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Recommendation Needed: Charging current for a 12V NiMH battery pack?

Discussion in 'Electronic Basics' started by QDR, Mar 24, 2005.

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  1. QDR

    QDR Guest

    Greetings all,

    Just recently I put together a 12.5 Volt battery pack using ten AA NiMH 1.25 Volt
    batteries in series. It has a current capacity of 2300mAh. After creating the first
    pack, I then created an identical second, and connected the output of that (in
    parallel) with the first. Then simply used epoxy to put the whole unit together as
    one 12.5 Volt, 4600mAh battery pack. (Total of 20 AA NiMH batteries.)

    My question is, what should now be the optimum charging current for this newly
    created battery pack?

    I know, at least with typical NiCAD's, that the charging current is supposed to be
    approximately 10% of the battery's current capacity (in this case, 460mA) for about
    14 hours. Would this be correct for this situation as well? What charging current
    would you use? Any advice and comments much appreciated.

  2. creating the first

    What is the manufacturer and model number of the battery.
    Optimal? You need to define optimal. Is it fast charging or long life?
    Most NiMH batteries should be able to withstand charge currents up to 1C
    (4600mA). Use what the manufacturer says in the datasheet.
  3. QDR

    QDR Guest

    Duracell AA/HR6/DC1500 NiMH/1.2V/2300mAh

    So I assume I can just charge it at 1C (when possible) for approximately 1 hour, or
    anything (lower current) below for a greater amount of time. When you say long life,
    are you referring more to the overall longevity and life-span of the battery, or to
    the length of time the battery will last for a particular charge?

    Since my smaller DC supply only puts out 1000mA, I'd probably have to charge the pack
    at around 1/4C or less anyway. Perhaps a good balance between long life and fast

    When I connected my DC supply (18VDC) to the battery pack through an ammeter, the
    charging current started out slightly above 1000mA. In about 35-40 minutes time, the
    current had dropped to around 805mA, if I'm not mistaken, and then started to make
    its way back up to 813, 814, 815mA and so on. It was at this point that I
    discontinued the charging. Is this normal for a NiMH battery?
  4. mike

    mike Guest

    It's not a good idea to FAST charge batteries in parallel. Small
    mismatches can cause almost ALL the current to one side. You don't have
    any way to measure the current in each string separately.
    You'll probably get away with it at low charge rates.
    Check the vendor specs on long-term slow charging. Early NiMH didn't
    tolerate this well. Maybe better by now.

    It's not a good idea to EVER connect a constant voltage source to charge
    a NiMH pack. You need CONTROL of the current in each cell.

    You need to monitor something to terminate fast charging.
    Temperature works if you measure ALL the cells and don't mind cooking
    them every time they charge. I prefer voltage slope sensing with
    temperature backup in case something fails.

    DO NOT attempt to fast charge based on time alone. You can never be
    sure of the initial state of charge. If you expect to time it manually,
    you'll make a mess the first time you get a phone call that diverts you.

    You didn't disclose the configuration of the pack. For fast charging,
    you need equal temperatures, meaning at least equal exposure to ambient.
    Depending on how you epoxied them together, you may have created a problem.

    There's a bunch of information on the cadex site.

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  6. QDR

    QDR Guest

    Very good advice, and it should be a concern. By severing the battery pack connection
    (no longer connected in parallel) and connecting them both to two separate ammeters
    while they are being charged by the same charger, I find the current difference
    between the battery packs to be generally less than 5-7%. The difference can start
    out larger as the batteries first begin to charge, and then becomes negligible the
    closer they get to being fully charged. What's your take on this?

    It could be that I got lucky and got some very closely matched batteries with very
    similar internal resistances.

    I'm not sure I understand this fully. May be a dumb question, but if a battery pack
    is comprised of, say, 10 cells, and the battery pack has only two connection points
    (one negative, one positive), then how would you control the current to each
    individual cell in the pack?
    What I've done thus far is this: I use a volt meter to closely monitor the voltage of
    the battery pack as it charges. In this particular case, the battery pack will reach
    a point where the voltage is at around 14.47 volts. It is at this point that the
    voltage begins to drop back down several one-hundreths of a volt. And also at that
    point that I disconnect the charger; Sort of a human intervention Negative Delta
    Voltage charging method, if you will.

    The configuration of the two packs is that they are comprised of 10 identical (in
    make and model) Duracell AA batteries to create 12.5 volts. These two packs are then
    connected in parallel to give a capacity of 4600mAh rather than only 2300mAh.

    The pack is put together in such a way as to create as equal temperature as possible
    between all 20 batteries. In fact, all batteries are all within physical contact
    within the pack. I have been charging it at approximately C/5. As far as temperature
    goes, it seems to be both equal and constant on both sides of the pack throughout
    charging, even though it never reaches anything near 140 degrees. There are no hot
    spots or cold spots. Very even temperature. A rate of C/5 I think might be still
    considered relatively slow, even for a technically termed fast charge.

    Recently I purchased the MAHA MH-777P-II Universal charger...

    ... to deal with packs (and others such as this), but as of yet have not used it
    with this pack. Generally upon buying (or creating) a rechargeable battery pack, I
    like to charge it manually first, with ammeters and voltmeters attached so as to
    physically see the real-world charging characteristics of the pack.
    Thank you for the info.
  7. I think mike is giving you good advice here, so I'm
    only clarifying what went before and answering
    your new questions.
    It's luck that you cannot and should not count on.
    That is part of the reason for mike's advice.
    To continue the luck would require similar histories
    for the cells. Varying discharge depths and rates,
    and varying recharge methods, will make the cells
    even less matched than when they were when new.
    The cells are in series, so controlling current in the
    string controls it in each cell, automagically.
    You may be seeing a heating effect as the cells reach
    full charge. When that happens, more applied charge
    is converted to heat rather than chemical energy.

    [Fast charge cautions agreed upon.]
    That connection is problematic because it makes the
    problem of unequal current sharing discussed above
    all the harder. Can you split the parallel strings for
    charging purposes? Or can you afford equalizing
    resistors among the strings? (slow discharge)
    You may want to provide a little extra "insulation"
    for the corner cells. They are more exposed to
    ambient than the middle cells.
    With that interest, you might want temperature sensors
    on the battery as well.
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