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Battery Question for the Experts

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  • Battery Question for the Experts

    I just read on a Forum dedicated to flashlight geeks, that if the voltage between two rechargeable lithium ion batteries is different enough the one with the higher voltage will discharge in the weaker one possible causing a fire/explosion. I know multiple cells are needed to make up one of our cordless tool batteries and I was wondering if this imbalance between cells can contribute to a safety problem?

  • #2
    Re: Battery Question for the Experts

    In general mixing batteries with different levels of charge is a bad idea regardless of the type. In the case of a parallel connection the tendency will be for the voltage to even out accross them so if you hook up a dead cell with a fully charged one you end up with roughly two half charged cells. At worse what should happen is a lower A.H. rating. In a series connection is where it gets a little risky. The higher charge battery will have to compensate for the one with the lower charge. That can cause the high one to get drained faster than its rated for and the dead battery can be overdrained and destroyed. This can be dangerous on any kind of battery. A little more on lithium ion since by definition they are the most dangerous kind. I'm not exactly sure about this part but one thing I've heard is that once the low battery dies, the good one will basically be reverse charging it from the negative side. THis is what could cause the dead one to fail violently.
    This shouldn't be a problem with most batteries since stuff like your powertools and electronics don't charge the cells directly. They have a processor that monitors each cell to make sure they stay balanced. Something like a flashlight can definately pose a risk since it would require individual cells that are individually charged unless they made some sort of proper battery pack for it with all the necessary safeties. In your tools for example if one cell in the series failed the charger would most likely consider that entire series defective and would not carge that part of the battery at all.
    Last edited by Velosapien; 04-21-2008, 01:19 PM.

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    • #3
      Re: Battery Question for the Experts

      Originally posted by Velosapien View Post
      In general mixing batteries with different levels of charge is a bad idea regardless of the type. In the case of a parallel connection the tendency will be for the voltage to even out accross them so if you hook up a dead cell with a fully charged one you end up with roughly two half charged cells. At worse what should happen is a lower A.H. rating. In a series connection is where it gets a little risky. The higher charge battery will have to compensate for the one with the lower charge. That can cause the high one to get drained faster than its rated for and the dead battery can be overdrained and destroyed. This can be dangerous on any kind of battery. A little more on lithium ion since by definition they are the most dangerous kind. I'm not exactly sure about this part but one thing I've heard is that once the low battery dies, the good one will basically be reverse charging it from the negative side. THis is what could cause the dead one to fail violently.
      This shouldn't be a problem with most batteries since stuff like your powertools and electronics don't charge the cells directly. They have a processor that monitors each cell to make sure they stay balanced. Something like a flashlight can definately pose a risk since it would require individual cells that are individually charged unless they made some sort of proper battery pack for it with all the necessary safeties. In your tools for example if one cell in the series failed the charger would most likely consider that entire series defective and would not carge that part of the battery at all.
      Thank you for that detailed explanation. I checked the individual voltage of the rechargeable lithiums since I use two in each of my tactical LED flashlights. To be on the safe side I will check the voltages after each charge.

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      • #4
        Re: Battery Question for the Experts

        If you are using a multimeter to test the batteries you'll need to make sure they are under a load. The resistance of a multimeter is high so even a dead battery will read like it's close to fully charged if you just stick the leads to the + and - ends.

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