Open Circuit Voltage

[mcsullinsberg]

Hi friends, first post here and a newbie on their way onto a shuttle bus conversion build. Hopefully I can manage to at least get some of this terminology correct. So I have purchased 4 - 320Watt Solar panels (https://www.ecodirect.com/Q-Cells-320-W-Mono-Solar-Panel-p/q-cells-q.peak-duo-blk-g5-320.htm) and I see that the open circuit voltage on the panels is 40.56V to total 162.24V. I was planning on purchasing the Growatt SPF 3000 all-in-one unit (https://signaturesolar.com/content/documents/GROWATT/1519008-specs.pdf) but in the datasheet for that unit, it says that the maximum PV array open circuit voltage is 145VDC. I know that these units will usually outperform their max specs, but that's not really something that I'm willing to put to the test. So my question is, would it be worth it to shoot for a larger all-in-one unit (I'd like to keep with that style vs. individual components) or should I just wire 2 of the panels in parallel and the other two in series? Thanks for any help you can offer!

 

[time2roll]

2s2p is fine. Sorry I have no feedback on the inverter.

 

[rmaddy]

or should I just wire 2 of the panels in parallel and the other two in series?

That's not a valid option. You would wire 2 in series. Then wire the other 2 in series. Then wire those two sets together in parallel. That's referred to as 2S2P.

 

[mcsullinsberg]

That's not a valid option. You would wire 2 in series. Then wire the other 2 in series. Then wire those two sets together in parallel. That's referred to as 2S2P.

Got it. Would you put an OCPD on the positives of a 2S2P array? I think I remember Will saying generally it’s not necessary for an array with only 2 parallel strings.

 

[rmaddy]

Got it. Would you put an OCPD on the positives of a 2S2P array? I think I remember Will saying generally it’s not necessary for an array with only 2 parallel strings.

You don't need anything until you get to 3P or more. Though it can be handy, for any setup, to have a PV disconnect switch close to the SCC. A 2-pole breaker is a common choice for this. Just make sure it's a DC breaker rated for the array's voltage and amperage. It's not their to protect, just disconnect.

 

[kdiver58]

I'm going to bump this thread up as I have the same all in one as the OP has stated the OCV of this unit is 145 volts. Does anyone have any input on how much you can go over? I have 8 panels ( coming) with an OCV of 39.9 volts. If I go 4S2P that puts me 10% over on the voltage. Or should I just run more wire, splitters and wire them 2S4P and not risk it? These are used panels I know the wattage will be below listed I'm just not sure of the voltage. Thanks .. K

 

[rmaddy]

Does anyone have any input on how much you can go over?

You must never go over the max PV input voltage. The temperature adjusted Voc of your panel array must always be less than the max PV input voltage or you risk damaging the charge controller.

39.9V x 4 = 159.6V. And that's at 25ºC/77ºF. You'll be at at 172V at 0ºC/32ºF. Don't do it.

Get 1 more panel and go with 3S3P. Then you'll be at 119.7V at 25ºC/77ºF and about 129V at freezing.

 

[73powerstroke]

Hi friends, first post here and a newbie on their way onto a shuttle bus conversion build. Hopefully I can manage to at least get some of this terminology correct. So I have purchased 4 - 320Watt Solar panels (https://www.ecodirect.com/Q-Cells-320-W-Mono-Solar-Panel-p/q-cells-q.peak-duo-blk-g5-320.htm) and I see that the open circuit voltage on the panels is 40.56V to total 162.24V. I was planning on purchasing the Growatt SPF 3000 all-in-one unit (https://signaturesolar.com/content/documents/GROWATT/1519008-specs.pdf) but in the datasheet for that unit, it says that the maximum PV array open circuit voltage is 145VDC. I know that these units will usually outperform their max specs, but that's not really something that I'm willing to put to the test. So my question is, would it be worth it to shoot for a larger all-in-one unit (I'd like to keep with that style vs. individual components) or should I just wire 2 of the panels in parallel and the other two in series? Thanks for any help you can offer!

You CAN NOT exceed the max open circuit voltage of the cc (charge controller) you need to wire them 2 in series max and parallel the other 2 . You can usually exceed the amps on most anything it just won't put out more, however exceed the voltage and poof.
Also cold weather can cause a voltage spike and cook the unit. So you want to have plenty of room below that rating . And make sure they don't spec two different ratings. Some times they will say like 150v max ocv at normal temps and 92v at 25celcius . With that in mind that 150v unit can really be 92v in places hot.
I'm not familiar with that unit. So I cant give advice there.

 

[kdiver58]

Thank you all .. I would love to add another panel but it wouldn't fit in my truck bed and allow me to close my roll n lock This is what is listed on the panels.

 

[rmaddy]

I would love to add another panel but it wouldn't fit in my truck bed

Then with those 8 panels you have no choice but to go with 2S4P. 4S2P will fry the charge controller. Note that in 2S4P you must fuse each of the 4 parallel strings. And you must properly deal with the 40A coming from the array. Most MC4 connectors are only rated for 30A. You might need to use a combiner box to avoid MC4 connectors.

It may be too late but other options include different panels and/or a different charge controller that supports more voltage.

 

[73powerstroke]

I'm going to bump this thread up as I have the same all in one as the OP has stated the OCV of this unit is 145 volts. Does anyone have any input on how much you can go over? I have 8 panels ( coming) with an OCV of 39.9 volts. If I go 4S2P that puts me 10% over on the voltage. Or should I just run more wire, splitters and wire them 2S4P and not risk it? These are used panels I know the wattage will be below listed I'm just not sure of the voltage. Thanks .. K

YOU CAN NOT EVER GO OVER.
Even if your a few volts under it, you will most likely fry it on a cold day.

 

[kdiver58]

Thank you all for the input. As you have all pointed out I never really factored the cold weather performance of the panels. It's for a boondocking setup and even in Georgia we get below 32 now and then. Thanks .. K

 

[73powerstroke]

Thank you all for the input. As you have all pointed out I never really factored the cold weather performance of the panels. It's for a boondocking setup and even in Georgia we get below 32 now and then. Thanks .. K

It's all good. We are all learning. It's out job to help each other and hope save others from making expensive mistakes.
Theres so much to know and learn here.
Whats your game plan now?

 

[kdiver58]

It's all good. We are all learning. It's out job to help each other and hope save others from making expensive mistakes.
Theres so much to know and learn here.
Whats your game plan now?

2S4P it will be easy for me to make up a combiner box for the setup.

 

[x98myers7]

The temperature adjusted Voc of your panel array must always be less than the max PV input voltage or you risk damaging the charge controller.

how do we calculate temperature adjusted Voc? I need to account for the same thing in michigan for the growatt 3k 24v i just bought

here's my basic math, for the ten 200w panels i'm going to run:
5 panels @ 24.3voc= 121.5voc which is less than the 145voc limit on the growatt....check good

so.... need help with winter temps. I recall reading somewhere on here needing to account for a winter temperature "spike", and suggesting a calculation factor of 1.2 for planning.

121.5 * 1.2 temp coefficient = 145.8voc which is right at the 145voc on the growatt... yikes?... not good? << help here please
------

 

[rmaddy]

how do we calculate temperature adjusted Voc?

School me on Temperature Coefficients

All: I am planning to buy a pallet of these panels from Signature Solar: https://signaturesolar.com/content/documents/Hyundai%20Solar/HiA-SXXXHI_M3_(380_405)_(19.12)_cs6.pdf Can someone tell me how to figure voltage, amperage, and wattage increase due to bright sun and cold temperatures...

diysolarforum.com

I recall reading somewhere on here needing to account for a winter temperature "spike"

There's no "spike". Voc simply goes up as the temperature goes down (and vice versa).

and suggesting a calculation factor of 1.2 for planning.

As explained in the post I linked, the 20% factor is a quick test. If you pass that test you are good to roughly -40º. If you don't pass that quick test then do the full calculations (as shown in the post I linked).

 

[x98myers7]

found it...

Adjusting Solar Panel Voc for temperature

When designing you solar panel system, it is important to adjust you solar panel Voc for temperature in order to ensure you do not over-voltage the PV inputs of your solar charge controller. This paper shows how to calculate the Temperature...

diysolarforum.com

 

[x98myers7]

got it.... thank you. so, I need to keep my panel Voc below 23v or I won't have enough headroom on my PV input voltage. that's cutting it pretty close already.

 

[rmaddy]

so, I need to keep my panel Voc below 23v or I won't have enough headroom on my PV input voltage. that's cutting it pretty close already.

Depends on the actual temperature coefficient Voc for your panels and the coldest temperature you expect to ever see.

 

[x98myers7]

yep, got it.. https://www.wrdsystems.com/max_voc_calc.php