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My first array

Discussion in 'Photovoltaics' started by John Hastings, Mar 26, 2008.

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  1. Just some quick calcs off the top, so don't get to heavy into

    I picked up two cheap and large surplus solar cell arrays that are about 3
    by 7ft each. The label says that each is 190 watts. Being an industry
    "EPA" type label, I am sure that is for high noon, totally clear day, far
    enough south that the sun is directly over head.

    190 watts would convert to about 1.55 amps at 120 volts, if this thing
    actually had that potential. Actually, leaning up against a rail and sort
    of aimed at the sun, I measure about 26 volts. Anyway, that converts to
    about 15 amps at 12 volts. Twice that amperage for both arrays in

    Since I plan to use it to help charge a 12 volt battery array, the voltage
    doesn't come into play since the battery would look like a direct short to
    the solar cells. It is the current that I want.

    My draw from my battery array (actually, it is two golf cart 6v batteries
    feeding a high efficency inverter) is about 3 amps on the average so these
    cells should go a long way in keeping the array charged. With a possible
    30 amp charge rate, tapering up from the morning and back to zero in the
    evening, I shouldn't even need a smart charger to keep batteries from
    being overcharged.

    However, all of my solar knowledge at this point is totally without
    practical experience. Anybody want to shoot holes in the above?


  2. Guest

    Here's a page that describes how to test modules.
    Be careful about going without a charge controller. 3A for 24 hrs at
    12V, plus inefficiencies, might be about 1kWh per day. Production of
    380W for 5 hrs might be double your consumption. Although you can
    probably expect a lot less, especially if the modules turn out to be
    24V. Simple charge controllers are relatively cheap MPPT controllers to match
    higher voltage arrays to lower voltage batteries are more expensive
  3. O
    That is a good link. Didn't have the assortment of power resistors in
    stock, but I whomped up an LM317 regulator as an infinitly adjustable load.

    Unfortunately, today in East Texas it is fairly cloudy, but at optimum
    load, one panel would dump about 3 amps into an ordinary 12v car battery.
    I don't have an electrolite tester at this location, so I can't tell what
    state the battery was in when I started, except for the rest voltage of
    12.1 volts.

    By the time the sun is supposed to come out in a couple of days, I will
    have the proper stuff here to do real tests.

  4. David French

    David French Guest


    Ooops - you forgot to 'square' the current (ohms law)!

    W = I^2 * R.

    so W = 10A*10A*0.1R = 10 Watts.

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