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Solar Panel Startup Sanity Check

rmaddy

Full-time Solar-powered Trailer Life
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Nov 16, 2019
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USA
Today I finally got my solar panels installed on the roof of my cargo trailer. Before I connected the PV wires I first turned on my charge controller which is all wired up to the batteries. I also turned off the PV breaker that I have between the panels and the SCC. The breaker is only on the positive PV wire. With the SCC on and connected to the battery and with the PV breaker off, I connected the 3 panels together in series and then connected the panels to the PV wires going into the trailer. Since it was getting dark by then I knew there would not be any power from the panels so I turned on the PV breaker. The SCC showed about 26V and 0A from the panels (I have a 24V battery). So far so good.

With the thought that I didn't want something unexpected to happen when the sun came up and started providing power to the panels while I wasn't there, I turned off the PV breaker for the night.

So now to my first question. If I head to the trailer in the morning with the sun beating down on the panels, is it fine to just turn on the PV breaker and let the power feed into the SCC? Is there anything else I should do first?

And once on, with power coming in from the panels, is it perfectly safe to turn the PV breaker off and on as needed if I want to stop and start charging with the SCC while the panels are under full sun?

While I'm sure I have it correct, I want to confirm my PV breaker is setup correctly. I have the MNEPV polarized breaker from Midnite Solar. I only have a breaker on the positive PV wire, nothing on the negative. The positive PV wire comes in from the panels and goes to the bottom (++) end of the breaker. The top of the breaker is then wired to the PV+ input of the SCC. The negative PV wire goes directly from the panels to the PV- input of the SCC. Does anyone see any problem with that setup?

Last question. The installation manual for my panels states "Do not connect or disconnect modules under load! Danger! Risk of serious injury or death from electric shock or electric arc flash!". What exactly is meant by "under load"? If I have my PV breaker off, does this mean my panels are not under load or are they under load regardless if they have any sun on them? Let's say I need to disconnect one of the MC4 connectors between my 3S panels. What steps do I need to take to do that safely during the day with the sun beating down on my panels? Is it just a matter of turning off the PV breaker or does it mean I need to cover the panels with a dark blanket?
 
Just triple check your polarities and wiring and off you go.

If you're worried about it, wake up early, turn on the pv breaker, and observe the output at the controller as the sun comes into play.

You can do the same with a blanket as you've stated, again if you're worried about it and new to solar. This is what I did my first time though it's wildly impractical on larger systems on roofs.

As for not disconnecting under load, just flip the breaker and do what you need to. Once the circuit is broken there's no load current and disconnecting is as safe as you can get considering there is voltage present.

If you need to change an mc4 connector due to damage or something, it's safe enough to just swap one connector at a time to ensure you don't short the two together. The beauty of the mc4 and similar types of enclosed connectors is that neither the male or female side is exposed to easily short on other things when disconnected.

However, with lower voltages like this you're relatively safe from a personal standpoint. Anything under 48v is "safe" for human contact in that in all but the strangest circumstances (pacemakers and stuff) its not high enough to injur or kill you, though a shock may still hurt a bit for some people.

You can still easily damage equipment though at those voltages, but that's just annoying and expensive rather than dangerous.


If your system was much higher series voltage or you used higher voltage panels this scenario changes but you're fairly safe here.
 
Also remember that if your battery is fully charged you may see low or no current. If so then make a point to put a load on the system that exceeds your panel output to the point that it dips into the battery.

Otherwise you may find out later when you need it to work that something is wrong even though the voltage looks correct everywhere.
 
If your system was much higher series voltage or you used higher voltage panels this scenario changes but you're fairly safe here.
These are 330W panels in 3S (990W total) so the combined Voc is over 120V.
 
Also remember that if your battery is fully charged you may see low or no current. If so then make a point to put a load on the system that exceeds your panel output to the point that it dips into the battery.
The batteries (LiFePO4) are at about 60% SOC at the moment so I should get some charging in the morning.
 
These are 330W panels in 3S (990W total) so the combined Voc is over 120V.
You have 40+ volt panels? Got it. For some reason I thought you were using smaller 12v nominal panels that are like 18Voc.

In that case you still have the higher voltage present to watch out for but disconnecting the mc4 connectors shouldn't be a big deal once the pv breaker is off, and then each individual panel is within a reasonably safe voltage once separated.

At any rate in series they'll all have to be disconnected with the pv breaker anyways if you need to work on one of them.

This actually makes it more prudent to turn the breaker on before the sun comes up (or while covered) so you can get your hands off and away from the wiring as the system comes online with the sun.
 
The batteries (LiFePO4) are at about 60% SOC at the moment so I should get some charging in the morning.
Perfect for a test run then.

It sounds like you're mostly good to go and just need to send it.

If you're really worried about having someone check over what you did, a quick ms paint wiring diagram can't hurt.
 
I did a quick test this morning while the sun was still lower in the sky. I flipped on the PV breaker and the SCC display showed about 112V and 0A for about 10 seconds and then the amps when up to a bit over 2A and the watts bounced around about 225W. That seems reasonable for 990W of panels in series, mounted flat, at 8am at about 40ºN latitude. About 45 minutes later I turned the PV breaker off and I was seeing about 280W.

So turning the breaker on and off at 112V but low amps worked well. No sparks. I have some work to do to tidy up and secure the cables today so I won't be turning it all back on until dark tonight.
 
I did a quick test this morning while the sun was still lower in the sky. I flipped on the PV breaker and the SCC display showed about 112V and 0A for about 10 seconds and then the amps when up to a bit over 2A and the watts bounced around about 225W. That seems reasonable for 990W of panels in series, mounted flat, at 8am at about 40ºN latitude. About 45 minutes later I turned the PV breaker off and I was seeing about 280W.

So turning the breaker on and off at 112V but low amps worked well. No sparks. I have some work to do to tidy up and secure the cables today so I won't be turning it all back on until dark tonight.
Report results!
 
Last night I got the panels all hooked up. When the sun came up this morning the batteries were at about 55% SOC. By about 2pm they were at 100%. There was only a 15W load on the batteries while being charged. Below is a graph of the watts from the panels. I started getting power about 5:56am and sunrise was at 5:49am so that's quite surprising. It seems the panels peaked at about 725W (990W of panels) midday. They produced 3.1kWh up to the batteries reaching 100% SOC. Today was clear but very hazy from the western fires (I'm in Denver).

20210720PV.png

The dip in the middle of the morning is likely due to shading from a telephone pole very near to my trailer.

I'll need to get the battery down to 20% SOC and see if I can fully charge by the end of the day.
 
Sounds like a good install.

If you find yourself at 100% every day with room to spare you may want to increase storage (depending on budget of course) to better make it through the rainy or cloudy days. Otherwise some folks like to dump the 'extra' power into electric water heating for their daily shower or something along those lines.
 
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