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Wiring a bunch of batteries in parallel, need advice!

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  • Wiring a bunch of batteries in parallel, need advice!

    I'm considering wiring (in parallel) a gaggle of UB1250 batteries. I want to move my starter battery to the rear without destroying the utility of the trunk. These 12v batteries are rated at 5Ah each but there is no rating as far as CCA (cold cranking amps). I'm pretty sure these will work if I wire enough together... it's just a matter of how many? These are AGP (acid gel) batteries so they should deliver a good current. Anyone have an idea how many of these needed to take the place of an OEM car battery?

    I've heard bad things about wiring car batteries in parallel. However, I believe I can avoid all those problems if all the batteries are installed new and all at the same time. Anyone have any experience with this? Thanks for the help.
    Palm Springs, California

  • #2
    If they're only 5Ah they obviously weren't intended for starting a car; thus no CCA rating. You'd most likely have to get info from the manufacturer to see if they're suitable for starting use.

    Its hard for me to envision that a bunch of those wired together is going to save any room in your trunk compared to installing a regular old car battery or something like an Odyssey which packs a lot of power into a pretty small space. Some batteries are smaller than others, but there is no magic solution that is going to save a lot of room and provide the same capacity (CCA and Ah) if all you're doing is replacing one lead-acid battery with another or a collection of individual batteries. (unless you're planning to distribute small batteries around your trunk into various nooks and crannies and deal with a bunch of wiring)

    I would think you could parallel them if they're all new if you REALLY have to go that route.


    • #3
      Moving even a single car battery to the trunk is problematic. The reason is that the starter motor requires very high current to operate. It's hard to turn the engine fast enough to start it.

      That means that high current has to be delivered over the wiring harness to the starter motor for a very short period of time. The ability of the starter motor is related to the voltage that it receives. At 12 volts, delivering high amperage is difficult and explains why the wire run is short and the battery cables are big and fat.

      Putting the battery in the trunk will drop the voltage due to the resistance of the wire. Piston powered aircraft often have batteries in the cabin due to little space in the cowling, but they are higher voltage - 24 volts rather than 12.

      Will this affect your ability to start the car or the ability of the alternator to recharge it? Perhaps. It depends on a variety of factors, including the internal resistance of the battery or batteries.

      Wiring batteries together can cause problems because batteries are of differing resistance. One battery may have 11.5 volts while another has 12.5 volts. That means one battery will be charged by the other at rest, lowering the overall voltage. It also means that one battery will get cycled more often than the other, causing it to heat up more and fail quicker.

      I only know enough about electricity to know that I don't fully understand all of the issues with a plan like this and I'm almost certain that the type of batteries you wire together will make a huge difference. Until you know more about it, I'd either stick with moving a single car battery and running the thickest cables you can or I'd wait until I understood it more.
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      • #4
        It can easily be done, and is. There are gel cells higher, Odyssey whatevers can go to 16+ AH, I don't like them. Just find what fits and use copper lugs with 2 ag or bigger to keep voltage drop within a volt under crank, .2v under run. Wire enough together for 40AH if you can, for your climate.
        With multiple batteries you increase the failure possibilities, with parallel a single dead cell will not reduce output voltage as much as amperage coming out, but will drive the charging system nuts. Any variation among the batteries will affect charging. Time for new troubleshooting skills...
        Without many of those baby batteries you will exceed their max current (40A for 5 sec)during start (80-160A), and quickly kill them.
        So - 40AH/5AH= 8 batteries minimum
        And 160A/40A=4 batteries minimum
        Not really worth it.....
        06 Silverado LBZ wants onboard everything...