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Unequal size Li-Ion batteries in series?

Discussion in 'Electronic Components' started by Thomas David Kehoe, May 16, 2005.

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  1. Is it possible to use two rechargeable Li-Ion batteries of unequal sizes in
    series?

    I'm designing a device that will run at 4V, 25-30mA. The device is pager
    size so a 9-volt battery is too big. I thought I'd found the solution in RC
    car batteries. These are flat so they can lie on top of the circuitboard.
    They are polymer li-ion, 3.7V. They are available in two sizes: 750mA and
    170mA. Two 170mA batteries lasts only 7.5 hours. I need at least 15 hours.
    The case can fit one large battery and one small batteries, or two small
    batteries. Possibly I could fit one large and two small, or three small
    batteries.

    I set up one large battery and two small batteries in series. The voltmeter
    read 11.9V. I set up the device and came back ten hours later. The large
    battery was at 4.1V - fresh - and the small batteries were at -0.1 volts.
    They had puffed up. I charged them but they never went over 0.1 volts. How
    could the large battery be fresh and the two small batteries be destroyed? I
    can't figure out a way to measure current draining from each battery.

    Now I have two test devices running. Each has one large and one small
    battery, in series. The difference is that one device has the large battery
    connected to the positive terminal and the small battery to the negative
    terminal; the other device has the small battery connected to the positive
    terminal and the large battery connected to the negative terminal.

    Any other suggestions?
     
  2. mc

    mc Guest

    I think by mA you mean mAH throughout.

    If you put unmatched batteries in series, one will discharge first, and then
    the other one will reverse-charge it by continuing to drive current through
    it. With NiCd cells this causes immediate severe damage. I'm not sure
    about Li-ion, but it sounds from your description as though the results were
    equally fatal.
     
  3. mc

    mc Guest

    BTW, if you needed 4 volts, why did you put them in series? Was 3.7 V not
    enough?

    In parallel, batteries of matched *voltage* are OK, regardless of whether
    the capacities match. The strong ones recharge the weak ones, rather than
    reverse-charging them. But if one battery shorts or becomes electrically
    leaky, it will discharge all the others.

    The 3.7-volt, 750 mAH (if it's mAH) battery will supply 3.7 V to your 30-mA
    load for about 24 hours.

    If you need technical assistance with your design, please drop me a line (mc
    at uga dot edu). I'm in speech science myself.
     
  4. You've already found out the hard way that the small battery won't work.
    If you must have a certain voltage, you should use a V converter to
    boost the single large cell up to what you need, and skip the multiple
    oddball cells.
     
  5. No - in general, it is a Very Bad Idea to use batteries of different
    capacities in series, as the lower-capacitiy battery will become fully
    discharged before the higher capacity one, then the higher capacity
    battery will start to reverse charge the low capacity one..
    What was the current drain? If it was 30 mA, your 170 mAH batteries
    would be fully discharged in about 5.6 hours, while the 750 mAH cell
    would be good for about 25 hours.
    The order of the batteries doesn't matter - the 170 mAH cell is only
    good for 5 hours, regardless of where you put it.
    --
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  6. mike

    mike Guest

    Do NOT charge or discharge equal lithium ion batteries in series without
    some monitoring circuitry to keep 'em from doing what yours did.
    NEVER, EVER charge or discharge unequal lithium ion batteries in series.
    Equal means same part number, same history, same batch.

    You don't want to do that with ANY battery technology, but at least
    other technologies are LESS likely to explode when you do. I said LESS
    LIKELY...

    If you intend using lithium cells, you need to do a LOT of reading on
    the subject. The Cadex site is a place to start.
    mike
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  7. ehsjr

    ehsjr Guest

    Thomas David Kehoe wrote:
    Others answered about the setup you have - don't do it.

    But say you had a series setup with identical batteries.
    To measure the current draining from each battery,
    just measure the current draining from the series
    combination. The current flowing in each battery is
    the same.
    Yes - since you are designing the circuit, why not design
    it to work from 3.7 volts instead of 4 volts? Otherwise,
    a DC-DC converter, as Watson suggested. These days they are
    unbelievaby simple - a chip and a few components.

    Ed
     
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