looking for a unit that could monitor solar panel potential power

MarkNewYork
MarkNewYork Registered Users Posts: 20 ✭✭
in Solar Beginners Corner #1
I have a 20 amp MPPT controller, 2-100 watt panels (in series) and a 100AH battery. Not grid tied. Wondering if there's some sort of unit I could hook to the system that would detect available unused power from the panels. If the battery is fully charged and the sun is out I'd like something that could switch on some load and put any available solar power to use! I don't want to tie into the grid at this point, just put 2-3 amps of 12v DC equipment - maybe more - on solar when there's extra power to be had. The unit would ideally provide a contact closure to switch on a relay. When the battery is charging the system can feed about 8 amps to the battery but I'd like to still put that 8 amps to use when the battery is full!
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  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    #2
    When panel voltage is above Vmp, excess power is available.    Of course Vmp isn't a fixed number, but this technique will probably be good enough.

    I am available for custom hardware/firmware development

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  • NANOcontrol
    NANOcontrol Registered Users Posts: 276 ✭✭✭
    #3
    This is just what a small microprocessor is for like the arduino. Panel and battery voltage can be monitored as well as temperature.  If you can say what you would do if you were watching the instrumentation, a program could be written. A 15 year old could do it. Most people are too old and fear drives their decisions not to try. A $3 board and FREE compiler, everyone wastes that much money before noon.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,641 admin
    #4
    Get a smaller solar panel (just a few Watts) that is sealed/designed for outdoor use.

    Connect it to a small power resistor "shunt". If the output voltage of the solar panel is near zero volts (near dead short across the shunt resistor), the resulting current is pretty close to the actual amount of sun hitting the solar panel (i.e., 0 amps = 0% sun, 50% amps = 50% sun, 100% amps = 100% sun).

    Use a small single board computer (or other device) to measure the voltage drop across the shunt resistor (Voltage = Current * Resistance).

    For example, say you have a 0.5 amp Imp/Isc panel and want a maximum of 1 volt across the resistor (panel voltage does not really matter as long as >~6 volts Vmp):

    R=V/I= 1.0 volts / 0.5 amp in full sun ~2 Ohm resistor
    P=I^2*R = 0.5a^2 * 2 Ohms = 0.5 Watt minimum rated resistor

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