equipment Needed

  • Battery
  • Battery strap & bolt
  • 17amp & 35 amp Wire
  • 30 amp FUSED relay
  • High amp battery cables
  • Battery terminals
  • Fusebox
  • Cable protector, electrical tape, zip-ties, spade connectors, scotch locks
  • Cross-head & flat-bladed screwdrivers
  • Wire cutters & strippers
  • Electrical crimping tool
  • Voltmeter
  • Small socket set and/or spanners

Fitting the Battery

First of all you need a battery, I used a leisure battery I bought from a local caravan dealer.

Then you need some way of fitting the battery. I got a generic battery strap kit (see picture, right) from a local car parts supplier. The main bar needs to be cut down to the correct size and only one of the rods needs to be used as there is an existing bolt-hole that can be used at the front of the Surfs engine bay.

 
     

I placed the battery in the engine bay and lined up the battery strap with the bolt-hole on the front of the engine bay (A), and marked where the battery strap bar needed to go.

Then I drilled through the bodywork (B) (this should come out in the wheel arch). After drilling, make sure you paint the area of the hole (both sides) to seal the metal and avoid any rust in future.

 

 
     

Then you need to feed your positive wire to where you want it. I wanted it fed to the cab, so I made a hole in the existing grommet in the passenger-side bulkhead (A). This feeds to the cab behind the glove box (see Lower Dashboard Removal - Passenger Side for details of removing the glove-box).

Even with the glove-box removed you will still have to reach up into a very tight space to get to the wire fed from the engine bay.

Once I was finished I used some clear silicone to sealed the hole around the wire in the grommet.

 

 
     

With the wire fed into the cab, I attached it to a 6-way fuse-box (A) which I mounted on an existing screw. This means I can take various fused feeds from the leisure battery.

I had to make a small metal bar to connect the high-amp live feed to the six terminals on the fuse box and I used self-tapping screws into spade connectors.

Obviously I will need to remove the glove box to check or change these fuses but I could not find anywhere else suitable that was out of the way, and it also means all the wiring will be well hidden.

NOTE: This picture was taken before I insulated the live feed. Do NOT leave live terminals exposed like this!

 
     

If you are wiring a split-charging relay then don't connect your battery yet - do this as a last step for obvious safety reasons.

When you are ready, connect the earth and live terminals and do up the battery strap. Make sure it is very well secured, especially if you are going to be going off road.

The earth just needs to be connected to somewhere on the chassis, I used an existing bolt to avoid doing any drilling.

 

Wiring the Split Charging Relay

There is a Wiring Diagram at the bottom of the page.

First find a suitable position for your relay. I mounted it on an existing bolt near some existing relays on the drivers side of the engine bay.

I connected the earth terminal of the relay (terminal 85) to the mounting bolt.

I then connected terminals 30 and 87 with 35 amp wire to the positive terminals of each battery.

   
     

Then I found an existing wire (A) in the engine bay fusebox that was positive only when the engine was running. It is important to ensure it is NOT positive when the engine is cranking during starting. This wire needs to be connected to terminal 86 of the relay.

In the picture you can see my relay switch wire (A) as well as the wire from my starter battery to my relay (B).

Finally I used cable protector and zip ties to tidy up all the wires and then inserted the two new fuses.

You can check the circuit is working by connecting a voltmeter to your new leisure battery and start the engine, then rev the engine and the voltage should increase slightly - if it does not then no charge is getting to your leisuire battery - check your fuses & wiring.

 

Wiring Diagram

Disclaimer

I have taken a lot of time to ensure the information above is correct, but please remember vehicle manufacturers make alterations and design changes during the production run of a vehicle. No liability can be accepted for loss, damage or injury caused by any errors in, or omissions from, the information given above.

If you think any information is incorrect, confusing, misleading or incomplete, please feel free to contact me.


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