Dual Battery Set Up

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.

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.

Thanks Rosco and Sandman. My minds at ease now. Cheers.

Roger B.....

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.

Just remember to fuse the cable at the start battery and the aux battery.

Thanks kindly gents. Your input is greatly appreciated. You've given me plenty to think about. Regards.

Roger B.....

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.

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.

I'm certainly now armed with a lot more knowledge than I had a few days ago. Thanks to everyone. Cheers.

Roger B......