Need Help with Maybe Adding 2 Solar Panels

[SolarHead]

I have read and heard where if you add more panels on a string, that the voltage/amp specs have to be very close or exact and if not, then the entire string can suffer or be de-rated down to what the lowest powered panel(s) is/are on that string. Not sure if I'm right about that. Anyway, what if I add a brand new separate string but bring it into my combiner box? My combiner box has room for more fuse holders, and can handle up to 1,000 Volts. Currently each combiner box is 367V and 15A total (this is 18 solar panels within one combiner box). The 2 that I want to add, I would have them on their own string, a DC disconnect, and bring the wires over into same combiner box. The 2 new panels would not affect the other strings would they? Since the 2 new panels would be facing West or SouthWest, I imagine they'd kind of power up later in the day while the other 36 are starting to decrease in output. I'm sure I'm not thinking of everything here and is why I'm asking for help. Since the 2 new panels are on their own string, they can be of any brand and specs correct? I dont want to over-power my MPPT either. I think one combiner box is currently sending (at full PV output) 5,505 watts if my math and logic is correct. My inverter (Sol-Ark 12K) has two MPPT and each can handle up to 500V and I think 6500W (need to verify). So, I am thinking it would be safe to be able to add 2 new panels (maybe 365w each), face them West or SouthWest and not cause any problems with my system. The output I think would decrease on the 36 panels as the 2 panels would be increasing around 4pm until dark, so I think the output is going to "balance" somewhat and not be just adding more total power. I'm aiming for longer lasting power , not more power.

 

[sunshine_eggo]

I have read and heard where if you add more panels on a string, that the voltage/amp specs have to be very close or exact and if not, then the entire string can suffer or be de-rated down to what the lowest powered panel(s) is/are on that string. Not sure if I'm right about that.

Pretty sure you heard that from me in one or more of your other threads.

Anyway, what if I add a brand new separate string but bring it into my combiner box?

You're still on the same MPPT input. It still has to be the same Vmp-Voc range as the other strings it's in parallel with. Since the same pair of wires are exiting the combiner, all inputs need to be in the same Vmp-Voc range.

Not 100% certain, but pretty sure you'd destroy the 2S string. Since it's MUCH lower voltage than the 9S2P array it's in parallel with, the 9S2P array would see the 2S array as a short circuit and dump all the 9S2P output THROUGH the 2S array. Best case is blown fuse/breaker. Worst case is fried 2S array.

If you plan to add strings to your existing MPPT inputs, your new strings need to have a compatible Vmp/Voc range as the existing.

Otherwise, you need another MPPT.

 

[SolarHead]

Okay that answers me question. I was wondering if having them on their own string was the way to do it, but since they would be paralleled, still wouldnt work. I wasnt sure how mis-matched panels would work in regards to strings. I'd have to basically have a 3rd MPPT.

 

[Supervstech]

Is there no way to get the new string up to similar voltage specs? Maybe add additional panels to get the specs to line up?

 

[SolarHead]

I haven't jumped into the math yet , but sounds like probably the most reasonable thing to do. The 36 panels I have , are discontinued, can't find more of them anywhere.

 

[SolarHead]

I even wondered about like adding a morningstar or midnite MPPT to handle a new separate string of panels, but then I don't think there's a way to connect to the battery bank once I go LifePo4 and BMS. Currently on AGM and I imagine acid batteries you can just connect multiple chargers to the same battery bank, but allow one inverter to manage the power from the batteries. Acid battery banks don't seem to care where they get their power from in regards to one or more chargers, but LifePo4 and BMS wouldn't like another charger in the picture, I imagine. Only other way would be to have separate string and get a plain (no frills) string inverter, or microinverters, bring the new panels into the Sol-Ark via AC coupling to the gen breaker.

 

[Supervstech]

I even wondered about like adding a morningstar or midnite MPPT to handle a new separate string of panels, but then I don't think there's a way to connect to the battery bank once I go LifePo4 and BMS. Currently on AGM and I imagine acid batteries you can just connect multiple chargers to the same battery bank, but allow one inverter to manage the power from the batteries. Acid battery banks don't seem to care where they get their power from in regards to one or more chargers, but LifePo4 and BMS wouldn't like another charger in the picture, I imagine. Only other way would be to have separate string and get a plain (no frills) string inverter, or microinverters, bring the new panels into the Sol-Ark via AC coupling to the gen breaker.

Lifepo4 won't mind multiple chargers either.
As long as the bank is large enough to handle all running at once.

 

[SolarHead]

does adding an additional charger to a LifePo4 battery bank mess with the BMS ? I was thinking the BMS on the batteries (I dont know much about LifePo and BMS yet and what they communicate back to the inverter with) would only be able to communicate with the inverter/charger and adding an additional and separate charger would be a monkey wrench. A less expensive MPPT charger (charger only) would not communicate to the BMS from what would guess, and it would just send charge to the battery terminals. I guess the BMS wouldnt know where the charge(s) are actually coming from, if its tops off sooner due to another charger, then the BMS just tells the inverter its full , etc. Correct? I got a TON to learn about a BMS, basically know that a BMS communicates the battery status (not sure exactly what) to the inverter. Dont want to get off topic here, I'll learn more about BMS's in the coming weeks/months soon as my batteries arrive. I plan to jump in and learn and see more then.

 

[Supervstech]

The chargers should be set to cut out BEFORE the bms does.
As long as voltage is below set levels, the chargers send the current.
The bms is strictly for cell management. Not pack voltage charger control.

If the bms is disconnecting, the charger is set too high.

 

[sunshine_eggo]

I even wondered about like adding a morningstar or midnite MPPT to handle a new separate string of panels, but then I don't think there's a way to connect to the battery bank once I go LifePo4 and BMS. Currently on AGM and I imagine acid batteries you can just connect multiple chargers to the same battery bank, but allow one inverter to manage the power from the batteries. Acid battery banks don't seem to care where they get their power from in regards to one or more chargers, but LifePo4 and BMS wouldn't like another charger in the picture, I imagine. Only other way would be to have separate string and get a plain (no frills) string inverter, or microinverters, bring the new panels into the Sol-Ark via AC coupling to the gen breaker.

Charging from multiple sources

One can have as many independent charging sources running concurrently as they like provided they are programmed properly for the voltage of one's battery bank. Examples (some or all at once, even multiples of a single type, different brands, etc.): Solar PV via MPPT Solar PV via PWM AC-DC...

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