Power in Flux
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Thread: Understanding a BMS -- reasons for and against them

              
   
   
  1. #61
    Seņor Member podolefsky's Avatar
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    Quote Originally Posted by lugnut View Post
    Doesn't the charge move from one cell plate through the external circuit to the other plate in the cell and not through the cell?
    Um...wait. That doesn't fit with my angel model of electricity

    Yes, in a sense that's right. You could think of it like a capacitor. That is, charge doesn't actually go "through", but it goes into one side and comes out of the other side. Either way, the total charge in the battery doesn't change.

    Either model is good, depending on what phenomenon you want to explain (they're all just models).
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  2. #62
    Seņor Member podolefsky's Avatar
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    In case this is at all interesting, I have a little piece of data for the discussion. My pack was full charged, and I read off the max and min voltage cells: max = 3.441, min = 3.422. All cells within 0.5% of each other. This is after balancing and sitting over night. Then I went for a 40 mi ride, and checked the max and min again. Max = 3.199, min = 3.166. So now all cells within 1%.

    These drifted around a bit for a few min, but stayed within 1% of 3.2V. In other words, any imbalance between cells was negligible after 1 ride. I'm not sure what would happen if I did this repeatedly without balancing, and I won't find out because my system automatically balances when charging.

    I could try to turn off the balancing algorithm, but I don't really want to do that...and my guess is that at the top the cells would still be within 1%. (I'd be surprised if charging at 10A unbalanced the cells more than riding at 60-300A). Bottom line - only thing I would worry about is any (possible) cumulative effect of not balancing over many cycles.
    - Noah Podolefsky -
    The GSX-E

  3. #63
    Moderator Nuts & Volts's Avatar
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    Just randomly found this video. I talks about active balancing with inductors. This means you are no longer limited to the capacity of the lowest/weakest cell. Pretty interesting stuff. Complicated and maybe costly, but it helps eliminate the concerns with battery manufacturing variation. enjoy

    Skip to 2:00 for the good stuff.
    Last edited by Nuts & Volts; 19 March 2011 at 1846.
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