Voc or Vmp Check my calc please.

[jaydee1445]

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Forgive my beating a dead horse. Also I cut and paste this from MS word (Dyslexia) and can't seem to get the bold font off. Sorry ​

I picked up 2 Delta Pro, DVH, and 6 Renogy 200 Watt Monocrystalline Foldable Solar Suitcase panels for emergency backup and avoid permitting for now. I’d like to add at least 6 more panels to max out input. I wish to use the Renogy suitcase for portability and durability, we have 2 wells and many out-building. ​

So far, my research seems to say use Voc to calculate max solar input to the DP. Seems like there is possibly a small over voltage protection in the MPPT but the fear of letting the smoke out has me being conservative.​

Running the numbers using Voc it looks like 6 panels in series are going to be just under the voltage limit and well under the amp limit(no debate over does the MPPT only draw it's max no matter what available amperage is). 8 would be way over and 7 panels would be unbalanced in a series / parallel config resulting in a 14% loss.(according to footprinthero) It seems the MPPT may have a small over voltage protection but I haven’t confirmed this.​

Still waiting Temperature coefficient % to fine tune maximium Voc values. I live in North Florida and we have a few freezes every year but temps warm quickly once the sun comes out so cold temp my not be a major concern. I would probably be ok if I disconnect PV on cold nights and wait for panel temp to rise before re connecting PV????​

How do my calcs look? Am I on the right track? Is temperature coefficent % going to screw me???

Renology 200Watt suitcase SPECIFICATIONS

Maximum Power at STC: 200W

Cell Efficiency: 22.5%

Open Circuit Voltage(Voc): 24.3V

Short Circuit Current (Isc): 10.42A

Maximum Power Voltage (Vmp): 20.4V

Maximum Power Current (Imp): 9.82A

Operating Temperature: -40℉ to +185℉



Here are my calculations Using Voc https://footprinthero.com/solar-panel-series-and-parallel-calculator

All results are at STC. 25 degrees C. Waiting on Temperature coefficient % from Renogy



2 stings of 3 panels each

All panels wired in SERIES at STC

• Max power output: 1428.84W
• Max power voltage (Vmp): 145.8V (max volt calculated using Voc)
• Max power current (Imp): 9.8A
• Loss: 0%

All identical panels wired in SERIES, then all series strings wired in PARALLEL: at SCT

• Max power output: 1428.84W (actually only 1093W @ 15amp current limit draw of MPPT)
• Max power voltage (Vmp): 72.9V
• Max power current (Imp): 19.6A
• Loss: 0%

2 stings of 4 panels each

All panels wired in SERIES:

• Max power output: 1905.12W
• Max power voltage (Vmp): 194.4V (well over voltage)
• Max power current (Imp): 9.8A
• Loss: 0%

All identical panels wired in SERIES, then all series strings wired in PARALLEL:

• Max power output: 1905.12W (actually only 1458W @ 15amp only a 30 watt gain for 2 more panels)
• Max power voltage (Vmp): 97.2V
• Max power current (Imp): 19.6A
• Loss: 0%

 

[kmin]

At freezing point your Voc would theoretically be around 25.5V so 6 in series is going slightly over Vmax. Probably they have put a little margin there and you would be ok. On the other hand, it's the only critical value in PV system.
Imp (NMOT) are typically -20% of STC, so 2 parallels would give 15.7A, quite close to Imax. Overall loss would be quite small.

 

[jaydee1445]

I got a Voc temp% from a tech supervisor @Renogy. He said .24mV and I was WTF until I realized he was reading the % as mV. Waiting on a firm answer from Engineering

The lowest temp I have seen was 18F at the coldest usually temps are above freezing once the sun is up.
the calc posted by Admin calc link gave maxV @ 0 degC 154.548 @-6degC 156.6475.

 

[kmin]

Yep, it's typically around 0.25%/'C.
Disconnecting for freezing days would eliminate the 0 'C 155V problem, but you would still have a lot of time at 10 'C, about 151V...
Either you believe there is margin, or you go with parallel.

 

[kmin]

Or you could put few diodes in series for winter period to lower the voltage couple of volts.

 

[jaydee1445]

All said and done 5 series suitcase would be 1190 W 121.5v x 9.8A. Pmax of 134 V @-17.7C/ zero deg F would still be 16 V under the MPPT
IMHO will be good short of nuclear winter and surely the wear and tear on the MPPT isn't worth a 238 watts.

Thanks for letting me talk this out......and keeping 6 Bennies in the solar fund

 

[kmin]

Good choice!
Like I wrote, Vmax is the only parameter to be concerned.... And who want's to live in Florida and worry about cold weather

 

[jaydee1445]

Also series strings don't need additional fusing, correct??

 

[kmin]

Absolutely not

 

[kmin]

There are people, who put fuses, breakers and switches all over the wiring. I see every unnecessary connection as a potential failure point. Between a series string and inverter only switches (if not incorporated to inverter).