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should amperage on back of 24v panel be doubled to choose charge controller size

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  • should amperage on back of 24v panel be doubled to choose charge controller size

    When it comes to choosing charge controllers for 12v solar panel arrays this is basically straight forward, you just add up the amps of the panels and make sure the charge controller is higher . However i seem to be a little confused when it comes to selecting controllers for 24v panels. Does the amperage displayed at the back of the panel reflect the TOTAL amps being passed to each battery attached to the solar panel (eg if it says 7.8A then the panel is only producing 7.8 amps in total so if 2 batteries are connected they are only getting 3.4amps each) so a charge ocntroller of say 10amps @24v would work (theoretically) or does it mean that it is supplying 7.8 Amps to each battery ( ie the panel is really producing a total of 15.6 amps) so a charge controller of atleast 20amps @ 24v is required

  • #2
    NO!

    Stop right there.

    If you use PWM controller, very stupid to do so, Yes you just add up the current and make sure the controller rating is equal or higher than the total panel current. Makes no difference what the battery voltage is. For each 12 volts of battery; you need no less than 16 volts and no more than 18 volts. This forces you to use very expensive 12 volt 36 cell battery panels, or 24 volt 72 cell battery panels. So say you want 300 watts, you buy 2-150 watt 12 volt battery panels and wire them in series. That will give a total panel Vmp = 36 volts and an Imp = 8.3 amps. You would need a 10 amp PWM controller. Cost is roughly $600 to $800 for panels and controller. But you would be a damn fool to do that. PWM is history along with expensive battery panels.

    PWM Input Current = Output Current. or panel current = battery current.

    Smart educated people use high voltage grid tied panels and MPPT controller. You still need a minimum of 16 volts per 12 volts battery, but you can go as high as 125 volts. Higher the voltage the. better within limits of the controller Voc upper limit. Example buy just a single 200 watt 72 cell panel which will have a Vmp 36 volt and Imp = 5.6 amps. However with MPPT you do not use the panel current, you use the Controller Output current.

    MPPT Output Current = Panel Wattage / Battery Voltage.

    So using the example above: 200 watts / 24 volts = 8.3 amps.

    A 200 watt MPPT system will cost you roughly $300 to $500.

    Did you catch any of that? It takes a 300 watt $800 PWM system to equal a 200 watt $500 MPPT System.

    Do not cheap out on a MPPT controller. A good and real 15 amp MPPT model will cost you $200. Get over it because it is cheaper than a $25 PWM controller.Figure it out.
    MSEE, PE

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    • #3
      The battery shouldn't affect the charge controller amperage, just the voltage (a 24 V battery needs a 24 V CC).

      If you are talking PWM charge controllers, the amps on the solar panel multiplied by the number of strings of panels in parallel is the minimum required amp rating of the charge controller.

      If you are talking MPPT charge controllers, take the total power of the array (DC Watts), divide by the battery voltage, and that is roughly the minimum required amp rating of the charge controller.

      In some cases, you might be able to get away with a slightly smaller CC (for example, if the calculation comes out to 31 A, a 30 A charge controller might be OK).


      What happens to the current when it flows to the battery?

      If you have batteries in parallel, the current is divided between them. With a pair of batteries, a maxed out 10 A charge controller would deliver ~5 A to each battery. With three batteries in parallel, the 10 A becomes 3.33 A to each. This is part of why parallel batteries are a bad idea... if the resistances aren't exactly equal, the current won't divide up exactly equally, and one of the batteries will get worked harder than the others.

      If you have batteries only in series, then each of the batteries receives the same current, 10 A in this example.

      CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

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