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Two battery related questions ...

Discussion in 'Electronic Basics' started by purple_stars, Dec 13, 2006.

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  1. purple_stars

    purple_stars Guest

    Hi folks,

    I use a bank of NiMH batteries to power some mobile equipment and I
    currently charge the bank with a small 6.5 watt folding solar panel.
    Works great. One nice extra of this setup is that the banks are made
    from AA batteries so I can swap out charged batteries from the banks
    and put them into electronics I use while in the outdoors, like my GPS
    receiver for example, and put the spent/drained batteries back into the
    bank where they eventually recharge. But using this system has brought
    up two questions in my mind that I would like answers to ...

    #1 - My battery banks are made out of 4 banks, each bank having 10 NiMH
    AA cells wired in series to make 12 volts. Those 4 banks are wired in
    parallel to supply the amps I want. They are all made of AA batteries.
    As I said above I sometimes swap batteries out and use the banks like
    a battery charger for personal electronics, very convenient. What I
    would like to do is make one of the banks out of AAA batteries instead
    of AA batteries for that very reason, so I could also charge up AAA
    batteries. Some of my electronics use AAA instead of AA.

    So the question is .... What will happen if instead of using 4
    identical banks wired up in parallel I instead use 3 banks made of AA
    batteries and 1 bank made of AAA batteries. They would all still be 12
    volt banks, but the AAA bank would have much less capacity. What will
    that do to my overall "battery" when it recharges ? Will it still
    fully charge using the solar panel even though one of the banks has a
    smaller capacity ?

    #2 - Right now I am using just the solar panel to recharge the banks,
    but it would be great to have the flexibility of using other charge
    sources that are available. If I was near an outlet I would love to
    use a 12 volt wall wart or even an automobile alternator to recharge my
    battery bank. Problem is I don't know if it will work. For one thing
    I do know that the NiMH batteries get warm if they are given a lot of
    current, how warm I have never tried to find out .. would they burn up
    or something ? Would they burn up a 12 volt supply by eating up too
    many amps while recharging ? The solar panel setup is kind of
    self-regulating in that it can only supply about 1/2 of an amp on a
    full sunny day, so the batteries can't recharge but so fast. But if I
    hook the batteries up to an automobile electrical system they could
    have all the amps they want, what is going to happen if I do that ?

    That is all rather long winded I know ... but I wanted to make sure I
    was clear about what I was doing. Does anyone know the answer to these
    two questions ?

    Thanks in advance!
  2. ehsjr

    ehsjr Guest

    It's not the best way to charge NiMh, but you can probably
    get away with it. The auto system won't overcharge the packs -
    unless there is a defect, like a shorted cell, in a pack - if
    you charge each pack through a diode. (If one cell is shorted,
    you can overcharge the other cells.) It also won't charge the
    packs fully. In general, you can figure the auto system will
    deliver ~ 14.4 volts when running. The diode will drop ~ .6 V
    so the NiMh pack will get ~ 13.8. (The diode also prevents
    discharging the NiMh packs into the car's battery.) For full
    charge, the pack needs ~ 14.1 volts. If the car is not running,
    the charge on the packs will be even less.

  3. jasen

    jasen Guest

    likely the smaller will overcharged and damaged.
    hook thre wall-wart to the input of your charge controller insted of the
    solar panel.
    boil dry.
    the charge controller should stop that.
    something like that
    first off it sounds like your charging setup is incomplete

    secondly series charging of different batteries, or ones
    with different histories, is not a good for them.

    get a 12V operated NiMh charger

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