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NiMH Self Discharge Test Charts!

Discussion in 'Electronic Design' started by CC, Aug 13, 2006.

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  1. CC

    CC Guest


    This file shows data on an experiment I performed on self discharge
    rates of NiMH cells.

    The cells had been used in a photoflash unit as well as an LED lantern
    (the RiverRock LED lantern from Target) over the course of months.

    Several incidents occured where one or more of the cells in the pack had
    reversed voltage due to depleting before the other cells, and yet still
    under load. This situation results in current flowing through the cells
    in the normal discharge direction, but with the cell voltage reversed
    resulting in electrochemical damage.

    There is a great deal of uncertainty amongst NiMH users about just what
    effect temporary bouts of cell voltage reversal have on cell
    performance. It should be fairly clear from the data that 3 of the
    cells are experiencing very high self-discharge rates. However, notice
    that their initial capacities are very close to normal.

    This is a very crude test which of course doesn't quantify the degree of
    voltage reversal experienced by the cells which are performing badly.
    In fact, it is not even known for certain which of the cells were the
    ones which experienced voltage reversal. Though it is a fair assumption
    to say that cells B and D probably were abused the most, with cell C not
    quite as bad, and cell A being the one that held up under load and may
    have never reversed.

    It would be nice to have measures of the actual remaining capacity in
    the cells at various points through the discharge curve.

    My experiences with NiMH cells have been disappointing in general. It
    seems that with multiple cell packs in consumer devices meant to work
    with AA alkaline batteries, there is no way to guarantee that cells will
    not experience voltage reversal. They most often do, unless one only
    operates equipment for short periods before recharging cells, thus never
    realizing the full capacity of the cells. This might still be
    preferrable to the costs of primary cells in some cases, but it is not

    I am convinced that these cells can only be properly used in series
    packs with some sort of intelligent supervisory electronics. For small
    packs of a few cells it may be sufficient to cut off the load when the
    pack voltage has decayed to some value. Though there are uncertainties
    about temperature with this approach. The best way is to have a circuit
    that monitors each cell voltage individually and cuts the load before
    any single cell collapses.

    The only bright side I can see is that cell capacity doesn't deteriorate
    from voltage reversal so much as self-discharge rate is compromized. I
    have experienced this as devices working fine with the freshly recharged
    cells, but after a little more than one week, the device shows weakened
    performance. Of course, as soon as the device is used with one or more
    weak cells, it is those cells which get reversed again. So the process
    is self-reinforcing.

    I wonder if a carefully matched set of NiMH cells would avoid this?
    Probably not. And who has the time to match them?

    Comments welcome.

    Good day!
  2. That looks promising as a way of weeding out bad cells, just monitor the
    voltage of each cell with a multi-channel A/D converter, and watch over
    around 15-20 days. Much easier than doing charge-disharge cycles and
    measuring the capacity.


    Adrian Jansen adrianjansen at internode dot on dot net
    Design Engineer J & K Micro Systems
    Microcomputer solutions for industrial control
    Note reply address is invalid, convert address above to machine form.
  3. Donald

    Donald Guest

    I think you should draw this out and have it checked first.

    I don't think it will work the way you think it will.

    Good Luck

    Its done this way because of the problems with your first suggestion.

    Good Luck x 2
  4. Phil Allison

    Phil Allison Guest


    ** An old trick that minimises reverse charging damage is fitting a reverse
    diode ( pref Schottky) across each cell.

    ( Going on better, why not use a MOSFET driven hard on when the voltage is
    close to zero or reverse? )

    ** Well designed battery powered devices have low voltage cut offs.

    Where the pack has no more than 4 or 5 cells this should prevent cell

    Using higher numbers of cells in series is just asking for trouble - a
    DC-DC inverter should be used to avoid this.

    ** They never remain closely matched for long.

    ........ Phil
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