Lookin for advice on a solar set up

[tanner0042]

I am upgrading a Renogy 400 watt (4 panels at 100 watts each) system. I have added Dakota lithium battery and inverter. I have also added 8 more 100 watt panels for a total of 1200 watts of panels.

Dakota Lithium Home Backup Power & Solar Energy Storage System, 5-20 KWh Battery, 3,000W Inverter​

(https://dakotalithium.com/product/d...tery-3000-watt-inverter/?attribute_pa_ah=5kwh)

Previously I have run different battery banks from lead acid to lithium with the 400 watt Renogy system. I just did not have the battery capacity I wanted with either of these battery chemistries. So this is the new upgrade.

The Dakota Lithium system can take an an MTTP photovoltaic input range of 120-250Vdc. My Renogy panels are 12 volt 100 watt panels. Here is the question I am looking for advice on. These 12 volt panels have an open circuit voltage rating of about 18 volts. When I put a volt meter on them (not under load) they all read over 20 volts, sometimes almost 22 volts. If I run all 12 panels in series, I have 12 volts * 12 panels for 144 volts, that's above the system 120Vdc input and well below the max of 250Vdc input. If I use the nominal voltage for my calculations of 18 volts * 12 panels for 216 volts, I am again well above the 120Vdc minimum for the system and comfortably below the 250Vdc max. But if I use my volt meter readings, let's just go on the high average and say 22 volts per panel * 12 panels for 264 volts, which is above the system max of 250 Vdc. But this is a meter reading on each panel under no load.

Is that with in specs for the Dakota Lithium inverter?

How would you recommend I configure my solar panels for the most efficient operation and set up? All 12 in series some configuration of pairing panels in parallels and in series?

A little more information. All panels are completely vertical. The ice house is always point due south so the panels are perfectly aligned to the south. The system is primarily used in Jan and Feb and the sun angle I calculated for that time of year is somewhere area 20-30 degrees for best panel angle. However, I am using this on a frozen lake and the amount of light reflecting off of the snow and ice is insane. I believe it more then makes up for running the panels vertically instead of at what the measured best angle should be. The benefits of vertical installation for me are ease of hanging panels on the side of the ice house and snow will not accumulate on a vertical panel.

 

[LakeHouse]

The first thing to do is to just totally erase from your mind any thoughts at all of these being '12V' panels. That's old terminology from when everyone used PWM controllers. The '12V' is a totally useless number for what you're trying to do.
I'm a little surprised by your voltage measurements of 20-22 volts on panels with Voc of 18V. There will be some effect from cold weather, and your Voc with temperature correction could get to 20V at -20C (-4F), but even at -40C (-40F), you shouldn't get to 22V, and I hope you weren't doing this outside at -40. You might want to check with a different multimeter, as I suspect you may be getting some incorrect readings.
Anyway, here's what you need to do:

1. Check that (Vmp * number of panels) is above the minimum voltage of 120 Vdc. Usually you want some additional headroom here.
2. Check that (Voc * number of panels) is less than the maximum allowable voltage of 250 Vdc. You must do temperature correction for this, since actual Voc will be higher than what is printed on the back of the panels. If you can find a data sheet for Voc coefficient for your particular panels, use that. Otherwise, -0.3% / degree C should be close to correct. Then figure out your minimum temperature and calculate the corrected Voc. Here's an example for -40C: Voc (@-40C) = Voc (@25C) * (1 - 0.3% * (25C - -40C)) = 21.16V for Voc(25C) of 18.0V. Using this number and 12 panels in series, you get 21.16V * 12 = 254V, which is too high. You may need to stick with 11, but it depends on the actual minimum temperature. Depending on your location in Minnesota, I suspect -40 is possible, so you may need to limit to 11 panels in series.

All panels are completely vertical.

This is probably the optimal arrangement in this case. Exactly as you say: they'll shed snow, and they'll be pretty close to facing the sun for the few hours that it's up each day.

One thing you definitely need to consider is low-temperature charging. LFP batteries shouldn't be charged below 0C (32F), it ruins them very quickly. Your setup hopefully has under temperature charging protection, but if they're sitting out in the cold all winter, they're just not ever going to charge.

 

[Q-Dog]

What is the VOC number printed on the label on the panels? Are all the panels the same?

 

[Mattb4]

I think you are confusing your Vmp with Voc for your panels. Typically RV or 12v panels will come in with a 22Voc and 18Vmp. Use the Voc number for allowable max and the Vmp number for the minimum.

Since your SCC is 120 to 250V this implies a max of 10 panels in series (220Voc and 180Vmp)
The minimum of 120V means a minimum of 7 panels in series (7 x 18v=126v).

With 12 panels you either are going to need using 2 less or adding 2 more for a 7S2P setup.

 

[tanner0042]

The first thing to do is to just totally erase from your mind any thoughts at all of these being '12V' panels. That's old terminology from when everyone used PWM controllers. The '12V' is a totally useless number for what you're trying to do.
I'm a little surprised by your voltage measurements of 20-22 volts on panels with Voc of 18V. There will be some effect from cold weather, and your Voc with temperature correction could get to 20V at -20C (-4F), but even at -40C (-40F), you shouldn't get to 22V, and I hope you weren't doing this outside at -40. You might want to check with a different multimeter, as I suspect you may be getting some incorrect readings.
Anyway, here's what you need to do:

1. Check that (Vmp * number of panels) is above the minimum voltage of 120 Vdc. Usually you want some additional headroom here.
2. Check that (Voc * number of panels) is less than the maximum allowable voltage of 250 Vdc. You must do temperature correction for this, since actual Voc will be higher than what is printed on the back of the panels. If you can find a data sheet for Voc coefficient for your particular panels, use that. Otherwise, -0.3% / degree C should be close to correct. Then figure out your minimum temperature and calculate the corrected Voc. Here's an example for -40C: Voc (@-40C) = Voc (@25C) * (1 - 0.3% * (25C - -40C)) = 21.16V for Voc(25C) of 18.0V. Using this number and 12 panels in series, you get 21.16V * 12 = 254V, which is too high. You may need to stick with 11, but it depends on the actual minimum temperature. Depending on your location in Minnesota, I suspect -40 is possible, so you may need to limit to 11 panels in series.

This is probably the optimal arrangement in this case. Exactly as you say: they'll shed snow, and they'll be pretty close to facing the sun for the few hours that it's up each day.

One thing you definitely need to consider is low-temperature charging. LFP batteries shouldn't be charged below 0C (32F), it ruins them very quickly. Your setup hopefully has under temperature charging protection, but if they're sitting out in the cold all winter, they're just not ever going to charge.

Thanks, the ice house is heated inside, even when I'm not there. I drop the temp when I'm not there, but the batteries stay well above freezing temp.

-40C is theoretically possible, but highly unlikely and only possible over night. During daylight hours we are unlikely to below -20C on the low end. Also, if the sun is out, no cloud cover, we are also not going to see those extreme low temps, that would be on a cloudy day.

Thank you for those calculations, I will do some math and go from there!

 

[tanner0042]

What is the VOC number printed on the label on the panels? Are all the panels the same?

The panels are up north about 2 hours away from where I am so i can't check them right now. All panels are Renogy panels with the same specs. 4 are about 3 years old and 8 are newly installed. The panels are slightly different as they were manufactured at different times and they changed the manufacturing process, but Renogy told me I can consider them all to be identical from an operational standpoint.

 

[LakeHouse]

-40C is theoretically possible, but highly unlikely and only possible over night. During daylight hours we are unlikely to below -20C on the low end.

The time of most concern is first light in the morning; cold overnight temperatures, and the sun hasn't had a chance to warm anything up yet. Often your daily low temperatures occur at about this time.

Also, if the sun is out, no cloud cover, we are also not going to see those extreme low temps, that would be on a cloudy day.

This is probably off-topic, but that is the exact opposite of what happens where I live. We only get -40C on clear days. Cloud cover tends to hold some heat in.

 

[Q-Dog]

The panels are up north about 2 hours away from where I am so i can't check them right now. All panels are Renogy panels with the same specs. 4 are about 3 years old and 8 are newly installed. The panels are slightly different as they were manufactured at different times and they changed the manufacturing process, but Renogy told me I can consider them all to be identical from an operational standpoint.

That's why I asked. I have some 100 watt panels with VOC of about 18 and some with VOC of 22. With one or two it isn't a huge difference, but with 8 or more it can be a significant difference.