Photovoltaics not putting out enough amps.

[shawname]

I have installed photovoltaics on my roof, and I must not understand something about how the system works. The problem is that I am not getting enough amps down at my charge controllers. For example, I have six 12Volt 2.9 amp commercial (but used Solarex) panels installed in parallel, (positive to positive, negative to negative) and I get 16 volts and only 2 amps down at the charge controllers!

I have tested the amps by shorting each panel and they all put out about 2.7 amps. I also shorted out the incoming leads from the group of panels, but the amps did not increase. I tested my digital VOM against the Cycle Analyst on my electric bicycle, and the two agreed on amps.

What am I missing?

thanks,

Shawn

 

[bobc]

You would expect more amps at short circuit than at high voltage, it's the nature of the beast.
Because power output is amps x volts, there is a maximum power point in the output V vs I curve & frankly 16V and 2A sounds pretty good to me!
Ah I re-read your post - you have 6 2.9A panels in parallel and are only getting 2A total.....
1) make sure there is no shadow falling on parts of the panels - shading just one cell in a panel virtually switches it off.
2) you call it a 12V panel and are pulling the power out at 16V (I don't know how your load is controlled) That is probably too many volts - if you can make the load get the panel output volts down you'll probably see the amps go up & produce a lot more power. Maybe a 3 parallel 2 serial connection would work better in your system?

 

[shawname]

Thanks. The panels have a voltage rating of 17.5 volts, and the voltages are higher in open mode. I meant 12 volt nominal panels. The panels are not shaded, in full direct sun, at 2:00Pm, (my solar noon).

I'm phflmuxed!

thanks,

Shawn

 

[liveforphysics]

The rated power of a pannel very rarely seems to occur in real life conditions.

 

[shawname]

Maybe so. I guess if I get a couple of people telling me so I'll have to believe it. When there is no other explanation.... How does that quote go?

Anyway, thanks.

 

[Russell]

Thanks. The panels have a voltage rating of 17.5 volts, and the voltages are higher in open mode. I meant 12 volt nominal panels. The panels are not shaded, in full direct sun, at 2:00Pm, (my solar noon).

I'm phflmuxed!

thanks,

Shawn

This is probably a dumb question but did you optimize the angle of the panels or simply lay them flat on the roof?

-R

 

[shawname]

Hi R,

The panels are just laid on the roof, which is a 4/12 pitch, which is 20 degrees. This happens to be pretty reasonable for my latitude of 25 degrees. I'm in Florida. I've taken panels off the mounts and tilted them and it doesn't change much.

Thanks, I am awaiting the right "dumb" question for the "Ahaa" moment.

Shawn

 

[Ykick]

I put 80W panel on my camper van years ago and remember something about a blocking diode so current only flows one direction? How long of wire runs do you use to the controllers?

 

[shawname]

Hey Ykick,

Cool electric bike, I ride one too, a cargo bike.

The longest wires run about 30 feet. The photovoltaics are mounted on the roof of my garage, and the batteries and charge controllers and inverters are underneath.

When I short the panels out, they are darn close to their rated amperage of 2.9 amps. When I hook them up in series, and check their amperage, I am getting .30 amps per panel. That makes for a whole lot less watts! I can't figure out why.

Shawn

 

[shawname]

Oh yeah, these panels do not have diodes.

 

[amberwolf]

If you have significantly lower current output when panels are in series than by themselves, either one or more of the panels has unusually high resistance, or your inter-panel connections do. You can test if it's a panel by trying them in pairs, and seeing if any pair is different than others, until you narrow down to the specific panel.

Connections can be tested with a meter, if the current drop is that dramatic it should show up even with a standard DMM, I think. Also, if this is not a new setup, but has been there for a while, it may be salt corrosion, if you have salt in the air around your area--there's enough moisture to do it even without the salt, if the metals of the connection points are different.


If it's a new installation, you might also want to be absolutely sure all the panels are wired in the right direction, because a reverse connection between sections might cause most of the current to be cancelled out.

Another issue is panel temperature, because unfortunately the best conditions for getting light to the panels (which increases the output capability) are also the best ones for heating them up (which reduces their output capability).

Given equivalent angle of sun, you'll probably see better output from them in the morning than the afternoon, because of temperature increases in the air and roof (or other mounting surface) that helps heat the panels themselves.

Any dust on the panel, or moisture, or any other thing on their surfaces, will cause less output. Dust or moisture in the air between you and the sun will do the same thing. Different angles to the sun, same thing, as well as different amounts of atmosphere (either because of lattitude or because of time-of-day).

I would never expect anywhere near rated capacity from any panel, because unless you're in a lab under completely controlled conditions, you are unlikely to ever get the conditions needed to see those ratings.


BTW: mounting the batteries and stuff under the panels directly may cause them to heat cycle a lot, and age them pretty quickly, compared to keeping them somewhere else. Depends on how hot it gets under the panels and how much airflow there is.

 

[CrazyJerry]

Depending on the distance from the panels to the controller you may be getting a voltage drop. Make sure the wire is heavy enough (since you are wired in parallel and amps are combined thus you need a heavier wire, and, you are running a relatively low voltage).
As a side note, You can use small speaker wire if you wire the panels in series and then use a good mppt controller such as a Outback......
~CrazyJerry

 

[shawname]

Thanks Amberwolf,

Say, are you that crazy guy that rode trans Canada on your electric bicycle? He had a rig a lot like yours.

I just checked the resistance on my sample panel and it is 1.8 ohms. I would think that if they were wired in parallel that a panel with more resistance wouldn't matter, but I am a newbie.

My panels are clean, but they do get quite warm in the afternoon sun here, but not too hot to handle.

My math told me I had 1.2 KW, and I am getting less than 300 watts. Maybe that's how things work in the real world.

 

[shawname]

Hi Crazy Jerry,

There is not much voltage drop. I am not using an mppt controller, but it's a decent Xantrex... I was wondering if the controllers can change how much the panels put out by varying the load... Because when I short the panels, I get almost the full rated amps.

Hey, that's what I'm trying to do, charge my ride from the sun.

 

[CrazyJerry]

shawname,
Is your Xantrex the type with the small knob type adjustment pots inside? If so make sure the bulk and float adjustments are correct. These adjustments will affect what the batteries see from the panels.
~CrazyJerry

 

[CrazyJerry]

"I would never expect anywhere near rated capacity from any panel, because unless you're in a lab under completely controlled conditions, you are unlikely to ever get the conditions needed to see those ratings."

----Maybe depending on the panel quality, but my experience is much different. In cold weather the panels are capable of full output. My panels are from 2000 (yes, 12 yrs old) and in the cold weather typically still put out a bit over their rated power. In the hot sun in the summer there is roughly a 10% drop at the most. Also, the Outback mppt has helped more compared to the old Heliotrope in the overall watts to the battery at the end of the day....

~CrazyJerry

 

[amberwolf]

Good to know that in the colder weather they'd be better; here in AZ that'll be a rare thing though (and probably in Florida, too).

Say, are you that crazy guy that rode trans Canada on your electric bicycle? He had a rig a lot like yours.

Nope; mine's even crazier than his. :lol: Justin_le here on the forum is who you're looking for. He runs Grin Tech, aka http://ebikes.ca and the thread about that trip is here:
http://www.endless-sphere.com/forums/viewtopic.php?f=29&t=7134

I just checked the resistance on my sample panel and it is 1.8 ohms. I would think that if they were wired in parallel that a panel with more resistance wouldn't matter, but I am a newbie.

In parallel, that's true, in that that panel simply wouldn't be contributing to total current output as much, but you were talking about having them wired in *series*, where all resistances add up, and any bad panel causes the whole string to reduce output.

I'm not sure if the resistance in a static test is goingt obe the same as when in operation, as they will heat up from passing current, too. You'd have to ask someone with more experience than I have on that one.

My panels are clean, but they do get quite warm in the afternoon sun here, but not too hot to handle.

Even a little warmer could cause a difference, though how much different mayb e dependeng on the panel design. You'd need to check it's curve specs. if they have any.

My math told me I had 1.2 KW, and I am getting less than 300 watts. Maybe that's how things work in the real world.

With solar, it can be, because manufacturers lie. Then distributors exaggerate those lies, and you just get a snowball effect on how good the panels really are, vs what they primised they'd be like.

Age of panels is an issue, too--older panels can be less efficient just because of design and materials, and also because of internal fracturing, connection problems, etc.,


I've hardly done anything with solar in a practical way, just because of that. Some people here have built their own panels from individual cells, though, so...

 

[shawname]

Is your Xantrex the type with the small knob type adjustment pots inside? If so make sure the bulk and float adjustments are correct. These adjustments will affect what the batteries see from the panels.

Yes my Xantrex does have the pots inside. I've never noticed them. I'll read the manual and figure them out tonight.

 

[Xanda2260]

What type of panel are they? Monocrystaline? Polycrystaline?
The panels I've played with have always been capable of nearly full output, but I've never had 30' of wire attached to them. Also, have you tried to measure current while the panels are feeding a load? I found the shorted current to be way down on what I expected. (this was just a 12v system for powering garden lights)

 

[bobc]

Panels are sold by max possible output under reference conditions, which is fairly close to the max you can get out of them in the sunshine. But you'll only get max output if you have an active maximum power point tracking controller forming the load - these things are very solar specific & a general purpose charger won't do it. Most low cost solar battery chargers are a shunt regulator, they're inefficient (get hot) and the panels are pulled down to battery voltage (say 14.4V) but they do work OK down to low sunshine levels because there is no "drop out" voltage problem like you get with a series regulator.
A panel for 12V SLA charging would usually have an open circuit output voltage of around 21V. If it had a short circuit current output of say 3A, it's maximum power point would be around 17V and 2.5A giving 42.5W. If it were used with a shunt regulator charger it would deliver (say) 2.7A at 14.4V which is only 38.9W (significantly less than its maximum capability). The panel datasheet should show it's I/V characteristics at different solar radiation levels.
Maximum power point tracking is hard to do without some sort of intelligence in the controller - if you ask for more amps than the sunshine can supply the panel output just collapses to ~0V. Your issue sounds different in that the load is not trying to draw enough current...

 

[shawname]

Hi Xanda,

They are polycrystalline. The voltage drop at the end of 30 feet of wire is not significant, the wire is a heavy gauge. My problem is the shorted load is just what I would expect, the unshorted output is one sixth of what I would expect.

thanks!

 

[dak664]

I suspect when you say "shorting the panel" you mean bypassing the charge controller. It appears you are charging a 12 volt battery, with the charging voltage around 14 volts. With the panels putting out 18 volts thats a 4 volt drop across your measured 1.5 ohms of wire resistance, not far from your 2.2 amp "short circuit" current for the whole array.

12 volts cant be sent any distance without big power loss, that's why most kilowatt inverters run at 24 or 48 volts. If you split your array into twice the voltage at half the current you would get nearly that full current at the battery; with a buck charger you would actually get more current since the bucking coil can convert excess voltage into more charging current. For large distances conversion to 128 volts AC at the array and using a line-powered charger can be more efficient.

If your current is limited by connection resistance you will see a large change from summer to winter. In winter the output will be a few volts higher and you could get full current.

 

[shawname]

Hi bobc,

Your issue sounds different in that the load is not trying to draw enough current...

I think this is a good way of saying it. So today I am going to remove and short out the leads from my panels down at the charge controllers, and measure the shorted amps. Crazy? If the load is not trying to draw enough current, this should give it plenty of load.

I should be drawing close to 39 amps- my voltage is 16.8- but I am only drawing three!

I appreciate the gains made in efficiency with MPPT, and if I do get high amps when shorted, I can assume that the load is not trying to draw enough current.

thanks!

 

[shawname]

I suspect when you say "shorting the panel" you mean bypassing the charge controller.

Actually I pulled one panel from the array and ran a lead between negative and positive, put it in full sun, and measured the amps. The panel put out close to it's rated amps.

12 volts cant be sent any distance without big power loss, that's why most kilowatt inverters run at 24 or 48 volts. If you split your array into twice the voltage at half the current you would get nearly that full current at the battery

The measured voltage of the array at the charge controller is 17.5. I ran 10 gauge multistrand, the max run is about 35 feet. I don't think it is the wire gauge or length because the current measured as the wires leave the panels is the same, and there is little voltage drop. Am I missing something? thanks!

 

[captain387]

I had a "Service call" on a solar micro solar setup my company had installed.

They had 4 panels and the amperage output dropped suddenly one day.
This scenario drove me crazy (Since I didn't know what I was doing and it just seemed weird)
I went up on the roof, took off the electrical cover and measured the amperage unloaded; all the readings were as expected. Voltage perfect.
Checked all the connections, double checked the inverter settings.
Checked the diodes in the panels.
Gave up.

Apparently one panel was defective and was drawing down the rest of the panels.

It was the difference between the no load vs meter reading shorted amperage.
Try each panel separately with a load connected.

Good luck.