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.
Those 75.6 Voc panels will bolt-in an early start feeding 12V batts. Solar panels are both weird & wonderful things. At first light they'll approach that Voc quite quickly with no current extracted. The mppt will detect that and attempt a start. If there's no A behind that V, the attempt will 'sag' that V heavily (mine go down to about half full Voc)
The +5V issue really should only impact those using panels with Voc < 2x Vbat. Yours well exceed that, and it's a non-issue.
Indeed, in your situation on a yacht, shading is likely to be a far more serious issue with spars, sails, wires (yes even wires). Paralleled panels will suffer far less from shade than a seriesed string.
The absolute optimum for you might well be a single 100/30 mppt on each panel. Each totally independent of the other shade-wise. Cost em out..
The +5V issue is really of no concern to you. Trust me? :)
But keep asking.. all good..
Thanks John.
Yes, I’ve looked at using three 100/30s; one for each panel. Or 100/50s to avoid the chance of current limiting.
I’m waiting on advice from SunPower re partial shading. To what extent does partial shading of some cells on one panel affect that panel? If wired in series, is there any effect from this on the string of three?
My understanding is that voltage is only reduced in proportion to the extent of the shading. Or is it worse than that?
Those are high-end panels, possibly the best on the market. Expensive, but purportedly as good as any with shade tolerance within the panel. Strung in series though, one shaded panel will likely still bring down the others to it's level. So all get reduced in output.
Panels can do strange things with V, but in the end it's Amps/Watts that deliver the grunt. My own (lesser) panels have a strung Voc of ~120V, and in poor light/shade the tracked Vmp can drop to about half that. Getting close to my 48V charge level, but still finds what W is there (minimal by then anyway).
Extrapolating that to yours, maybe 35Vmp *might* be reached, but it's still well above the charge V of your batts. Paralleled panels would work for you.
3x 100/30's would work fine too. No point going bigger, the 30A equates to 13.3V under charge, and you'll almost always be above that under charge. /50's would be a waste of money.
The independent mppt's would work even better than panels paralleled into a single. There the tracked Vmp is a compromise, it'll come close, but not perfect.
Thanks John.
I'm in touch with a user of three of the 360W versions of these SunPower panels. He has them set up just as you suggest, independently, with three separate controllers. He's delighted with the performance. Three 100/30s would indeed cost a little less than a single 250/100. I must admit it would be nice to save a few bucks. There's not much of that happenin' roun' here at the moment. :-)
