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dual batteries in off-road vehicles

Posted by mike1161 (My Page) on
Sun, Oct 29, 06 at 1:34

I am currently adding a second battery in parrallel to my electrical system in preperation for a winch and lightbar. Charging both batteries simultaneously will be a large draw on the alternator and I am scared to overload it and damage/break it.
Anyone know how I can do this? I have thought of keeping the second battery on standby (not functioning) unless I needed the extra CCA for winch operation. Also I have installed a battery isolation switch allowing each one of the batteries to be operated seperately, in parallel, or disconnected entirely from the electrical system of the vehicle.
A friend mentioned the use of an in-line resistor coming off the pos (+) leg of the alternator to ensure that it is never overloaded.
I'd appreciate any tips you all might have regarding this project, preferably from someone who has done it before. I know it's not everyday. Thanks.


Follow-Up Postings:

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RE: dual batteries in off-road vehicles

Might want to look at some of the Boating Supply stores in your neighborhood. See what that have for a dual battery system Automatic Charging Relay.

Lots of boats have dual battery systems and lots of ways to handle them.

Here is a link that might be useful: Creating a Reliable Battery System / West Marine


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RE: dual batteries in off-road vehicles

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RE: dual batteries in off-road vehicles

Don't sweat the small stuff. Install a battery isolator and there will be no problems. You don't need separate cutoff switchs or anything else. Wire the winch (and light bar if you want) to work off the 2nd battery. The isolator will automatically keep your starting battery from going dead should you use the winch excesively or with the engine off. If your alternator does wimp out some day, replace it with a heavy duty one regardless of what your vehicle came with. The biggest continuous strain on your system as you describe will be while you're driving around with the light bar on. The alternator works with voltage and currant limiters, so it doesn't know or care how many batteries it's charging. Wired up a bunch of these this way in the 70's. It's no big deal. People always want to make things harder than they have to be for some reason. jmo


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RE: dual batteries in off-road vehicles II

The one I was refering to is pictured here as the 'diode type'. It's as tough to do as it looks.

Here is a link that might be useful: isolator


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RE: dual batteries in off-road vehicles

I understand that some larger RVs use dual battery setups. You may find dual battery systems at RV dealers.


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RE: dual batteries in off-road vehicles

If your installing your lightbar and winch to use for an
" excessive " amount of time with the engine off you might have a point. If it is for normal use with the engine running don't waste your money on a second battery. Two batteries in a vehicle work fine as long as they are IDENTICAL. Problems start when one battery goes bad and the other is still good. Goodby alt. A normal alt. will charge two batteries with no problem. Some trucks have two
twelve volt batteries or four six volt batteries with a
normal alt. and have no problems. They use a series - parallel switch for discharge but charge all at the same time. Might be over your head because of the cost. Tell your friend to stay away from your alt. If he or you put a
resister on the + post of your alt. the alt. will not fully
charge the batteries. You can kiss your alternator goodby in due time. Bottom line. You don't need 2 batteries
or switches unless your going to play around for 4 - 5 hours with the engine off. Dual batteries are mostly used
for hard start conditions and are not isolated. Even motor
homes only use one battery at a time. If you want to spend your money DO NOT use a diode for isolation. WRONG. Use a
continuous duty solenoid. Diodes go bad and pop real fast
plus they can leak. Just some info from someone that does
this for a living. That was your request, right ?


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