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  1. #1
    mtbr member
    Join Date
    Jan 2013

    Unequal battery packs in parallel?

    I have two 7.2v battery packs that I carry with me for my helmet light. Tomorrow night I'll be doing an extended night ride and was thinking of connecting both packs to my helmet in parallel. I figure it would be nice if I didn't have to stop to switch packs in the occasion that I outrun one pack. The problem is that one is a 5000mAh pack and the other 2100mAh.

    Conventional wisdom tells me that I should roughly get 7100mAh out of this configuration in a perfect world. The caveat I have.... Is that one pack is a few years old. Do I risk damaging the newer pack if the older one discharges down first? Obviously my concern is being without light 15 miles from civilization/cellphone reception at night.

  2. #2
    mtbr member
    Join Date
    Oct 2006
    Do not do it. You will mess up the packs especially if there is no cell balance built into the packs.

  3. #3
    mtbr member
    Reputation: HEY HEY ITS HENDO's Avatar
    Join Date
    Feb 2008
    if they are both fully charged, you can connect them together for more capacity,
    because they are connected together, they will run down equally as they are now one battery pack,
    ...Scun.thorpe, UK

  4. #4
    mtbr member
    Join Date
    Oct 2006
    No the lower capacity pack will discharge faster and voltage will drop faster on the old pack. Do the packs have a PCB low/overcharge voltage built into the packs?

  5. #5
    mtbr member
    Reputation: mattthemuppet's Avatar
    Join Date
    Jul 2004
    from what I've read and been told by mtbmacguyver on here, if you have unequal capacity batteries in parallel then the weaker battery will float at either the discharge or charge voltage until the stronger one is also discharged or charged. The reason being that the weaker cell will be unable to contribute (or "absorb" if charging) any current, so once it's done it's done and won't get pulled down to a lower voltage as it would if the batteries are in series.

    A better question would be why you wouldn't just carry the smaller one in your backpack as a reserve in case you need it? Much simpler

  6. #6
    Join Date
    Sep 2008
    I would have thought that they would discharge pretty much equally unless the current load is at the limit of one of them to source. Now I'm going to have to go try it.

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