I have a mppt 250/85 and currently have two strings of 4 panels in parallel. The panels produce 10.7 A Isc and 52.9voc for each panel, so all in all 21.4 Isc and 211.6voc. The current arrays are SW facing. I plan to add 8 more panels in the same configuration of 21.4Isc and 211.6voc but in a SE orientation.
Is it possible to connect the new SE array onto my existing SW array making a maximum Isc osf 42.8 Amps at 211v oc? Using my mppt 250/85 ? Thats 4x 4 panel strings as 4 in parallel.
The 250/85 has a maximum Isc of 70A per the data sheet, so yes.
You absolutely need a combiner box with fuses or DC breakers because you will be running a 4s4p configuration. This isn’t needed for 4s2p configuration you have now. Basically this is because a short in one string will take 3x the rated current from the other panels and could catch fire. This assumes you have the terminal version of the 250/85.
You also need to make sure your cabling is up to the task if everything goes down one cable. At least AWG 6 in the usa (maybe even 4), and 16 or 25 mm^2 elsewhere.
If you have the MC4 version of the 250/85, you must use two separate feeds because the MC4s are only rated for 30A each. Then you can use 2x AWG 10 or 8 if in usa. But you still need DC fuses or breakers.
That said, the 250/85 can only put out 85A of battery charge current, about 4900W assuming you have a 48VDC battery. You didn’t say what wattage your panels are rated for, but multiply that by 16 and compare to 4900W. Over-panelling is fine if you are expecting it. SE/SW facing will keep it from being too overpaneled.
Thank you for your good information. I have indeed fused each string with 10a fuses and two strings (the SE arrays) are combined to a single 6mm pv cable run to the plant room, and the other two strings (SW arrays) are on two separate roofs and each have 6mm pv cable going to the plant room.
In the plant room i have 3 cable runs from the 4 strings which each have a breaker, the output side of the 3 breakers are combined to the mppt. Forgot to say that the panels are rated at 415w each, which would be 6640w in total, i know this is more than the charge controller can handle, so will the charge controller simply ignore the extra available power? And being they are SE and SW orientation, all 4 strings will never have full sunshine on them anyway.
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OK with 6mm^2 PV wire. that’s good.
Yes, you will never see the 250/85 provide more than 85A of battery amps, it self limits.
What that works out to in PV wattage depends on the battery voltage, but you’ll never get to 4900W.
With my 250/100 over-paneled, I would typically see 5300W as my battery tends to be near 53V. With your 250/85, I would expect around 4500W. Keep in mind the mppts get hot when running at 100% capacity. I put a small quiet fan under mine.
Your SE/SW use case is exactly why people like to over-panel. Your solar day will be extended earlier in the morning and later in the day. At noon you will probably hit the limit of the MPPT, but that’s ok. You’ll see it in VRM or Victron Connect as a flat part of the PV production.
Great, thanks. Oh and yes my battery is also a 48v system, my batteries are ususlly around 54v and 52v when low soc. I dont mind the fact that the mppt cant give as much as the panels can produce as the new SE array is really only there for adding extra power for winter and cloudy days, the rest of my panels are more than enough for the summer months.
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