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Building AA NiMH solar charger to use with iSun

Discussion in 'General Electronics' started by Andrew, Dec 4, 2003.

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  1. Andrew

    Andrew Guest

    I am thinking of building a battery charging pack to use with iSun
    Solar Charger.
    It will charge at least 3 AA NiMH batteries (the more the better, what
    ever makes sense). I will use the Energizer AA NiMH 2100mAh
    rechargeable batteries.
    Since I am an electronics/rechargeable batteries newbie, I would like
    to ask for some pointers on how to construct an appropriate circuit to
    safely charge the batteries, without damaging them or the iSun.
    ISuns peak output is: [email protected] or 145 [email protected] .
    I suspect in imperfect world I would never get that anyway.

    I am thinking about putting 4 AA 1.25v batteries in series and maybe
    putting a diode to protect against revers current flow. What diode
    would you recommend?

    How do I ensure that I have appropriate voltage across the batteries?
    I guess I need about 4*1.25 + (about 0.2V to maintain voltage
    difference when they are fully charged). That would mean I would need
    to ensure that the voltage never gets above 5.2V.
    The current: if we follow C/10 that would mean I need about 210mA.
    Again, I would need to ensure it does not get any higher then 210mA.
    Or is it too high for NiMH batteries already?
    How can I limit the current and voltage? What would you recommend?
    Is there a way of simulating the solar charging of AA NiMH batteries
    using software like CircuitMakter, Multisim or others?
    Thank you for your help,
  2. Ian Stirling

    Ian Stirling Guest

    1n4001 is cheap and will work fine.
    290mA forever is not good for 2100mAh cells.
    However, it's not too bad, and unless you leave the cells in for
    weeks in full desert sun pointing the panel directly at the sun there
    will not be a problem.

    Set the panel up to 7.6V, connect a diode in series, and you can
    safely charge from 1-4 cells.
    In theory, you might just charge it in one day in the desert if you keep
    the panel pointed at the sun.
    In practice, 2 sunny days in most places, or more cloudy.
    Put the cells in the cool away from the cells.
    Don't leave the cells in for more than a few days at a time.
  3. Use any diode capable of handleing the current, with max curent output of
    290mA and 4 cells NiMh 2,1 Ah. no ekstra circutry are necessary.
  4. Andrew

    Andrew Guest

    Thank you for the replies.
    I thought I'll take this opportunity to learn something about
    electronic circuits. So I have read a little and found a wonderful
    component called LM317.
    I thought – it would be great if I can limit the voltage and ensure a
    limited constant current. Here is what I have come up with:

    Fig 1. Constant Current (~210mA) and regulated Voltage (~6V)

    | LM317 | 6
    *---------*-------|IN OUT|---/\/\/\--*------*---------*
    | | | | | | |
    | | | COM | \ | |
    | | ----------- / 265 | |
    | Solar | | \ | |
    +| Panel | 1uF | / | 1uF |+
    --- --- | | --- --- 4AA
    - 7.6V --- *-----------------* --- - NiMH
    --- | | | ---
    - | \ | -
    | | / 1k | |
    | | \ | |
    | | / | |
    | | | | |

    Fig 2. Constant Current (~210mA) only

    | LM317 | 6
    *---------*-------|IN OUT|---/\/\/\--*------*---------*
    | | | | | | |
    | | | COM | | | |
    | | ----------- | | |
    | Solar | | | | |
    +| Panel | 1uF | | | 1uF |+
    --- --- | | --- --- 4AA
    - 7.6V --- *-----------------* --- - NiMH
    --- | | ---
    - | | -
    | | | |
    | | | |
    | | | |
    | | | |

    I also realised that putting extra circuitry, especially diode wastes
    the Solar panel power. Therefore I am inclined to connect the
    batteries in series directly to iSun.
    Apparently iSun already has a reverse current protection so I do not
    need a diode.
    Don't have to worry about voltage drop caused by adding a diode.
    It probably does not make sense to limit the current to 210mA. There
    is not much difference between 290mA and 210mA. As long as I limit the
    time the batteries are being charged it should be OK.
    So the remaining questions are:

    1) What is the minimum and maximum voltage I can charge the 4 AA
    batteries with? Can I charge them with iSun set to [email protected]? Maybe
    15V iSun setting and then reducing the voltage to 7V or 8V is more
    efficient.? If there is not enough sunlight and the solar panel drops
    to 10V or less the batteries will still get charged. Do I need extra
    circuitry to regulate/limit the voltage or can 4 AA batteries take it?

    2) Can I charge 1 or 2 AA batteries connected to 7.6V/290mA ?

    3) What if the batteries have some residual charge and they just need
    a top-up? How will I know when they are fully charged and I should
    take them out?

    4) What if some batteries in the pack have different residual charge?
    Some are fully discharged and some need just a top-up, is this a

    Thank you for your help,
  5. mike

    mike Guest

    I've failed at guessing what you're trying to do.
    Sounds like you want to plug in random cells with
    varying state of charge and charge them in the sun??
    You don't have much choice but to use constant current
    if you want to keep it simple.
    First thing to do is to plot the current vs voltage
    for the solar panel at varying light levels.
    You may discover that under normal circumstances
    the current output is way less than you expect.
    When I tried that experiment, I just gave up cause
    there wasn't enough output from my panel to charge
    anything unless it was noon in June. The slightest
    angle to incident made a dramatic difference in output.

    There's no substitute for nor better learning tool
    than real measured data.

    A light bulb is a PTC resistor that can be used to
    advantage to limit charging current to something
    the cells can stand.

    Another low-tech solution is to use a lead-acid battery
    charged from the sun. Now you have a pretty constant
    source of juice to charge the NiMH any way you want.
    Clamp the lead battery at 14V (or 7V) so it can't be overcharged.
    Then you can use a commercial fast charger that will
    optimally charge and protect your NiMH. Or you can build
    one similar.

    A charge pump is another interesting method to transfer
    charge from one battery to another.

    If you want efficiency, your problems multiply.


    Bunch of stuff For Sale and Wanted at the link below.
    Toshiba & Compaq LiIon Batteries, Test Equipment
    Honda CB-125S
    TEK Sampling Sweep Plugin and RM564
    Tek 2465 $800, ham radio, 30pS pulser
    Tektronix Concept Books, spot welding head...
  6. N. Thornton

    N. Thornton Guest

    15v or 7.6v is your peak output V. Much of the time you wont get even
    close to that out. So if you charge a 6v pack on a peak 7.6v solar
    panel you'll get no charge at all for much of the time. Using the 15v
    output will get things charging, and also provide current limiting.

    The batteries and solar cell combination will limit the voltage
    already. Solar cells have significant source resistance, and batteries
    have a zener like characteristic. So that already covered.

    And the solar cells will also limit the current, theres only so much
    they can put out.

    right ok. Diode already built in.
    It would be wise to look at the charge current requirement for your
    cells, make sure youre not going to nuke them or blow them up, or

    minimum is going to be a bit above their nominal voltage. Theres no
    maximum since your source is i limited.
    right, no reg needed tho.
    yes if they can take the charge current, if not no. Its better to
    charge battery packs as a complete pack though, to avoid mixing
    chagred and half charged cells.

    Yes: Dont divide the pack. If your appliance uses 4 AA cells, charge
    that 4 cells as one unit.

    Regards, NT
  7. Harshana

    Harshana Guest


    Just two weeks back I made a NiCad charger (AA 1.25v 650mAh x 2) with
    a LM317, like what Andrew has drawn. But there was a small difference.
    I had a diode in series in the LM317 o/p (otherwise battery
    dischages). Then I though diode is no good for a charger, cause its
    I/V characteristics. So I put 50 Ohm resistor also in sereis.

    I had two batteries in series. These are my observations:
    1. Batteries had starting voltage of .7V each.
    2. I was pumping around 65mA (I was monitoring the current and varing
    the LM317 o/p voltage so I get this current all the time)
    3. If I don't increase the o/p voltage of LM317, charging current
    drops below 10mA.
    4. Within minutes battery votage increased upto 1.4v each (2.8 both)
    and it stayed there (reason for 3).

    So I figured this is some sort of stable stage for about 10hour
    charge. But with this cct configuration, when starting up from very
    low battery voltages, it would take lot of current which might damage
    the batteries.

    That's why I thought I should put a little R in series so it gives
    desired I/V characteristics (for larger V change small change in I)
    this is actually the opposite of a diode I/V and can be easily obtain
    with high R. but then U run into the problem high R means U desipate
    lot of W in the R itself and Ur i/p should be kept high.

  8. Bill Cotton

    Bill Cotton Guest

    I goggled "Charge pump" and found many varation. I am interesting in
    fast recharging my lead acid lawn mower battery from an external
    battery safty. My current plan is to use small wire size to limit the
    current. I also have a stand-by AC power system, using an inverter,
    for my home and plans to connect the deep cycle battery to my car
    battery for recharging, using the same small wire.
    Where can I get more information about the charge pump you mention?
  9. mike

    mike Guest

    You just need a couple of switches and diodes. Charge a cap from the
    source battery and dump it into the load battery. Probably need an
    inductor to limit peak current. C, delta-V and rep rate define the
    average current. It's not a lot different from a standard switching supply.

    Bunch of stuff For Sale and Wanted at the link below.
    Toshiba & Compaq LiIon Batteries, Test Equipment
    Honda CB-125S
    TEK Sampling Sweep Plugin and RM564
    Tek 2465 $800, ham radio, 30pS pulser
    Tektronix Concept Books, spot welding head...
  10. David Wood

    David Wood Guest

  11. Steve Spence

    Steve Spence Guest

    for $20 I bought a NiCd/NiMH charger from Walmart, that included 4 AA NiMH
    batteries. It has a 12vdc as well as a 120vac interface, so I just plugged
    the 12vdc interface into my 15 watt solar panel with a ups battery as a

    Steve Spence
    Renewable energy and sustainable living
    Donate $30 or more to Green Trust, and receive
    a copy of Joshua Tickell's "From the Fryer to
    the Fuel Tank", the premier documentary of
    biodiesel and vegetable oil powered diesels.
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