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When I plug details for 3 x SunPower Maxeon 3 400W panels in series for a 12V battery bank into the MPPT calculator, the only recommendation is for the 250/85, which the calculations indicate will be overpowered at max and min temperatures. Why isn't the 250/100 being recommended? Is it included in the calculator? Panel parameters are: 400W, 75.6Voc, 6.58A Isc, 65.8Vmpp, 6.08A Impp, V temp coeff 0.27%/C, I temp coeff 0.04%/C, 12V system. What are the calculation results if a 250/100 is used?
Hi Valden.
The calculator won't be using a nominal 12V to work it out. Typical charge V's are higher than that. I don't know what V it would normally use, but 14.1V matches a 250/85 with 1200W of panel. Rarely would you see 1200W from those panels, so the recommendation is fine.
The 'temp coeff' would be used only to check the strung Voc is lower than 250V. Nothing to do with panel output W.
No reason you couldn't use a 250/100 if you chose.
And if you wired those panels in parallel, rather than series, a 150/85 would do too. All would work equally well into 12V batts.
Hi John. Thanks for this explanation of the calculator's workings.
Let me explore this question of series vs parallel connection of the panels (in this case only three in number because they'll be on the stern arch of a yacht).
The MPPT charge controllers I've been considering all have a charging initiation voltage of battery voltage + 5 volts. So, using 12V to keep things simple, 12V + 5V = 17V needs to be sensed as an input by the charge controller before it starts to send charge to the battery.
Let's say the light level is quite low and a single panel can't produce enough voltage to initiate charging. However, if three panels are in series, the summed voltage could easily be over the charge controller's threshold. I plan to take my yacht to places with low light levels and I think this series connection approach will help. Charging can start with one third as much light.
John, your reply prompts a question. Would using a 150/85 with the three panels connected to it in parallel somehow get around this problem of each individual panel not producing enough volts to get over the MPPT's working threshold? I think not, but I'd be happy to hear otherwise.
