Circuit Breaker: Which Way Round?

**sam_d** · 8th April 2009, 05:16 PM

Just a quick question regarding circuit breakers in Dual Battery set-ups.

I've done a few searches but still can't quite find the answer I am looking for and the best answer I did find didn't quite make itself clear.

Anyways, with the Easter Holidays approaching I can finally finish my dual battery set up in my Disco but I just need to clarify which way round the circuit breaker should go.

In the diagram below should the circuit breaker be mounted in Position 'A' (with the longer connector attached to the aux. battery), Position 'B' (with the shorter connector attached the the aux battery), or does it not matter:

Does the circuit breaker allow current one way but not the other? Obviously I need current to flow one way to charge the aux battery but need overload prevention the other way in case the cable between the isolator and the Aux battery gets damaged...

Thanks in advance!

miky · 8th April 2009, 05:58 PM

A circuit breaker allows current to flow in either direction. If the current flowing through it exceeds 30A it will open, like a switch.

As for how it should be connected... I doubt that it matters.

That said, if you are worried about the connection, why not check with the manufacturer or your local auto electrician? He might think you are a dope but so what if it gives you peace of mind.

**feral** · 8th April 2009, 06:06 PM

It should be stamped showing 'Batt' or "AUX' on the bakalite.

EDIT:

Short copper pole goes to the main battery end.

Long silver pole goes to the Aux battery.

Cheers.

**Blknight.aus** · 8th April 2009, 06:28 PM

it depends on what type of circuit breaker it is....

thermals work either way, as mentioned.

magnetics also work the same (although some have a shunting diode that will make them either trip immediately or not at all if flow is reversed depending on the internal layout)

you can get specialized electronically controled circuit breakers that break if the current flow is reversed but they're exxcy.

**hook** · 8th April 2009, 07:29 PM

Make sure there is one between your main Bat and isolator as well

I think you will find 30 amp to small.

I use your type to run acc. (fridge-lights etc)

this is what I use on my dual bat

100A Circuit Breaker - Jaycar Electronics

When my 2nd bat is run down, the Disco (100amp) alt has only ever pushed out 60amp (gauge), normal 30amps, so you may get away with thier 70amp.

**rovercare** · 8th April 2009, 07:31 PM

CB will go either way, not polarity sensitive, to have the current "flow" on way only, you'll need a diode

**drivesafe** · 8th April 2009, 07:57 PM

Hi sam, in the situation that you are using them, it doesn’t matter which way the go.

Most stud type automotive circuit breakers are marked “BAT” on the short post and “AUX” on the long post.

This does not mean they are polarised ( directional ) they are simply made with a short post to connect the single cable that would come from the battery and the longer post is such so allow a number of cables to be joined to the AUX post.

In a single auxiliary battery set up, a 30 amp circuit breaker is more that sufficient and is actually a lot safer than fitting a bigger current rated device.

Also if both batteries are going to be fitted under the bonnet, most people don’t bother fitting a circuit breaker, I personally prefer one circuit breaker be fitted in this type of set up, just for overload protection.

If the second battery is to be fitted anywhere else in the vehicle and/or caravan or camper trailer, then you MUST fit a circuit breaker at both ends of the positive cable, as close as practical to each battery’s positive terminal.

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