# Multiple battery wiring?



## THASQGOTME (Jul 6, 2006)

I am helping a friend with his install. He is adding a 240 amp alternator and a battery in the rear.

Here are the questions....

1. Do I use a circuit breaker or fuse holder underneath the hood from the + to the + of the rear battery. Or do I just use circuit/fuse holder from the rear battery to the amplifiers? Or both?  

2. Where exactly should I install the battery isolator? Under the hood after the circuit breaker/fuse holder. Or in the back of the vehicle right before the battery?  

Thanks for the help


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## bobditts (Jul 19, 2006)

you should have a fuse/circuit breaker within 18" of the + terminal of BOTH batteries. This means you will have atleast 3 fuses/circuit breakers (between the 2 batts and before the amps). The isolator would probably be easiest to install under the hood.


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## Genxx (Mar 18, 2007)

Can someone draw a simple schematic for this. I have seen it asked a few times but I don not have the answers. Then people could search and find it easy.


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## bobditts (Jul 19, 2006)

ill see what i can come up with. thasqgotme, which isolator did you buy?


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## bobditts (Jul 19, 2006)

found these on google.


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## THASQGOTME (Jul 6, 2006)

So where do the circuit breakers/fuse holders go in this schematic? 

Heres what I was thinking, obviously not right I guess?


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## QtrHorse (Oct 17, 2007)

I would personally put one on both batteries and one on the wire that feeds the isolator. I made a nice drawing and now cannot load it to my webshots. I will work on it more tomorrow.


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## THASQGOTME (Jul 6, 2006)

Any luck with the schematic? Anyone else want to chime in with a schematic?

Thanks


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## kcermak (Jan 30, 2008)

In the configuration above, isn't a voltage regulator required that will sense the current requirements of each battery and send them the different amount of current they each need?


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## QtrHorse (Oct 17, 2007)

This is what I came up with. Just curious why you want a battery isolator? Does your friend want to use one battery when the vehicle is off? If not, just hook them together without the isolator. The fuses for the batteries should be no more than 18in away and the fuse for the isolator if you choose to install one there should be the same from it. As you have in your drawing, the ideal thing to do would be to run a ground from the main battery back to the accessory battery but some don't like to do that because you need more wire.

I would say your drawing is good, just remove the isolator if not needed.


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## GlasSman (Nov 14, 2006)

Theres no reason to run an earth wire through the car. 

Your chasis is an earth.

I was planning on relocating my battery to the trunk....but it's too much trouble since it's my daily driver and I have no spare vehicle.:blush: 

Does your friend listen with the car off often? SPL competitor?


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## bobditts (Jul 19, 2006)

an isolator is needed when using 2 different batteries, 2 batteries that are not the same age, or when you want to isolate one battery for your auxilliary items and one for the vehicle itself.


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## Aaron'z 2.5RS/WRX (Oct 24, 2007)

May I bite on this threas for one second... I'll be running dual batteries, not sure of the "main" yet, but thinking Diehard Platinum or similar oddyessy, sitting next to it will be a Batcap 800... 

Am I going to need a bunch of hardwear to get this working properly? (I have a bunch of fuse holders and whatnot, but no isolators.. )


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## THASQGOTME (Jul 6, 2006)

My friend does want to compete in some SPL comps. The main reason I thought he needed the isolator is because it keeps both batteries charged at the same time correct?


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## WrenchGuy (Jun 10, 2007)

The isolator seperates the start battery from the accessory battery. So your start battery doesn't get used while you are parked and jamming. Then it lets current to the battery which needs it most during start up and driving. The isolator isn't totally necessary. If you use your car everyday I wouldn't worry about it. If it is parked for 2 days plus without running I would get one cuz the batteries will pull or drain on each other. There is more to it but thats some of the logic.


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## QtrHorse (Oct 17, 2007)

THASQGOTME said:


> My friend does want to compete in some SPL comps. The main reason I thought he needed the isolator is because it keeps both batteries charged at the same time correct?


What wrenchguy said.

No, the isolator is not needed to charge both batteries. My diesel truck comes stock with two batteries and they are wired in parallel. One positive to the other positive and each battery is grounded seperately or could be grounded to one another with only one ground to the block/ body if they were close to each other.


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## chad (Jun 30, 2005)

The isolator should be contactor based (relay) not solid state to avoid the PN junction loss if the diode.


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## qstarin (Mar 25, 2009)

I'm still a tad unsure after searching and reading.

If I want to use 2 batteries only to avoid voltage drop below 12v, cause lets say my amps draw boat loads of current, and I only ever use the stereo when the car's running.

Do I need any sort of managing electronics or can I simply hook the batteries up parallel a.) 2 different batteries and b.) two identical same-age batteries.

I'm thinking parallel is fine for two identical batteries. Not sure about different batteries, but don't care as much since I'd probably just get 2 new DieHard Platinum group 31's.


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## djr (Mar 10, 2009)

my understanding of this if you are listening to car a lot while engine off get a isolater and if listening while driving is you thing no need for isolater provided your batteries are the same 

think of it like this 2 battrey setup.. batt in engine bay.. batt then fuse through firewall.. boot.. fuse.. second batt.. fuse.. amp.. 

ground in boot

boot = trunk


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## qstarin (Mar 25, 2009)

I keep seeing vague references to some parasitic effect where two different batteries in parallel supposedly try to charge off each other or something similar and in so doing run each other dead.

That doesn't make sense to me, though. I would think they'd reach an equilibrium in potential and then no current would flow at all.

I'm only curious cause it would be much cheaper if I could add a single new battery - like a DieHard - in the trunk, and keep the fairly new and decent but nothing special battery under the hood.


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