mppt parallel pv input with different voltages

solarking
solarking Registered Users Posts: 67 ✭✭
in Solar Beginners Corner #1
What happens if we connect a 12v 100 watt panel in parallel to a 48v(80Voc) 1000 watt array which is fed into  a MPPT(48v batterybank). Is it useless?
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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #2
    I suspect the 12v panel sees 80v and yes, becomes useless.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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  • Nila
    Nila Solar Expert Posts: 173 ✭✭
    #3
    Please describe the specs of the 1000 watt array , may be since this is a MPPT controller you can try to series this tiny panel in if the specs allow?

    You cannot parallel different voltages that too in this big difference 80VOC to 17 the smaller panel may get killed

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  • Gene.243
    Gene.243 Solar Expert Posts: 35 ✭✭
    #4
    If the diode on the 12 V panel survives the 48V applied against it nothing happens.  The 12V panel will never contribute to the system.  If the diode fails to a short then the battery dumps through the 12V wiring until something melts.  That current would be limited by any fuse in series with that part of the circuit.  If the diode fails open (luckily) nothing else happens.  You only loose the diode on the 12V panel.    There is no good way to do it. 
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  • Nila
    Nila Solar Expert Posts: 173 ✭✭
    #5
    @Gene.243 what would happen if he adds this tiny panel in series ? Still gets killed?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #6
    Back to basics... For series panel connections, the panels need to have Imp withing 10% of each other for "near optimum" operation. Within 5% is even better.

    For parallel connected panels/series strings, the Vmp (of the paralleled single panels in parallel or strings of panels in parallel) needs to be withing 5% to 10% between high and low Vmp-strings.

    Consivably, if you have current miss-match, if you have an 8 amp panel in series with a 3 amp panel--Either 3 amps will flow, or the other panel with "overwhelm" the smaller panel with 8 amps. There will be no power added by the smaller panel, and, very possibly, you will ruin the smaller panel and even possibly start a fire (8 amps in a 3 amp panel).

    Voltage wise--Small miss-match, the lower voltage string will produce less power (if the MPPT controller tracks the higher Vmp-string voltage)--Or the MPPT controller may track the slower Vmp-string voltage--And the higher voltage string will produce full current, at the lower voltage. Not as bad--But how will each MPPT controller "track" the "right" voltage (you can have 2-3 peaks with just two different string voltages).

    Very high voltage difference between strings--Just like any reversed biased diode, high enough voltage will ruin the diode junction(s) in some cells.

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