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Thread: 18650s in parallel and reverse charging danger

  1. #1
    Flashaholic
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    Default 18650s in parallel and reverse charging danger

    What are the dangers of using 18650s in parallel. I know if they are used in series, reverse charging is a danger. How about in parallel setup, such as the Fenix TK35? Is there any danger of reverse charging or explosions?





  2. #2

    Default Re: 18650s in parallel and reverse charging danger

    It won't be a problem. All cells see the same voltage. I used 3 18650's in parallel to run a digital camera for hours at a time. It was taking time-lapse photos.

    Charging took a while, it was before purchasing a hobby charger.

  3. #3

    Default Re: 18650s in parallel and reverse charging danger

    The TK35 is a series setup, not parallel.

    18650 safety in series is best achieved with quality protected cells. This way if one cell starts to get too low the protection kicks in and everything should be good.

    In parallel there should be little concern with reverse charging being a danger (that is more of a concern for primaries). If you have say 3 x 18650 cells in parallel and one has slightly less capacity that the other it will start to sag in voltage a little quicker. The great part about using rechargeable cells in parallel is that the stronger cells will essentially charge the weaker cell keeping the cumulative voltage of the system at a constant level.

  4. #4
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    Default Re: 18650s in parallel and reverse charging danger

    Quote Originally Posted by ShineOnYouCrazyDiamond View Post
    The TK35 is a series setup, not parallel.
    Hmm, interesting. How is series vs parallel determined? From the pictures on BatteryUniversity, I just assumed that if the cells were back to back, they were "in series", and if the cells were side to side, they were "in parallel".

    As for 18650s in series, I'm kind of wary of relying on the protection to kick in.

  5. #5
    HKJ's Avatar
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    Default Re: 18650s in parallel and reverse charging danger

    Quote Originally Posted by aimxplode View Post
    How is series vs parallel determined?
    From the electric connection between the batteries, usual this is also reflected in the physical layout:

    Series:
    +battery-+battery-

    +battery-
    -battery+


    Parallel:
    +battery-
    +battery-
    My website with flashlight, battery and charger information: lygte-info.
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  6. #6

    Default Re: 18650s in parallel and reverse charging danger

    Like HKJ said. If you look at two batteries side by side like the TK35 and both positive ends pointing the same direction it is likely parallel. If you see them in reverse directions it is likely in series. BUT - this is not always true - some battery holders may point all batteries in the same direction but have cross wiring underneath that wires them in series.

  7. #7
    Enlightened
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    Default Re: 18650s in parallel and reverse charging danger

    Quote Originally Posted by HKJ View Post
    From the electric connection between the batteries, usual this is also reflected in the physical layout:

    Series:
    +battery-+battery-


    +battery-
    -battery+


    Parallel:
    +battery-
    +battery-


    HKJ
    Then can you confirm if the TK35 is series or parallel?

  8. #8
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    Default Re: 18650s in parallel and reverse charging danger

    Quote Originally Posted by Enzo View Post
    HKJ
    Then can you confirm if the TK35 is series or parallel?
    It is in series and the battery holder does include part of the driver (or maybe all of the driver). This means that the output voltage from the battery holder is not the voltage of two batteries in series, but about Vf of the led.
    My website with flashlight, battery and charger information: lygte-info.
    More than 200 battery reviews and 80 charger reviews.
    Compare 18650 LiIon batteries or smaller (RCR123, 16340, 14500, 10450) LiIon batteries.

  9. #9
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    Default Re: 18650s in parallel and reverse charging danger

    Quote Originally Posted by HKJ View Post
    It is in series and the battery holder does include part of the driver (or maybe all of the driver). This means that the output voltage from the battery holder is not the voltage of two batteries in series, but about Vf of the led.
    Thanks. Love your reviews!

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