Hooking up two panels with different amperages in series

gkerlin
gkerlin Solar Expert Posts: 27 ✭
in Solar Beginners Corner #1
Install is on a 5th Wheel RV 12v System with 4 Trojan T-125 6v 240ah each. Tristar TS-MPPT-60 Controller

I have 3 UniSolar ePVL-144 panels (144w 33v 4.4a MC4 connectors) Plus 3 UniSolar PVL-68 (68w 16.5v 4.1a MC3 Connectors)

Numbers are rounded to simplify.

Due to space limitations I'm unable to put 4 of the 144's on the roof so I've opted for the following:

wire a 144w and 68w in series because they are different voltages. The 144 is 33v and 68 is 16.5.

That gives me a voltage of 49.5v per string. I would have 3 of these strings and they would be wired together in parallel.

49.5v is supposedly ideal for this MPPPT controller to reduce to 12v. That is the sweet spot for efficiency (35v - 50v)

The higher voltage also allows me to use smaller wire. I can use regular #10 pv wire down to the controller.

The 144w is 4.4 amp and the 68 is 4.1w amp

So.. If I wire a 144 and 68 in series the voltage combines to 49.5 but the amp would be the lower of the two (4.1 amp) as I understand it. This means that my 144 would really have the output of a 136w.

The 3 series strings (144w+68w panel each string) would be wired in parallel. That means 49.5v at 4.1 amp

The way that I see it I would have 3 stings of 136w + 68w for a total of 612w. I would lose about 8w from each of the 3 144w panels due to the mis matched amp rating of the 144 and 68. Is all of this correct? any issues thus far?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Hooking up two panels with different amperages in series

    Welcome to the forum.

    The current on these two panels is so close that I wouldn't worry about it at all. Yes, the lower current panel will limit the output of the higher one. But it's about a 10% difference.
    Also since this is feeding an MPPT controller the numbers it picks may not be the panels' rated output; it will take whatever Voltage * current it thinks is best to provide optimum charging.

    So you will have three parallel strings, each with a 144 Watt and a 68 Watt panel. As far as the TriStar is concerned you will have 636 Watts from which it will be able to produce around 40 Amps of current peak. You can look at that as 480 Watts. It might do better. There are bigger variables here than 0.3 Amps per panel.

    For those who like math: 49.5 Volts * 4.1 Amps = 202.95 Watts * 3 = 608.85 Watts for the whole array.
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