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Charging NiMH with Solar Panel

Discussion in 'Photovoltaics' started by FlashGordonDownUnder, Nov 13, 2004.

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  1. Hi,

    I want to be able to charge my 1.2V NiMH batteries while I am bushwalking.
    Or more correctly I should say that I just want to be able to top up their
    capacity while I'm walking as there will not be an oppotunity to be in the
    one place for 10 (20?) hours with correct orientation to get the best
    output.

    My digital cammera and poratble HDD use 4 x 1.2V NiNH so I'd like to top up
    the batteries as a group using a 12V 5W solar panel (Uni-solar flexible
    type).

    Could someone tell me if I can connect the panel directly to my batteries
    (they will be in series = 5V) with a diode to stop reverse current back to
    panel or should I be limiting the voltage with a zener or very basic
    regulator to get the voltage capped at say 6 - 8 V. I'm not too worried
    about the current as I'll never get more than about 230mA out of it and even
    at this rate would be for a few hours maximum hence never reach capcaity of
    batteries (which are 2300mAh).

    As I'll be having a number of sets of batteries (4 per set) I could put two
    sets in series and then charge that way but would my top up charging be
    halved this way and become rather useless?


    Thanks Gordom (from Downunder in sunny Sydney, Australia)
     
  2. Gymmy Bob

    Gymmy Bob Guest

    NiCads and NiMh batterries are designed to take a current charge forever.
    Usually this the 10 hour rate. 550mAh / 10 = 55mA for 14 hours for full
    charge (40% loss in charging)

    Their voltage does not increase much and therefore is a lousy way to control
    charge. Maximum current is what you want to control. They also get warm when
    fully charged. Feel them and see if they are done.
     
  3. Bob,

    Thanks for the info.

    But could you tell me if I can connect a 12V solar cell directly to 4 1.2V
    batteries connected in series as I know the solar cell can put out anything
    from 0V to about 24V.

    Thanks Gordon
     
  4. Gymmy Bob

    Gymmy Bob Guest

    Again. The voltage of the open circuited solar cell does not matter. The
    circuit voltage will be the voltage of the battery bank. You will have to
    connect them experimentally and measure the current and adjust with a series
    resistance.
     
  5. OK, I shall try that.

    I thought the voltage was important also, but I did measure with 8 batteries
    and noted that it was around 10.5 - 11V I think.

    Thanks Gordon
     
  6. boB

    boB Guest

    After the batteries are charged, a big enough un-regulated PV module
    can bring the battery voltage too high and destroy them.

    I'd use some kind of charger to control the end of charge. Otherwise
    you have to watch them closely.

    boB
     
  7. John Rowell

    John Rowell Guest

    Might use a voltage regulator. You can get an LM317 chip at Radioshack for a
    couple bucks. Add two resistors to adjust the voltage to 6 volts. This will
    make sure the voltage stays within a safe level. The current from the solar
    panel is low enough you shouldn't have to concern yourself with
    overcharging.

    --John
     
  8. Gymmy Bob

    Gymmy Bob Guest

    These are NiMh. If he doesn't have to wory about overcharging, he doesn't
    have to insert a regulator chip either. NiCads and NiMh are never charged
    with constant voltage.Voltage does not indicate state of charge well.
     
  9. I'll be trying it this weekend if it does not rain on me!

    Will be using some oldish NiCd's so if they get distroyed it will not matter
    too much, they have had a hard life, been shorted out a few times with the
    holder catching alight, luckily was around when this happened, otherwise may
    have burn the house down!!
     
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