Panel and Load Voltage Mismatch

dherscher
dherscher Registered Users Posts: 17 ✭✭
in General Solar Power Topics #1
I received a 260W, 24V panel as a gift. Unfortunately, all the loads I would conceivably power with this panel are 12V. 
One is a brush-less motor that I would power directly from the panel if I could find a way to step down the voltage. The other would involve a battery bank (lighting for nightime). 
Suggestions? Would an MPPT controller work for this?
Thanks!
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    It depends on what your loads are and when you need them... Only during sunny days, or in dark/stormy weather. Obviously, batteries for non-sunny times... But batteries add cost, complexity, and battery replacement every 3-5 years (for less expensive Golf Cart deep cycle batteries in warmer climates).

    For smaller daylight loads--You could look for "buck mode" (step down) switching power supplies... They will take the variable high voltage/lower current and efficiently step them down to 12 volts @ whatever power the panel is outputting (say you draw a maximum of 120 Watts---Then 120W/12V=10 amps for your 12 volt loads). Solar panels do not have surge capability or the ability to supply more power than there is sun available. Generally, for a daylight only operation, you probably want the panel ~2x the Wattage of the loads you want to "somewhat reliably" want to run during the day.

    There are literally "tons" of these devices out there (different inputs/outputs) and some latch off when power fails, some auto restart... Some have meters, some are adjustable, etc., on Amazon, EBay, and Chinese Vendor websites.

    https://www.amazon.com/Aideepen-Constant-Current-Adjustable-Converter/dp/B0747QDRW9

    They are not battery chargers... If you wanted to charge a 12 volt battery bank, then a real MPPT type charge controller would be what you need. And these are all over the place in price and quality (and a fair amount of "fake MPPT controller" are there too).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • dherscher
    dherscher Registered Users Posts: 17 ✭✭
    #3
    Thanks Bill. The daytime PV direct load I would like to power is a 12V brush-less motor to circulate water during the winter. This "buck mode" switching power supply sounds like the sort of thing I could use for that. 
    During the summer I would re-purpose the panel to power some lights and I know that would require a battery, which I have several 2nd life deep cycle 12V batteries that I could sacrifice to the cause.
    I've just been trying to figure out how to overcome the mismatched voltage thing.  
    An MPPT charge controller will step the panel voltage down to charge the batteries? If so, is it possible I could use one for both applications and simply plug the motor into the battery ports of the controller? Or do I need to buy a separate Switching power supply and MPPT controller?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #4
    An mppt controller is perfect for the battery+load on a battery bank.

    In general, mppt solar charge controllers need a battery to turn on and operate correctly.

    There is an optimum size for the battery bank. It runs around 5% to 13% (and as high as 20%) rate of charge from the solar array to the battery bank:

    260 Watt array * 0.77 panel+controller deratings * 1/14.5 volts charging * 1/0.05 rate of charge = 276 AH @ 12 volts maximum
    260 Watt array * 0.77 panel+controller deratings * 1/14.5 volts charging * 1/0.10 rate of charge = 138 AH @ 12 volts nominal
    260 Watt array * 0.77 panel+controller deratings * 1/14.5 volts charging * 1/0.13 rate of charge = 106 AH @ 12 volts cost effective minimum
    260 Watt array * 0.77 panel+controller deratings * 1/14.5 volts charging * 1/0.20 rate of charge = 69 AH @ 12 volts minimum functional

    5% rate of charge is the minimum solar array for a 276 AH battery bank (weekend/bacup/sunny weather operation).

    10% rate of charge is the "nominal/optimal" suggested maximum 138 AH battery suggested for full time off grid operation.

    The 13-20% rate of charge is the smaller suggested AH capacity for a 260 Watt array.

    You could also use a PWM controller rather than a MPPT controller. PWM controllers are much less expensive (you need a 12/24 volt capable PWM charge controller)... But it will only harvest about 1/2 of the solar panel energy vs the 100% an MPPT controller can (and will support 1/2 the size of batteries suggested in the above calculations). PWM controllers cannot "match" the Vmp of ~30-36 volts of the 260 WAtt panel to the "12 volt" battery bank.

    So, it depends on how much energy you will need (a smaller amount from cheaper PWM charge controller), or you want the maximum possible (MPPT, a more expensive charge controller).

    If you are not planning on using a bunch of power every day, a pair of 6 volt @ ~200 AH "golf cart" batteries in series would not be a bad fit for a system that you want to play with... Otherwise, a 12 volt @ ~138 AH or smaller battery bank (Flooded Cell Lead Acid) would be better for a system that you want to cycle more deeply every day.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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