Thread: Dual Battery Wiring

Yes. They need to share a common earth.

It is best to run earth back to starter battery but should still charge ok if aux has been earthed properly I would have thought.
For the record on my TD5 I had I ran 2 x heavy cables + & - (+ via a solenoid obviously) from the aux in the back of my vehicle to the POS & NEG on the starter battery.

Both batteries just need to be grounded to the chassis or body. There is no need to run a separate ground wire between the batteries.

I am not sure how the TJM system works, but on most of the systems (traxride excluded) you need to mount the controller as close to the primary battery as possible. If you dont you can get modulating (probably not the correct term) voltage and it will not charge properly. Most controllers also have a separate earth on them.

If you have NO charge happening, then I suspect that you have wired the controller back to front.
The controller is usually just a voltage sensitive relay that cuts IN at 13.5 seen on the starting battery side or whatever and OUT at 12.0 ( Drivesafe) , 12.4 (Sidewinder) or 12.7 most others.
Even with only 8B&S wire I would expect the starting battery voltage to be over cut in at the boot, although I would use at least 4B&S and maybe 2B&S on such a long run to prevent voltage drop.

Check that you have the wires on the correct terminals.
Regards Philip A

The DBS controller says batteries are linked, controller is earthed on car body with 2nd battery in the boot. Starter battery isn't earthed to car body.

Will checking voltage with a multimeter at positive wire end work without a negative wire?

So 8 b&s cable is too thin, we're only talking a few meters here?

Starter battery must be earthed to body/chassis. If it wasn't, nothing on the vehicle would work.
Where have you earthed the battery down the back?

Well yeah the negative terminal (usual) is on the starter battery, the car works.

Battery down the back is earthed to the chassis with Tjm on a bolt behind seat belt cavity.

Wiring negative back from the second battery gives you a more predictable connection with a known voltage drop.

8b&s will be ok if you are running a fridge and a couple of accessories. I have no problems with a small waeco, which draws 5ah when the compressor is running. It will also happily run a hard wired bushranger black max compressor.

Best thing to do is work out what your maximum load is and then calculate the voltage drop over the circuit - this includes both negative and positive cable runs. You can then select cable on the basis of how much voltage drop is acceptable at maximum load. It saves you from either undersizing or oversizing the cables.

Cheers
Paul