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Thread: How to wire and use 2 Car battiries?

  1. #1
    Join Date
    Jun 2008

    How to wire and use 2 Car battiries?

    I will be installing 2 battires in my car one under the hood the secound one in the trunk is it ok to run a the postive to the on in the hood to the postive to the trunk and then hook the neagtive to the ground in the trunk??

    To simplify postive Hood to postive trunk
    neagtive trunk to ground/chassis

  2. #2
    Confusion Master
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    Sep 2003
    If you go down to the woods today, You're sure of
    Do a search for Split Charge Relay.

    there have been a number of threads discussing the best way to do this.

  3. #3
    Join Date
    Jun 2006
    Calgary, Alberta
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    Use a battery isolator.

  4. #4
    Join Date
    Jun 2008
    is it ok to use the switch idea ?? so the batterys won't fight

  5. #5
    Variable Bitrate
    Join Date
    Jan 2008
    Southern Calif
    The way its done is, when the cars engine is turned on, both batteries are connected together so that they can be charged up with the alternator. When you turn off the car, the heavy duty relay (isolator) kicks off and now the 2 batteries are not connected.

  6. #6
    Constant Bitrate MindDrive's Avatar
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    Feb 2008
    Shorewood IL
    click the '97 seville link... what you should do is use a battery isolator

    the under-the-hood battery does nothing BUT power teh functions of the car, while the 2nd battery in the trunk (or wherever) powers nothing but auxilary componets (pc, amps, sterio etc)

    the altenator connects directly to the isolator, its the isolators duty to determine which battery needs a charge, cheap ones just use diodes to direct flow at a certain voltage, where as some of the more $$ ones will by default start the charge on battery 1 (the under the hood) and once that is maintained properly, will direct charge voltage to the 2nd battery. the isolator im using i purchased at napa for like 70 bucks plus a $15 install connector kit for the GM altinators,


    the whole purpose of using an isolator rather than just connecting 2 batteries together (which can be done and does improve power output by connecting the +'s together and the -'s together) BUT if one battery slowly fails (due to bad cells, unstable fluid etc) then it will drain the other battery and you will eventually have 2 completely worthless batteries.

    One way to combat this is by using 6V batteries in series **= 12V** and double that.. which will give you the equivalent of 2 12v batteries, but far more amps BUT with the extra weight and added wiring and overall cost its not something you would want in a common vehicle

    One tip, the 2nd battery should only be a ***" dual duty deep cycle"*** marine style battery for several reasons:
    they take massive drain beatings
    they hold a charge longer
    low maintenance
    and if relayed right, or for emergencies, you can start a vehicle using jumper cables or your own vehicle by wiring a solenoid into the ignition or a switch

    GL adding a 2nd battery

  7. #7
    Admin. Linux loser.
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    Bugbyte's Avatar
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    Sep 2004
    Corning, NY
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    Easy answer as stated: Battery isolator and the accessory battery should be deep cycle.
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