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Our current set up on our boat is 2x30W panels wired into one PWM charge controller and one 70W panel wired into a separate PWM charge controller. I'm away from the boat at the moment but am assuming the two smaller panels are wired in parallel. 12V system on board.
The new set up will be 2 x 60W and 1 x 55W mono-crystaline panels with 0.08A difference.
Assuming new panels all wired in parallel, do I need one MPPT controller or two? I = 3.22 and 3.30A respectively. If so, what size?
With PWM controllers, you must use the correct "nominal" panels in parallel, i.e., for a 12V system, you need 12V panels... which range from about 17-22V, but force the panels to operate at battery voltage losing about 20-30% of potential power.
With MPPT controllers, they allow the panels to operate at their optimal power voltage - typically 17-19V for a 12V panel - and then converts the higher PV voltage to the lower battery voltage.
Since you're wiring them in parallel, their current doesn't matter. Their Vmp does. Your array will experience a % loss roughly equal to the % difference in Vmp. Each panel will output whatever current it can, and in parallel, those currents will add to each other.
If you place the panels in series rather than parallel, then current matters. In this case, the higher current panel will be forced to operate at the lower current panel. 3.22A vs. 3.30A is only a 2.5% penalty. Personally, I regard this as acceptable and would not want to add a second MPPT for that 2.5%.
At 175W of total power, you would only need a 175W/14.5V = 12A MPPT. The Victron 75/15 would be more than adequate.
Thanks for the quick response. That's very helpful. The Vmp difference is 17.10 v 18.20 so around 6%. We will go with one 75/15 as you suggest.
