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Old 31-10-2022, 03:12   #1
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Solar Panels Outputting Limited Amps

I have 3 250-watt panels hooked up in series to a 30 amp controller (I know, leaving a lot on the table). I noticed that I wasn't getting the expected 30 amps from the controller, so I ended up measuring the output of each panel and found the following:

Short Circuit Current: 14.14 amps
Open Circuit Voltage: 22.5 V

Panel 1
Current: 7 amps
Voltage: 22 V

Panel 2
Current: 10 amps
Voltage: 22 V

Panel 3
Current: 10 amps
Voltage: 22 V

Based off some other pieces on the boat, I assume these panels were the cheap ones available some number of years ago, so I'm not expecting the greatest in terms of quality.

More out of curiosity, is dropping amperage like this a sign of aging panels?

I'm in Sydney, Australia, so the sun is full bore and always trying to kill us. Currently, the half producing cells are able to charge the battery, but it takes all day and I don't get anywhere near full power until well into the day.

Thinking of getting some cheapo 300-watt panels off ebay, another 30 amp controller, and seeing how things go.
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Old 31-10-2022, 03:17   #2
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Re: Solar Panels Outputting Limited Amps

Quote:
Originally Posted by workmaster2n View Post
I have 3 250-watt panels hooked up in series to a 30 amp controller (I know, leaving a lot on the table). I noticed that I wasn't getting the expected 30 amps from the controller, so I ended up measuring the output of each panel and found the following:

Short Circuit Current: 14.14 amps
Open Circuit Voltage: 22.5 V

Panel 1
Current: 7 amps
Voltage: 22 V

Panel 2
Current: 10 amps
Voltage: 22 V

Panel 3
Current: 10 amps
Voltage: 22 V

Based off some other pieces on the boat, I assume these panels were the cheap ones available some number of years ago, so I'm not expecting the greatest in terms of quality.

More out of curiosity, is dropping amperage like this a sign of aging panels?

I'm in Sydney, Australia, so the sun is full bore and always trying to kill us. Currently, the half producing cells are able to charge the battery, but it takes all day and I don't get anywhere near full power until well into the day.

Thinking of getting some cheapo 300-watt panels off ebay, another 30 amp controller, and seeing how things go.
Your voltage is near Open Circuit Voltage (VOC) which usually means the batteries are nearly fully charged.

You might what to turn on all or most of your loads and see what you get then or after the batteries are down a bit.

Mine a 65 watt panel and a 50 watt panel have about the same Open circuit voltage but are usually only near that when in Float Mode or disconnected
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Old 31-10-2022, 03:21   #3
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Re: Solar Panels Outputting Limited Amps

Quote:
Originally Posted by thomm225 View Post
Your voltage is near Open Circuit Voltage (VOC) which usually means the batteries are nearly fully charged.

You might what to turn on all or most of your loads and see what you get then or after the batteries are down a bit.
Sorry for the confusion, I should have explained better.

I disconnected the panels from the controller and measured the Open Circuit Voltage and Short Circuit Amperage at the panels (well, at the leads coming from the panels).
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Old 31-10-2022, 03:47   #4
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Re: Solar Panels Outputting Limited Amps

WHy do you expect to get 30 A from the controller, that's just a maximum rating.


Based on your figures, you are getting 22 x 10 = 220 W from two of your 250 W panels. Depending on time of day and conditions, That's not unreasonable.
The 22 x 7 = 154 W is a bit low, but there could be all sorts of reasons for that.


More importantly:What sort of controller? MPPT or PWM ?
If it's PWM, you will be losing a lot if you have anywhere near 22V going into the controller.
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Old 31-10-2022, 04:08   #5
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Re: Solar Panels Outputting Limited Amps

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Originally Posted by StuM View Post
WHy do you expect to get 30 A from the controller, that's just a maximum rating.
Perhaps I have a misunderstanding of how the controllers work.

I've got a 720 A/h AGM battery bank. I have a Renogy 30 amp controller.

I was about 100 amp hours down at the time (83% capacity), so I think the controller would be in float mode (and I wouldn't expect huge current).

I then turned on the microwave to heat up lunch. This put a load of over 100 amps on the system (as shown by the shunt). I would expect the controller to see a drop in house battery voltage due to the massive load and pop out of float mode. But perhaps the voltage doesn't actually drop in the house bank that much and therefore the controller remains in float?

Quote:
Originally Posted by StuM View Post
Based on your figures, you are getting 22 x 10 = 220 W from two of your 250 W panels. Depending on time of day and conditions, That's not unreasonable.
The 22 x 7 = 154 W is a bit low, but there could be all sorts of reasons for that.


More importantly:What sort of controller? MPPT or PWM ?
If it's PWM, you will be losing a lot if you have anywhere near 22V going into the controller.
This was Short Circuit Amperage, which the data sticker says should be 14.14 amps. Is missing out on 30%-50% of the Short Circuit Amperage concerning?

It's an MPPT controller.

The panels are hooked up in series, so voltage is about 54 V when everything is hooked up to the controller.
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Old 31-10-2022, 10:30   #6
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Re: Solar Panels Outputting Limited Amps

First off, Renogy sucks, stop just buying more of the cheapest gear, not a sound approach.

But in general, the short circuit / open circuit specs are more theoretical than anything seen IRL.

The amps flow rate between the controller and battery is what really counts, never mind between the panels and controller for now.

When the bank is at lower SoC yes amps will climb to some extent, but under 90% the difference is much less than what you see with lead.

Putting the panels in series and using just one SC makes the system as a whole very vulnerable to ANY flaw in any panel, or even the slightest partial shading, dust etc

My reco is, buy a good model controller like the smart Victron ones. That panel would be handled fine by the 15A model if you want to go 1:1 or if you want just one for all three, maybe the 50A is lots cheaper there...

If you want room to grow with a single controller handling more (matching!) panels then spend lots more go to big amps

but the 1:1 approach is not only more efficient but with Victron not expensive, and lets the MPPT algo deal with any variations optimally.

Also makes it easy to spot faulty panels, keeping everything exactly the same and measuring output while swapping between the three.

If you go to a 24V system each can handle double the watts, but really higher voltage panels in future would perform better.
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Old 31-10-2022, 11:03   #7
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Re: Solar Panels Outputting Limited Amps

The OP's questions and testing are quite reasonable.

The Voc of the panels should generally be reached under just about any light conditions, and it seems like that is the case.

The Imp is specified (nearly universally) at STC (standard test conditions, panel temperature 25C, 1000W/mē of irradiance normal to the panel). Sydney is at 33S and the sun's declination today is ~14S, so maximum solar elevation is 70° above the horizon. Maximum available power (and Isc) to horizontal panels (most boat mounts are near horizontal) is therefore ~94% of rated, or ~13.3 (and that's at a panel temp of 25C). Not knowing how the panels are mounted, or at what time the measurements were taken, or the weather conditions at the time, I would guess that 10A is a pretty reasonable measurement for Isc since it is directly related to amount of sun available at the time of the test.

If all three panels have the same mounting and orientation, and measurements were taken under the same sky conditions then I would suspect the panel with the 7A measurement. Since that is ~1/3 lower compared to the other panels I could suspect a burned out diode or connection on that panel resulting in only 2/3 of the panel being active. I'd want to do a lot more testing before coming to that conclusion, but I could see that being a potential cause.

If one panel is damaged to the point that it is only passing 2/3 current, and with the panels all connected in series, that means all the other panels will also be limited by that current - panels in series are limited by the lowest current panel. So once you connect them back in series you lose the 10A you are measuring and drop to 7A on all three.

As a test, I would take the two 10A panels and connect them in series to the controller with the third panel disconnected, if the power delivered by the two panels is nearly the same as with three (under all the same sky conditions) then it is a nearly certain indication that the third panel is restricting current (so long as the batteries are in a state that they can accept all available power).
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