batteries in series, different discharge rates

Discussion in 'General Electronics Chat' started by strantor, Jun 8, 2011.

  1. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
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    Ok, i will set up an example:
    let's say you take 4 12V batteries and put in series to run a 48v motor continuously, (assuming they are all 4 exactly the same) the 4 batteries should all discharge at the same rate, correct?

    Now, you tap between the first and second battery and add a 12v motor there (in parallel with the first battery), will the first battery discharge faster than the other 3?
     
  2. tom66

    Senior Member

    May 9, 2009
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    Yes, and doing so is a bad thing because it can lead to cell reversal: a particular cell can have a negative voltage forced across it if it is discharged quicker than the other two cells. See the screenshot of a simple capacitor divider showing a similar effect. The second cell from the bottom has been discharged 100x faster than the other cells and has a -63mV potential across itself (look at the cell tap voltages), which would ruin almost any type of battery. In some packs, this action can even overcharge other cells in the battery, damaging them too.
     
  3. iONic

    AAC Fanatic!

    Nov 16, 2007
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    Although not the most efficient solution, but using a 12V Voltage Regulator (LM7812) across the 48V supply (the 4 batt's in series) would get around the issues presented above.

    [​IMG]
     
    Last edited: Jun 8, 2011
    strantor likes this.
  4. strantor

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    cool drawing? what program did you use? I usually use paint.
     
  5. tom66

    Senior Member

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    A 7812 will not be happy with 48V and will likely break down, allowing the full 48V to hit the 12V motor.

    Not to mention the inefficiency of that solution. Per amp the motor wants, 36W will be dissipated. So it will be less than 25% efficient.
     
  6. iONic

    AAC Fanatic!

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    I used an old version of Paint Shop Pro, back when it was Jasc Inc.

    Wasn't sure what the max Vin on the 7812 was, but you are probably right about your assessment.
     
  7. tom66

    Senior Member

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    Max Vin is 35V, according to this datasheet. Some 7824 regulators can handle 40V input.
     
  8. strantor

    Thread Starter AAC Fanatic!

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    Ah, I remember that, I used to use Jasc Paint Shop Pro 7, then 9. It was alot easier to use than the new ones IIRC.
     
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