What power should I be seeing from my PV?

[thecampulanceman]

I hope someone can lend a hand - I installed my system last November on to my truck. I have 6 Rich Solar 190w panels. 3 in series, 3 in series, then those 2 sets wired in parallel on to a 12v Epever Tracer 10420AN 100amp controller and 3x Renogy 100Ah 12v Gel batteries. The panels are:

Rich Solar 190w panels RS-M190
Maximum Power(Pmax): 190W
Maximum Power Voltage(Vmp): 20.4V
Maximum Power Current(Imp): 9.31A
Open Circuit Voltage(Voc): 24.3V
Short Circuit Current(Isc): 9.86A
Maximum System Voltage(Vmax): 1000VDC

When I look at my PV output I'm not seeing higher than 550ish (max) watts during full sun straight above. My concern is that I have a panel or panel wiring problem. Based on my series/parallel wiring, can anyone tell me what the (max) output should be (V, Amp, Watt)? I'm embarrassed to say that I don't know how to calculate that info...

 

[gnubie]

Unless you manage to wire a panel in backwards (connectors put on incorrectly) you can simply add up the paper watt specs so 6 x 190W = 1140W peak. If you hook them up as 2 x 3 you should get an array of Vmp 61.2V, Imp 18.62, ie 1140W. Into a 12V battery that comes in at nominal 95A so within the abilities of the controller to provide to the battery.

What's the state of charge of the gel batteries? Without somewhere for all that current to go you'll not actually see 1140W even if your panels are actually capable of producing it at the time. If you place a load on the system that can take the full output of the controller how many watts do to see coming from the array under optimal sun?

Keep in mind that your panels' output decreases as they heat up. The stated specs are likely with the cells at 25C. There is a derate figure that you can apply to calculate their output based on their actual temperature. Usually the voltage derate is around 0.3%/degree C. The current rating increases marginally with temperature but the change is so low we can ignore it and just use voltage change.

*edit adding ...*

Pmax = Imp x Vmp x ( 1 - (cell temp - 25 x 0.003) )

Also it doesn't take much off angle to reduce your panel's output a bit. One way to get a good idea of how square on the sun is rather than looking up is to place a can on a panel and look at the shadow cast. Any other reasonably level thing on the roof will guide you too, a radio or TV antenna etc.

 

[400bird]

For voltage sources (batteries or PV panels)
Series: add voltage (current is the same through the circuit)
Parallel: add current (voltage stays the same)

So for your 3 panels in series, VOC would be just under 75 volts (24.6*3) for each string of 3 panels. SSC for each string of 3 would still be 9.86 amps.

When you parallel the two strings, the voltage stays the same. Current is added, so just under 20 amps short circuit current.

And watts is just amps*volts. So you can figure that out at any point in the circuit with a little math.

But, the manufacturer's specs are done with laboratory conditions. Beyond perfect sun, cold panel, perfect angle. Basically, impossible situations in real life. I don't know what you should expect. But, on a van where the angle is never going to be correct, my guess would be correct

 

[thecampulanceman]

Unless you manage to wire a panel in backwards (connectors put on incorrectly) you can simply add up the paper watt specs so 6 x 190W = 1140W peak. If you hook them up as 2 x 3 you should get an array of Vmp 61.2V, Imp 18.62, ie 1140W. Into a 12V battery that comes in at nominal 95A so within the abilities of the controller to provide to the battery.

What's the state of charge of the gel batteries? Without somewhere for all that current to go you'll not actually see 1140W even if your panels are actually capable of producing it at the time. If you place a load on the system that can take the full output of the controller how many watts do to see coming from the array under optimal sun?

Keep in mind that your panels' output decreases as they heat up. The stated specs are likely with the cells at 25C. There is a derate figure that you can apply to calculate their output based on their actual temperature. Usually the voltage derate is around 0.3%/degree C. The current rating increases marginally with temperature but the change is so low we can ignore it and just use voltage change.

*edit adding ...*

Pmax = Imp x Vmp x ( 1 - (cell temp - 25 x 0.003) )

Also it doesn't take much off angle to reduce your panel's output a bit. One way to get a good idea of how square on the sun is rather than looking up is to place a can on a panel and look at the shadow cast. Any other reasonably level thing on the roof will guide you too, a radio or TV antenna etc.

Ahh - you've just given me another piece of understanding; thank you! What you're saying if I understand - even if the panels can give 1140w, if there is not a need for it (low discharge of batteries, a high load [Air Conditioner, microwave]) then I won't see that high level of PV power. I didn't know that. Now based on this, tomorrow when the sun is high I will run the Air conditioner or microwave and look at the PV to see what the output shows. I'm in the tree shadows now and the sun is getting low so midday tomorrow I'll take a look. This is making some sense now - and I'd hoped that I didn't have a problem on top with the panels and/or wiring. There is a lot of great info in your reply and thank you for that.

 

[snoobler]

+1 to everything @gnubie said, and yes, you got it on the battery. Next time you're in what you perceive as optimal sun, run the A/C or microwave and create a load and see if your panel output increases.

Once your battery is at about 14.4V, it's voltage limits the allowable incoming current meaning even if there is more power/current available, the battery can't accept it.

Lastly, shading. Even a very small amount of shading on one CELL in a single panel can severely impact all panels in that string. You are currently configured for 3S2P. If shading is a factor, 2S3P might be more advantageous.

 

[thecampulanceman]

This is the type of info I was looking for - thank you for breaking it down for me.

Series: add voltage (current is the same through the circuit)
Parallel: add current (voltage stays the same)
So for your 3 panels in series, VOC would be just under 75 volts (24.6*3) for each string of 3 panels. SSC for each string of 3 would still be 9.86 amps.
When you parallel the two strings, the voltage stays the same. Current is added, so just under 20 amps short circuit current.
And watts is just amps*volts. So you can figure that out at any point in the circuit with a little math.

 

[thecampulanceman]

Yes, tomorrow I'll get a look at the output while I'm pulling a load on it and see what it shows. The shading has been an issue for me at times and I pick my locations based on direct sun because of it. So far it hasn't been enough of a problem to entertain a change but...

Thank you.

+1 to everything @gnubie said, and yes, you got it on the battery. Next time you're in what you perceive as optimal sun, run the A/C or microwave and create a load and see if your panel output increases.

Once your battery is at about 14.4V, it's voltage limits the allowable incoming current meaning even if there is more power/current available, the battery can't accept it.

Lastly, shading. Even a very small amount of shading on one CELL in a single panel can severely impact all panels in that string. You are currently configured for 3S2P. If shading is a factor, 2S3P might be more advantageous.

 

[snoobler]

By shading, I meant anything else on the truck itself that might cast a partial shadow on the panels.

 

[thecampulanceman]

Thank you all for your info and taking the time to reply.
I am at peak sun right now and I have the A/C running. On the Tracers' MT50 I'm seeing:

834w
16.4a
50.9v

So, it looks like I do not have a panel/wiring problem and that both strings of 3 panels are operating.

 

[snoobler]

Are you in float mode?

That can be another limiter. If your batteries get fully charged early on, and you're in float mode, the array will only produce that which is needed to run the load and keep the batteries floated. 50.9V is a little low for a float voltage, but I could see it hanging around there. If it was in bulk mode, you have your number.

Regardless, that's way better than it seemed before. I would concur that you don't have an underlying issue.

 

[thecampulanceman]

I was in bulk mode when I did that reading. I wanted to be in bulk and run the A/C for 'max' load.

Are you in float mode?

That can be another limiter. If your batteries get fully charged early on, and you're in float mode, the array will only produce that which is needed to run the load and keep the batteries floated. 50.9V is a little low for a float voltage, but I could see it hanging around there. If it was in bulk mode, you have your number.

Regardless, that's way better than it seemed before. I would concur that you don't have an underlying issue.

 

[snoobler]

Perfect. Probably just the best the panels can do in a presumably flat orientation.