Fitting A Twin Battery System

February 10, 2006

Have you ever wanted to have two batterys on your 4x4 either to give that extra bit of cranking power on those cold mornings or to run a winch?

I have fitted a second Battery on my vehicle and as i was doing the work i took pictures at intervals of the fitting so others can do this to their vehicle if they wish.

Firstly you will not have to change your alternator to do this as most alternators can easily charge two batterys.

The equipment you will need is:

1: Some heavy duty Battery cable (i used about 3 meters)

2: some thinner cable to trigger the relay

3: A heavy duty relay able to carry 100+ amps (the one i used was rated at 180amps)

4: A switch (optional)

5: A Battery earth lead

6: A positive battery teminal clamp

7: A battery (of your choice but i would use one at the same ratting as your primary battery)

Find a location for your battery earth strap and fit it to the body of the vehicle making sure it has a good contact with the body and making sure that you will be able to connect the clamp to the negative teminal of the battery when you fit it.

Now the battery if in location you can run the cables from your main battery to the location of where you will have your relay and from the relay location to the secondary battery positive teminal.

When running the cable remember to run them where they will NOT be able to chaff and short out and making sure that the cable is secured (i used heavy duty plastic cable ties). But do NOT connect the cable to the primary battery yet.

Next you will need to find a location for the relay,try to place it in a location were you can get at it with some ease.

Once a location has been found connect the two battery cables to it.

Now you will have to wire the switching circuit for the relay, the way i did mine was to connect into a positive feed from the ignition curcuit and then to a switch i fitted to the dash and from the switch to the relay and from the relay to an earth on the vehicle body, (the only reason i wired it this way is so i had more control over the secondary battery but if you wish you could just wire from the ignition or from the thin cable that powers the alternator warning light),

I found the easiest way was to wire the relay whilst it was lose and then fix it to its location

After all the connections are made to the relay then you can connect up you positive battery teminals on both batterys and then connect the earth clamp to your secondary battery.

Then your ready to test the system, how i did was to connect a multimeter to the secondary battery without the engine running and check the voltage

Then start your engine and check the voltage and make sure the secondary battery is charging.

And that should be it you now will have a twin charging system.

I hope this write up expains how to do it simply and easily.

February 11, 2006

Top stuff mate, This is just what I need!

Out of interest, where did you get the relay from? Is it something I might be able to get from hellfrauds or is it more specialised?

Thanks again!

February 11, 2006

I got the relay from a company called X-ENG as they seem to specialise in this sort of stuff but i didnt wire the system as to their wiring diagram as i didnt like the fact that using their curcuit if the primary Battery was to go low enough to not be able turn over the engine and you would have to open the bonnet and run a jump lead from positive to positive.

Where as i have wired it if the primary Battery has 9v in it(not enough to crank the engine) you can turn the key to IGN, Flick the switch and then the seconary Battery will jump the primary battery and you havent even had to get out the car.

Heres the web site for X-ENG

http://www.x-eng.co.uk/X-Charge.asp?MID=32

with this kit you will only use the relay and thin wire you will need to buy some heavy duty battery cable as the kit only comes with two peices about a foot long with is usless for the application you will need for a Prado or Surf.

HTH :thumbsup:

February 11, 2006

Thanks again! I've needed this for the past two years and never found the info to help!

The two batterys wont short if you leave it turned on will they?

February 11, 2006

Thanks again! I've needed this for the past two years and never found the info to help!
The two batterys wont short if you leave it turned on will they?

They shouldn't do no the only way you would get short is if one of heavy duty Battery cables was able to rub on the body and wear through the insulation thats why its vital that all the heavy duty cables a secured really well.

if your going to use the relay with i would say is a must (just joining the two batterys together is a crude method of twin Battery charging) then you really need to find a wire that is live only when the Ignition is on and cut into that wire using a scotchlock and take a wire to one side of the low tension (switching) side of the relay (or if you want more control over the system to a switch and then to the LT side of the relay) and then to a good earth point.

By doing it that way when you turn off the IGN the two batterys will be isolated from each other.