Cold weather and Voc?

[Jims94vmx]

Is there an actual formula? I am running an experiment and have 4x180w panels currently in 4s. I have seen 82v so far. Temps 50 to 70.

Tomorrow morning in the upper 30's. my panels are 21.6 plus or minus .5%

My controller is 100/50

Could not find anything to show. Only cold weather blanket statements

 

[sunshine_eggo]

Your panels have a temp coefficient something like, -0.33%/°C. It's likely on their datasheet online - lookup brand and model.

Generally speaking, with a 18V margin, you'd need to be -40°F/-40°C to tickle the 100V limit.

Official numbers from NEC:

PV array temperature correction table (NEC 2017)

There are calculators like this one made by @upnorthandpersonal which help you calculate PV array voltage and power for low temperatures based on the specific specifications of your panels. These are great tools and will give more precise...

diysolarforum.com

 

[Jims94vmx]

Thanks. Should be @ 90V in morning. Will keep close eye out and disconnect if getting too close.

 

[sunshine_eggo]

Thanks. Should be @ 90V in morning. Will keep close eye out and disconnect if getting too close.

Don't be too paranoid. You'd need to drop another 30°C to close the gap from 90V to 100V

 

[Jim Burrow]

In general, as the ambient temperature goes up, your pannel VOC will degrees linearly. But the current will increase exponentially.

 

[fafrd]

In general, as the ambient temperature goes up, your pannel VOC will degrees linearly. But the current will increase exponentially.

Uhhh, no. Current increases linearly with increasing temperature, but by a factor far smaller than the factor by which voltage decreases with increasing temperature.

This is the reason that output power (Vmp x Imp) decreases with increasing temperature (Imp increases by less than Vmp decreases with each degree temperature increases)…

 

[MichaelK]

Here is the link to Midnight's string calculator. Plug the values for your panels into it.

MidNite Solar - Classic Sizing Tool.

MidNite Solar is the industry leader and manufacture of quality Renewable Energy System electrical components and E-Panels.

www.midnitesolar.com

I found the specs of a representative 180W panel and plugged them in. With THOSE specs it appears the string would exceed 100V at about -10F, or 10 degrees below zero.


So, you are likely to be OK, though I strongly suggest your plug in your own documented specs to double-check.

 

[Jims94vmx]

Temp hit 34* with a real feel of 27* Voc hit 86.45 and seems to be heading towards 81 on average. So it all worked out.

Clouds will roll in around noon so hoping this experiment results in overall higher wattage.

Thank, hated to see a scc get toasted

 

[sunshine_eggo]

What make/model panels?

 

[OffGridInTheCity]

I like the Midnite Tool as well - I have Midnite Classic charge controllers so I used it pre-purchase.
I also found this the other day - https://gold-coast-solar-power-solutions.com.au/posts/solar-panel-maximum-voltage-calculator/ It focuses on Voc - has a simple calculator near bottom of the web page that looks like this:

 

[upnorthandpersonal]

I like the Midnite Tool as well - I have Midnite Classic charge controllers so I used it pre-purchase.
I also found this the other day - https://gold-coast-solar-power-solutions.com.au/posts/solar-panel-maximum-voltage-calculator/ It focuses on Voc - has a simple calculator near bottom of the web page that looks like this:

Similar to the one I made some time ago. Calculates power diff as well.

 

[12VoltInstalls]

The calculator with your actual panel specs will keep you safe.

hated to see a scc get toasted

That’s why I ‘adjusted’ to a 150V controller- because in 4S I could potentially be 104.xV+ at the realistic one or two days a winter lowest temperature. Not a good gamble.
Down the road you might consider a 150V controller as it will give you another panel of safe input. Alternatively two strings of four of those panels will increase your cloudy or all-day harvest, albeit potentially with some “unharvestable” watts in the middle of the day come spring/summer/fall or clear skies. Depends on what your needs are mostly.

 

[Jims94vmx]

What make/model panels?

3x Bougerv 180 and 1 180 from Continous Resource.

 

[sunshine_eggo]

Can't find any data on the Bougerv.

Continuous Resources is:

Hightec Solar 180W 36 Cell 12V Nominal Solar Panel

180W Solar Module. Made in the USA. Free Shipping in the continental US! Specifications Hightec Solar 180W 36 Cell 12V Nominal Solar Panel Specifications: Power: 180 Watt Vmp: 18.95V Voc: 23.90V Imp: 9.50A Isc: 9.87A Maximum System Voltage: 600V Module Efficiency: 17.0% Temperature Coefficient...

www.continuousresources.com

-0.32%/°C

Going to assume -0.33%/°C for the string.

Variables:
V = measured Voc
T1 = measured temp, °C
T2 = Temp @ 100Voc, °C

T2 = T1 - (100-V)/V *100 / 0.33

Conservatively (since you reported 82V @ 50-70°F):

V = 82V
T1 = 25°C (77°F)

T2 = 25 - (100-82)/82 * 100 / 0.33 = -41°C

Panel Voc has a range, typically ±5%, so measurement of your own Voc gives the best starting point. Just make sure it's true Voc. If you're reading it off the MPPT output, you can't trust it. If you disable charging and then measure Voc at the PV input terminals, that can be trusted.

 

[Jims94vmx]

Thanks everyone. Will end up going 2p 2s after the experiment if we get good sun. Kind of interesting how i am still seeing some charge this late and overcast. Even on a sunny day the sun is behind trees around 3 or so.....

 

[Jims94vmx]

Hit 34 this morning. Highest Voc 87.61

Put back to 2S 2P this evening. I needed a new Y connector.