Off Grid for small Cabin

[12VoltInstalls]

^^^^let the experts answer - but generally you can over-watt stuff (solar or not) but over-voltage is basically bad.
It’s like certain plastic water pipes: you can use them for air pressure way over their intended max pressure (still not a good idea btw) but if you get anywhere over their temperature limit even at rated pressure and kaboom. Failure. Over volts is like over temperature. Aren’t good no it ain’t.

 

[Hedges]

Your Growatt spec says maximum PV array open circuit voltage 145VDC
PV panel says open circuit voltage 44.3V

44.3V = 132.9V
145V/132.9V = 1.091, so Voc can rise 9.1% before reaching 145V limit

http://www.solarpanelsplus.com/PV-Specials/LDK%20275-285.pdf

"Temperature coefficient of Voc -0.33%/degree C"

9.1% / -0.33% per degree = -27.6 degrees (relative to 25 degrees ambient)

25 degrees C + -27.6 degrees = -2.6 degrees C.

From those figures, three panels in series is OK down to -2.6 degrees C.
You should be OK if record cold temperature for your location is not colder than 2.6 degrees below freezing.

(I'm not getting MichaelK's 149V figure. My earlier comment was "Assume 1.2x Voc for a safe figure, or get temperature coefficient from data sheet and your location's record cold temperature for a more accurate value." That conservative multiple gave 159V. I've now done the math with data sheet temperature coefficient, but we need your location's record cold temperature. Not typical, but coldest ever recorded.)

 

[pbennison]

Well this is northern NY.........typically it can get well below freezing even in October..

I guess that answers my question...Thanks

 

[corporate.joe]

$150 for a stamped metal box, a din rail and some bus bars? Anyone have recommendations other than the MNPV6? MNPV3 is a little better but leaves no room for future expansion with minimal savings in comparison.

 

[iamrich]

$150 for a stamped metal box, a din rail and some bus bars? Anyone have recommendations other than the MNPV6? MNPV3 is a little better but leaves no room for future expansion with minimal savings in comparison.

I got mine off eBay used (in mint condition) for $87 I think. There is always someone getting out while you are getting in Just a quick eBay search found this one. I almost bid on it and I don't even need it.

MidNite Solar MNPV6 Combiner Box for sale online | eBay

Find many great new & used options and get the best deals for MidNite Solar MNPV6 Combiner Box at the best online prices at eBay! Free shipping for many products!

www.ebay.com

 

[corporate.joe]

I got mine off eBay used (in mint condition) for $87 I think. There is always someone getting out while you are getting in Just a quick eBay search found this one. I almost bid on it and I don't even need it.

MidNite Solar MNPV6 Combiner Box for sale online | eBay

Find many great new & used options and get the best deals for MidNite Solar MNPV6 Combiner Box at the best online prices at eBay! Free shipping for many products!

www.ebay.com

Current bid:
US $76.00

That didn't take long from ~$55

 

[iamrich]

Current bid:
US $76.00

That didn't take long from ~$55

Yeah, the breakers are $20 each, so I imagine it will go for over $100, it was just an example. The one I got was a buy it now auction, so maybe I got lucky...

 

[MichaelK]

(I'm not getting MichaelK's 149V figure. My earlier comment was "Assume 1.2x Voc for a safe figure, or get temperature coefficient from data sheet and your location's record cold temperature for a more accurate value." That conservative multiple gave 159V. I've now done the math with data sheet temperature coefficient, but we need your location's record cold temperature. Not typical, but coldest ever recorded.)

Sorry bout that. I usually select freezing as a starting point because frost is easy to observe, and almost every spot in the US will drop to freezing at some point. So, with a conversion factor of 1.12X at 0C the three panels in question would be 44.3Voc X 3 panels X 1.12 = 148.85 ~149Voc.
It's easy to plug 1.12 into a calculator and spit out a number.

When I'm serious about determining the highest winter Voc, I always recommend a string calculator like Midnight's.

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

It's just that I don't want to take the extra time to determine each and every individual's max Voc, when I'm just taking about winter weather in an illustrative way.

 

[pbennison]

Last weekend I finally hooked everything up. I ran my table saw and a few other things and was producing power. I was not there long so other than a few hours of solar charging the battery bank I do not have much data to share. I hope to do that soon. Of course I have one more question that is nagging me. I have 8 panels set up in 2S4P.

Question: My roof is set up so that one side gets morning sun and the other side gets afternoon sun. When I connected them I did not really think about how they should go. With the lack of knowledge I simply hooked the one in morning sun in series with one in afternoon sun X 4. Now I have a nagging suspicion I should have connected morning sun in series and afternoon sun in series?

 

[12VoltInstalls]

Yes- two series sets in AM, two series sets PM. At best, wildhat math says you’re only getting 25% of the potential, but reality could be as little as 5%.

Although solar is funny sometimes. At a previous location I got leaf-filtered sun an hour-ish early AM, no full sun until 12:15pm to sunset. So I positioned SW facing at roughly 55* above horizontal. I would get a an amp or two all morning - shaded and facing the wrong way. That was interesting to discover, but it’s not a good plan.

 

[Bluedog225]

Great thread. Subscribed.

 

[pbennison]

My system has been operational for a couple months now. I have not been able to use it much other than some power tools, lighting, charging phones, 18V cordless tools, etc. I have had no issues and it seems to be working fine.

My plan was to take the batteries home and keep them in my basement for the winter. However, I was thinking of leaving the system connected so that if/when I go there during the winter it would be nice to have some power. These would be short 2-3 hour visits just to check on the cabin, dig out snow and generally just make sure everything is ok.

Is there any downside to doing this? It is not a huge deal either way, but, my thinking is that I would prefer to keep the batteries charged. The roof has a good slope with fairly good sun cover so I don't think the snow would stay on the solar panels. If during one of my visits I see the system not charging I can always take them at that point.

Looking for any advice, suggestions, etc. Next year I want to start pulling data. I want to add another 24V (4-6V batteries) to increase capacity.

 

[12VoltInstalls]

Lead acid batteries? It might be much better for them to leave them connected then to leave them in the basement for four months. But you do have to be aware of electrolyte levels. I always seem to lose fluid level way more in winter.

 

[pbennison]

Lead acid batteries? It might be much better for them to leave them connected then to leave them in the basement for four months. But you do have to be aware of electrolyte levels. I always seem to lose fluid level way more in winter.

Yes, sorry, lead acid batteries. I will plan to monitor them. Thanks.,

 

[Wellbuilt]

I just charge mine to full and turn the system off .
I get snowed out for 5/6 weeks some times .
If the battery stay cold they don’t discharge much
I think the could sit for 3 months + if cold .
I would not bother keeping the system running

 

[740GLE]

At what max temp will your idle batteries see? Any issues if they see -20/-30f?

 

[pbennison]

At what max temp will your idle batteries see? Any issues if they see -20/-30f?

Northern NY. Anything between -10F and 40F.

 

[Wellbuilt]

I am not 100% but I think a fully charged battery could freeze at -79 ?so you are fine.
My batterys are in a insulated box in my garage ,and get down to 25o some times when it - 20 out side .
The colder it is the better , I think at 30o they can sit for 6 months with out a charge .
My coldest temp is -27 I’m on the top of a Mountain in the middle of nowhere in the Catskills

 

[MichaelK]

I am not 100% but I think a fully charged battery could freeze at -79 ?so you are fine.

What I've read about the temperature low for a completely charged lead-acid is -40F, which just so happens to also be -40C. -79 is colder than any other statement I've ever seen.

I'm in California though, and just about the only place I think might get to -40 is the top of Mount Whitney, so I'm not talking from experience.

 

[Horsefly]

What I've read about the temperature low for a completely charged lead-acid is -40F, which just so happens to also be -40C. -79 is colder than any other statement I've ever seen.

I'm in California though, and just about the only place I think might get to -40 is the top of Mount Whitney, so I'm not talking from experience.

I'm pretty sure the accepted minimum temp for a fully charged lead-acid battery is -70°F. I know I've seen that many places, but it may vary. The important thing is "fully charged": A dead empty lead-acid battery will freeze completely and be ruined at 32°F, but then at dead empty it was already mostly ruined.