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MPPT controller Open Circuit Voltage calculation

D

davelondon

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I'm planning to use three of these 550W JA Solar panels in series:


There's a couple of different Open Circuit Voltages listed on the spec sheet - "STC" and "NOCT":

STC: 49.9V (149.7V for three in series)
NOCT: 46.68V (139.44V for three in series).

I had imagined a 150V MPPT controller would be fine, but looking in the Victron spec sheet, they say 145V max for "start-up and operating maximum":


Should I be using STC or NOCT for the calculation? Is it good practise to have some headroom in the maximum voltage? Or can you take it right up to the maximum with no extra wear on the MPPT controller? Do I need to step up to a 250V MPPT controller for this array?
 
'NOCT' is their test setup for conditions of lower light, slightly lower temperature, and a slight hint of wind. I'd definitely size for STC - especially if your expedition vehicle will ever be located at high altitude without any shade, in cool conditions. With higher temperatures V(oc) goes down within these panels, but it doesn't go down very fast.

I've read that the Victron product line can survive at least 20-30% more V(oc) than the rated 145 Start-Up Volts, but that they respond by temporarily shutting down and refusing to restart until V(oc) is back below 145 Volts. If you lived in the Western USA Desert, as I do, you'd possibly be losing the best parts of many great days. But if you can count on a bit of shade, your Voltage (and output) will be greatly reduced - and you'll be fine. And maybe on those "perfect" days, you won't even need the most perfect hour, with the AM and PM providing more than enough power to reach100% SOC?
 
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