parallel battery setup

You can... but :)

Ensure you have an _excellent_ earth for your 2nd battery and ensure it doesn't develop corrosion over time. If the batteries are in parallel during starting you may be drawing huge currents from the 2nd battery which is not a great idea without a cable connection - but I think Mini's use to do it, didn't they?

Mike Harding

Even if both batteries are identical, you won't get equal current sharing between the two batteries.

With the several hundred amps of starter current flowing, the extra resistance in the leads to the rear battery plus Isolator Switch will mean it will supply a lot less current.

Unless you make the wiring to the rear battery FOUR TIMES the section of the wiring from Starter to front battery.

Yes, extra starting power would not be a straight doubling of the two batteries because of the limited capacity of the cabling. But I don't think this is what Stiphodon wants to do.

If you want it as an aux for running a fridge or lights, then what you want to do sounds fine, but it may pay to at least have a good earth direct to the chassis, not just the tray.

Hi Stiphodon, in any professional dual battery set up, no matter how sure you are that having the earth return through the body will work, running both the positive and the negative guaranties that you KNOW you have good power connections between the two ( or three ) batteries.

In your case, the potential of a bad earth return is much greater if you want to try and use the tray as part of the earth return.

No matter what set up you have, there is no way the properly test to see if a secure earth return can be made via the body of the vehicle. Simply measuring for any resistance between the two battery locations is next to useless because the problem of a resistance will only show up under a high current loads.

If you have a good earth return through the body, you will actually have a better set up with a lower potential voltage drop than you would when running two battery cables but because of the possible problems as explained above because there is no way of knowing for sure, what you can do is to run both the positive and the negative cables between the batteries and also run earth straps at both batteries. This way if you have a good earth through the body, you get the advantage of it. But if the is a resistive path back through the body, it won’t effect your batteries operation.