Thread: Wiring for battery box

Originally Posted by drivesafe;3026562[COLOR=#000000

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When running 6B&S, you need to use either 50 amp auto resetting circuit breakers or 60 amp fuses.

These must be located near the positive ( + ) terminal of each battery in the system.

I think this is worth highlighting. I have seen quite a few systems that have no protection at the auxiliary battery end of the charging circuit. People seem to forget that the power will flow back the other way in the event of a short circuit.

Originally Posted by Dorian

Hi Dan,
I've been off the radar a few days with a commissioning job. After reading Tim's post I checked my actual installation and I have 6 B&S not 6mm.
I am using a 50 amp auto reset breaker though. But in anything on this topic, you would be a fool to argue with Tim.

Cheers Glen

Hi Glen and on one hand, thanks for the vote of confidence, but on the other, I not only like a good argument ( who would have thought ! ) but HONEST arguments can be very educational, for all involved ( that’s me included ), so better to argue and get is right/safe then to keep quiet and end up looking foolish.

I emphasis “ HONEST “ arguments. There are times when people, because of undisclosed ulterior motives, will post up porkies and that is one thing I will not stand for, as was the case some months back, about a lithium battery install, and why it was done.

To the subject at hand.

There is a science to how you set up a dual battery system. It is not a complicated science but it does help to know about it.

For example, I hate the use of Auto Resetting Circuit Breakers, with three exceptions.

In headlight circuits, where they can still proved intermittent lighting at night, if a problem occurs, and they use to have heaps of problems in old Fords and Holdens, with fuses blowing, leaving the vehicles blacked out in the middle of the road.

The eventual use of relays solved this problem.

With electric trailer breaks, it is a LEGAL requirement to use Auto Resetting Circuit Breakers and not fuses, for the very same reason as with headlights.

Lastly, in dual battery circuits but for a more complex reason, which I will not cover here.

Knowing exactly how different devices work is imperative, so some basic fundamentals may help.

Cabling first, and 6B&S cable ( 13.5mm2 ) can tolerate around 100 amp continuously and not exceed the safe 70C temperature limit. NOTE, this is when used as dual battery cabling, where the cabling can be in confined areas that can cause the cable to run hotter than in open air situations.

But 6B&S cable can also SAFELY handle up to 400 amps, for short periods of time, like when used as start motor cable. Horses for courses!

“GENUINE” 50 amp Anderson plugs can safely handle up to 95 amps for long periods of time. So they are ideally suited for use with 6B&S cable.

Now to fuses and circuit breakers ( CB ). These are a tad more complex.

The marked rate on a fuse and CB is what they are designed to carry for 4 hours without going open circuit, without tripping.

To achieve the rated current at 4 hours, The fuse or CB will, for a number of reasons, carry higher currents for shorter periods. For example, fuses and circuit breaker will carry roughly 135% of their marked rate at around 30 minutes, without tripping.

Thats a little over 65 amps for a 50 amp device.

They can carry a few hundred amps for a couple of seconds and not trip.

An example of what you can do when you know how devices work.

With a short Length of 6B&S Twin cable, fitted with a 50 amp circuit breaker in the positive ( + ) cable near the battery’s positive ( + ) terminal, and a 50 amp Anderson plug on the other end of the 6B&S Twin cable, you can then use this lead for a number of purposes.

One use is, using another length of 6B&S Twin, you can make yourself a set of jumper leads with battery clamps on one end and a 50 amp Anderson plug on the other.

Then if you ever need a jump start or are asked yo jump start some one else’s you are all set up to do it and to be able to do it safely, and the circuit breaker will not trip. I have carried out this very operation on numerous occasions and the circuit breaker has not tripped.

But if a CONTINUOUS overload will eventually blow a fuse or trip an auto resetting circuit breaker. The obvious advantage with the auto resetting circuit breaker, is that it does just that and allows to continued operation of a dual battery system, all be it in a controlled manor.

With a fuse, you will probably not know the fuse has blown until you arrive at your destination and find you have a low or flat auxiliary battery.

With a dead short, because of the thickness of the cable, a fuse or CB will go open circuit the instant the short occurs. With an auto resetting circuit breaker, it will continually trip, take some time to cool down, reset and instantly trip again.

If a circuit is set up properly, the cabling will never be effected ( melt and/or catch fire ) in the above situation.

I had a case about 20 years ago, where an auto electrician had fitted one of my systems to a customers Ford Falcon.

The customer had come down from Brisbane to the auto elect on the Gold Coast, and once the job was done, the customer head back to Brisbane.

He decided to fill up at the servo at Coomera and will filling, he could hear a loud click coming from under the bonnet every so often. The noise seemed to be coming from one of my isolators.

He rang the auto elec who asked him to come back so they could replace the isolator.

The auto elec then contacted me so I could be there to see what was going on.

I arrived just as the customer was leaving and the auto elec explained that there was no issue with the isolator, but that after he originally tested the setup after he installed it, he then fixed the Anderson plug in place at the of the vehicle, but he didn’t realise the positive ( + ) cable had looped around inside the body and when he screw the Anderson plug to the body, he screwed into the positive ( + ) cable.

With the screw mount fixed, all was well.

We guesstimated that the circuit breaker tripped somewhere between 700 and 1,000 time before the short was removed, but the cabling was not even warm to handle.


Again, it is simple science but knowing what to use will make an installation safe