Dual Battery Set Up

Good evening all. Just a small query. In a dual battery set up where the auxiliary battery is located in the rear of the vehicle,eg. the back of a ute or station wagon, does the isolator unit have to be installed near the main starting battery, or can it be installed at any convenient point between the two batteries? Look forward to your replies. Cheers.

Roger B......
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Reply By: Member - Rosco from way back - Wednesday, May 22, 2013 at 17:50

Wednesday, May 22, 2013 at 17:50
G'day Roger

From my understanding it;s better, but not essential to be located near the main battery.

Perhaps more importantly is the fact that you should (must by law I believe) have 2 fuses/circuit breakers ... one at each end, with a long run of wiring like this.
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Follow Up By: Member - Roger B (VIC) - Wednesday, May 22, 2013 at 18:09

Wednesday, May 22, 2013 at 18:09
Thanks Rosco. I thought as much. I've got plenty of room in the back of my vehicle for a battery etc. But under the bonnet no room to even attach a small isolator let alone a second battery. I'm aware of the need for fuses or circuit breakers.Thanks mate. Cheers.

Roger B....
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Follow Up By: Member - Rosco from way back - Wednesday, May 22, 2013 at 18:20

Wednesday, May 22, 2013 at 18:20
If you're tight for room tryhttp://www.traxide.com.au
He makes a very smart, but small dual battery manager.
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Follow Up By: Bonz (Vic) - Thursday, May 23, 2013 at 22:33

Thursday, May 23, 2013 at 22:33
Hey Rosco, I am not aware of any law governing 12v wiring, can you point me to a reference mate.

Thanx
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Follow Up By: Member - Rosco from way back - Friday, May 24, 2013 at 09:14

Friday, May 24, 2013 at 09:14
G'day Bonz

I'll look into it cobber and get back to you. It may be I'm getting confused ... (oldtimers disease). I recall when I was setting mine up the bloke who supplied all the bits and bobs stressed the need for a cb/fuse at each end of a long wiring run with a battery at each end (my aux is in one of the boxes under the canopy). It may be it was good practice rather than mandatory. I'll drop him a line and ask.
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Follow Up By: Member - PJR (NSW) - Friday, May 24, 2013 at 10:15

Friday, May 24, 2013 at 10:15
Hey Bonz.

Who cares whether there is a law or not. It makes sense to normal people.

We had a bloke on a trip with us with a breaker in the bonnet end only but the cable through the body of the ute earthed out putting the positive of the rear battery direct to earth. The breaker in the bonnet tripped but that was not enough.

Phil
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Follow Up By: Bonz (Vic) - Friday, May 24, 2013 at 18:03

Friday, May 24, 2013 at 18:03
Actually I care Phil, cause I do wire up a few of these and want to know what the legislation specifies, fuse, resettable breaker, auto breaker or whatever. I have my way of doing things and if there legislation that is involved then it'd be sort of nice to comply. If not then anything else that you or someone else says is "not enough" is merely their opinion.

Thanx Rosco, look forward to seeing what comes up mate.

Regards

GWBonz
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Follow Up By: Member - PJR (NSW) - Friday, May 24, 2013 at 18:11

Friday, May 24, 2013 at 18:11
Thats marvelous Bonz. It didn't look that way.

Phil
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Follow Up By: Bonz (Vic) - Saturday, May 25, 2013 at 07:25

Saturday, May 25, 2013 at 07:25
you might need to get your eyes looked at Phil
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Reply By: Sand Man (SA) - Wednesday, May 22, 2013 at 19:57

Wednesday, May 22, 2013 at 19:57
Hi Roger,

In my experience, it doesn't matter where the isolator is located.
A smart isolator provides two functions in a dual battery setup.

1. It gives priority to the starting battery to ensure it is at an optimum state of charge before opening the auxiliary circuit. Once the starting (primary) battery has reached approximately 13.4v the isolator will open the auxiliary circuit so that charging of batteries on this circuit is enabled.

2. It keeps the starting battery and auxiliary battery electrically separated so that devices on the auxiliary circuit can not drain the starting battery.

As an example, my auxiliary battery is located in the tub of my dual cab, contained in a Sidewinder Flyer box. This box includes an isolator which enables a standard dual battery system to operate, when the charge cable is connected to the starting battery terminals.
In this situation, the isolator is at the remote end but still functions successfully.

Bill


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Reply By: Member - Roger B (VIC) - Wednesday, May 22, 2013 at 20:12

Wednesday, May 22, 2013 at 20:12
Thanks Rosco and Sandman. My minds at ease now. Cheers.

Roger B.....
AnswerID: 511620

Follow Up By: Member - J&R - Wednesday, May 22, 2013 at 20:22

Wednesday, May 22, 2013 at 20:22
There's no law regarding circuit breakers.

Common sense dictates one near the pick point for the aux feeds.

However, you don't need any circuit protection in the main feed from the underbonnet battery to the aux. Any circuit protection is only going to introduce voltage drop, and you want to be able to utilise the override function on the isolator.
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Follow Up By: Allan B (Member, SunCoast) - Wednesday, May 22, 2013 at 22:07

Wednesday, May 22, 2013 at 22:07
J&R, Unless I have totally misunderstood are you are saying that no over-current circuit protection is required at the under bonnet (cranking) battery in the feed to the rear-mounted auxiliary battery. If so then this is a very dangerous practice. If that cable were to short to body then a very heavy fault current would be delivered from the cranking battery with the real possibility of fire.
The same applies to fault feed from the auxiliary battery so circuit protection is required at each end of this cable. Well specified and installed over current protection will not introduce any significant voltage drop.


As for utilising isolator override, the long cable in question would need to be very large cross-sectional area to carry cranking current and if it is not sufficiently large it may be overloaded to a point of failure and possible fire.


Or hav
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Allan

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Reply By: Allan B (Member, SunCoast) - Wednesday, May 22, 2013 at 22:29

Wednesday, May 22, 2013 at 22:29
Roger, A dual battery isolator typically engages at 13.2v and disengages below 12.7v. This works fine where the isolator is positioned reasonably close to the alternator or cranking battery. However if the isolator is positioned at the far end of a long cable run it is possible for the high current draw of the auxiliary battery when discharged to introduce sufficient voltage drop in the cable to fall below the 12.7v drop-out threshold of the isolator causing it to disengage. With no current flow the voltage will now rise and the isolator will re-engage and continue to switch off-and-on indefinitely. This scenario is less likely with sufficiently heavy cables.
Accordingly, it is preferable to position the isolator closer to the supply source.
I am surprised that there is insufficient space for an isolator in your engine bay.
Cheers
Allan

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Reply By: olcoolone - Wednesday, May 22, 2013 at 22:55

Wednesday, May 22, 2013 at 22:55
Just remember to fuse the cable at the start battery and the aux battery.
AnswerID: 511637

Reply By: Member - Roger B (VIC) - Wednesday, May 22, 2013 at 22:57

Wednesday, May 22, 2013 at 22:57
Thanks kindly gents. Your input is greatly appreciated. You've given me plenty to think about. Regards.

Roger B.....
AnswerID: 511638

Reply By: Member - J&R - Thursday, May 23, 2013 at 09:56

Thursday, May 23, 2013 at 09:56
Roger B
Additional clarifications….
There is no legal requirement for circuit protection for auxiliary battery wiring.

And yes, wiring can be a dangerous practice if not done correctly. Note I say correctly. For many years cars have had batteries located in many areas, sometimes nowhere near the engine. A lot of modern cars have batteries under front/rear seats, in boots, near the boot....you get the idea. And all done without main feed protection. (in the automotive environment, how do you economically protect a feed that needs to supply up to 500 amps cranking current....) Yes ok there are ways, but, as I said, they’re very expensive to do properly.

But you can do it by doing the wiring job properly.
By people (or machines) who know what they're doing.

And here is the GREAT DISCLAIMER.
You take advice from forums such as this as just that....general advice. There are many experts on this, and all other public forum sites and some even know what they're talking about. However most do not. Their experience is from their experience, or an armchair, and again, forums. So the circle of misinformation/folklore/old wives tales continues.

However you are correct. If the current were to be above the capacity of the cabling, it could overheat. And if you do want to utilise the override function, you will need large cabling. 2 B&S would be suitable. If no override, 4 B&S and you can, if you feel a great need, also put in a 50-80 amp circuit breaker or fuse, but keep in mind this will be the weak point, both physically and electrically. Solder all connections and seal. Crimp, solder and seal if you like. Do not just crimp unless your crimping tool and connections cost you the equivalent of a small house. Insulate the cable run with split tubing or similar, use good quality grommets and clean cut holes (no burrs, sharp edges) where cabling passes through metal panels and make sure you secure with at least good quality uv, heat and oil/petrol stabilised/resistant cable ties. Mount the battery box firmly, not with the 2 plastic clips they usually come with.

Done this way, you minimise the complexity of the system, safeguarding its integrity without introducing components where dirt etc may compromise it (either electrically or physically).

I think you get the idea.

A number of other points have been made. Mount the isolator (if it's a smart one) close to the main charging battery. It’s voltage sensitive so give it the best chance of doing its job properly. If it's just a solenoid (100A for example) wired to the ignition feed, mount it anywhere.

Do put overload protection in the feeds from the auxiliar battery. A simple fuse usually suffices, or a fuse for each item is also acceptable though a bit of overkill as most devices have their own.
AnswerID: 511655

Reply By: KenInPerth - Thursday, May 23, 2013 at 20:08

Thursday, May 23, 2013 at 20:08
Roger

My input - with everything in agreeance with Allan and Olcoolone.

In one of my situations I run 8B&S Tinned (marine cable) from a cranking battery (Fused or CB at the take off) over about 4 meters to a D250S Dual located with a battery to run a fridge.

If you are not concerned about pumping huge current into the battery for faster re-charge (the D250S limits it to 20Amp) then this reduces cable sizing and increases distances.

I have the battery in a battery box with the D250S strapped on top and some Anderson Powerpole connectors mounted in the side wall of the battery box lid for Aux power take off (fused inside the box). I also connect the main feed to the D250S via powerpole connectors. Put's everything in the back, is reasonably compact, and somewhat portable.



AnswerID: 511688

Reply By: Member - Roger B (VIC) - Thursday, May 23, 2013 at 20:58

Thursday, May 23, 2013 at 20:58
I'm certainly now armed with a lot more knowledge than I had a few days ago. Thanks to everyone. Cheers.

Roger B......
AnswerID: 511698

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