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NiMH battery paralleling

Discussion in 'Electronic Design' started by Spehro Pefhany, Jul 18, 2007.

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  1. Hi, all:-

    I want to combine some (6 or 8) series strings of a bunch of NiMH
    batteries in parallel to get more W-h. The voltage is high enough that
    a diode drop won't affect the efficiency much.

    Do you think it is better to just parallel them or to use a Schottky
    diode to "or" each string? There will be a fuse or polyfuse on each
    string, and a single string can supply the entire current if
    necessary. Charging will be done independently, it's only the
    discharge path I'm concerned with here.

    I don't see any specific guidance on this matter from the battery

    Thanks for any suggestions.

    Best regards,
    Spehro Pefhany
  2. I would be worried about a cell failing short-circuit. Isn't that the
    normal failure mode? Then the other strings will discharge themselves
    through the string with the faulty cell.
  3. Leaving aside right or wrong question, it is pretty typical solution for
    battery packs to have cells hardwired in parallel for both charging and
    discharging. First, the cells are connected in the parallel one to
    another, and then the paralleled pairs are connected into one string.

    Vladimir Vassilevsky

    DSP and Mixed Signal Design Consultant
  4. I think the main thing to watch out for is when one chain is empty,
    that the max charge current is not exceeded.
    So when the others charge the empty ones.
    Any chance of reverse connection?
    Diodes would make me feel safer.
    Or something with MOSFET swicthes perhaps to reduce losses?

  5. Hi, John:-

    With NiCd cells shorting is the usual failure mode, I am not sure
    about NiMH. The end-of-life is defined as a decrease in storage
    capacity under specified conditions. I don't think I've seen one that
    has failed completely (unlike NiCds), in my own use of them for
    digicams etc.

    That's my idea behind putting "or" diodes in there, so if the voltage
    drops on one string due to a dead cell (open or short) then the
    survivors should share the load (more or less).

    But even without diodes, in case of a catastrophic (maybe a chain
    reaction failure due to overcharging from other cells) the internal
    fuse & vent in the cells chould go (but before that, the external fuse
    should isolate the problem string and break the current). I suppose in
    the case of a polyfuse it would cut in and out with some rep rate,
    which isn't good.

    Best regards,
    Spehro Pefhany
  6. Hi, Jan:-
    They'll be charged in individual strings (one charger per string), so
    at least that's not an issue.
    Shouldn't be, but putting diodes in the right location could act as
    insurance. It wouldn't be catastrophic if string was reversed.
    Yes, the old bass-ackwards MOSFET trick. It would add a handful of
    parts since the string voltage is more than the gate can stand and
    there's a lot of strings. For lower voltages it would be nice.

    Best regards,
    Spehro Pefhany
  7. Gibbo

    Gibbo Guest

    You can get away with paralleling many types of rechargeable cells for
    the simple reason there is a "dead band" before charging starts. For
    instance a flat lead acid cell at 1.9 volts will not start to charge
    until the terminal voltage is raised to around 1.95 volts. 1.92 volts or
    so will do nothing.

    This doesn't happen with NiMH (or Nicads). The tiniest increase in
    terminal voltage will result in a charghe current.

    This in itself is, of course, not an issue but it becomes one when it is
    realised that the terminal voltage of different NiMH cells varies
    slightly for the same state of charge.

    Two cells in parallel will never have the same open circuit voltage for
    the same state of charge so when connected in parallel the one with the
    higher voltage will discharge slightly into the lower one.

    During the use of the battery pack this means that the cell with the
    lower terminal voltage (yes lower, not higher - because it will be in a
    higher state of charge for the same voltage and therefore able to
    produce more current) will be worked harder during discharge and charge
    and result in much less than double the capacity of a single cell on its

    That's why they should not be connected in parallel.

    The diodes are a must.
  8. Hi, Gibbo:-

    Sounds reasonable. Done!


    Best regards,
    Spehro Pefhany
  9. If a single series string can handle the load, I'm assuming that what
    you are after is the higher capacity, not increased peak current due to
    the parallel strings.

    I think your diode/fuse solution will work OK. There are some multiple
    battery controllers (search Maxim, I think) that switch between two or
    more battery packs. They allow for hot swapping single packs, control
    charging, and provide some battery status signaling, IIRC. Needless to
    say, there will be additional $$ involved.
  10. That could be a problem. But the diodes will fix that. You might not
    even have to polyfuse the individual strings if you use diodes.


  11. The isolation diodes alone will allow hot swapping.

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  12. Fred Bloggs

    Fred Bloggs Guest

    Looks like combining NiMH in any way is an art form, and you're taking a
    chance of early failure even with the series string, especially if the
    customer has access to it. Google turns up a few papers describing UPS
    backup systems using hundreds of NiMH arranged into parallel banks but
    they don't go into any detail regarding matching and monitoring methods.
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