Low Vmp, problem with panel or controller?

[TechnicalLee]

Hello,

I have the Renogy 50W (12V) panel connected to a Genasun GV-4 MPPT controller. In full sun I am getting 21.5V open circuit and 2.95 amps short circuit current. However when the battery is charging the Genasun is running the panel at about 14.45V @ 2.45A which is only about 35W. From what I read the max power point of a 12V panel should be around 16-18V? The sticker says 18.5V. My question is if this is normal, and if not, does it mean I have a bad panel or a bad controller? The voltage changes slightly so it does seem to be actively tracking.

Oh, and I also tried connecting the panel directly to the battery, current went down to about 2.05A @ 13.0V.

I tested at solar noon on a cloudless day, sun was normal to panel, 15ºC temp.

[inetdog]

It is perfectly normal for the current to be less than Isc when the panel is delivering power to a load.
But a drop in current from 2.45A to 2.05A is not expected.
If the battery charging voltage set point of the CC is such that you battery is not capable of absorbing the full power of the panels (SOC is too high for Bulk charging to apply), then I would expect the current to be even lower when the CC is operating to reduce the power output of the panels.

I do not see a single problem with either the panel or the CC which could cause the overall set of symptoms that you describe, so it is possible that you really have one or more problems on each side of the picture.

Since the panel delivers Isc when tested but does not deliver a current greater than Imp when Vout is lower than Vmp, it is possible that you have one fourth or one third of your panel defective and activating its bypass diode as the current being drawn increases.

Actually, now that I think more about it, that sort of panel problem combined with the normal MPPT action of the CC could very well explain entirely the symptoms you see.

[TechnicalLee]

I attributed the lower current to moving down on the power curve for the panel? The panel has two strings of 18 cells (two blocking diodes). This particular battery sits at about 13.0-13.1V open circuit after drawing off the surface charge (higher voltage AGM), and the voltage was essentially stable so I'm almost sure it was in bulk/CC.

What I'm trying to understand is how a panel can produce rated Isc but have parts of it defective? So the two strings are in series and the bypass diodes are in parallel with each string so I would think all the cells would have to be good and "accelerate" the current to reach Isc. But then again, each string taken alone would be half the voltage yet the same Isc right? The open circuit voltage would check out as long as there was a tiny bit of contact between cells, but the voltage drop off as the current increased like a badly sulfated battery? Now that you mention it some of the connections between cells looked poorly aligned, so I guess you are telling me to go check the short circuit current for each separate string tomorrow?

[Mike90250]

many MPPT controllers want to see Peak Battery Voltage + 50% at their input. A "12"V system would see a peak of about 16V while in EQ, and adding 50%, (another 8V), your panels should provide at least 24Vmp. While a MPPT controller is efficient, it is designed to reduce the voltage. So you may need to search for a special controller that can work with reduced overhead voltage, or simply use a PWM controller with such a low power system.

[inetdog]

I attributed the lower current to moving down on the power curve for the panel? The panel has two strings of 18 cells (two blocking diodes). This particular battery sits at about 13.0-13.1V open circuit after drawing off the surface charge (higher voltage AGM), and the voltage was essentially stable so I'm almost sure it was in bulk/CC.

What I'm trying to understand is how a panel can produce rated Isc but have parts of it defective? So the two strings are in series and the bypass diodes are in parallel with each string so I would think all the cells would have to be good and "accelerate" the current to reach Isc. But then again, each string taken alone would be half the voltage yet the same Isc right? The open circuit voltage would check out as long as there was a tiny bit of contact between cells, but the voltage drop off as the current increased like a badly sulfated battery? Now that you mention it some of the connections between cells looked poorly aligned, so I guess you are telling me to go check the short circuit current for each separate string tomorrow?

They are not blocking diodes, they are bypass diodes.
If one section of the panel cannot produce the same current as the rest of the panel, then when that current level is drawn from the panel, instead of a reverse voltage building up on the defective section as the rest tries to force current through it, the bypass diode simply conducts. The effect is a panel with 2/3 of the rated voltage, but still able to produce the rated current.

If there were no bypass diodes the voltage would drop even more, the current might be reduced and the defective section might be further damaged.

(When I say damaged section, what that usually means is a section that contains at least one damaged cell or interconnect. That is all it takes to limit the current from that section.)

[Wy_White_Wolf]

You have a fake MPPT controller that is actually a PWM controller.

WWW

[Sunking]

You have a fake MPPT controller that is actually a PWM controller.

WWW Genasun is legit. They make small Marine Grade Charge Controllers. Quality wise they are one of the best.

Technicallee what you are seeing is normal. MPPT controllers are both MPPT and PWM. A MPPT controller is only MPPT in Constant Current Bulk mode. Once in PWM Mode Input Current = Output Current. All other modes are PWM. What you are seeing and not thinking about is your battery is already fairly well charged up, thus your charge controller has no reason to go into Bulk Mode when all the battery needs is Float because it is already charged up. You cannot charge a full battery or nearly charged up.

As soon as charge current drops to Imp or, your controller is in PWM mode. Perfectly normal and is suppose to do that or it would burn your battery up. Genasun is unique in that you have no control of the voltage setting. That is done when you order the controller and tell the factory what to set it at. You cannot change it.

What you should see once charge current tapers off toward Zero Amps, is panel voltage will go above Vmp to Voc

[TechnicalLee]

Alright, so I went out and tested the current and voltage of each string of 18 cells on the panel. They both produced 2.95 amps. One string was 10.9V and one was 10.7V. However I also got my FLIR out and took some thermographic images of the panel which are VERY interesting:

IMG_2486.JPG

IMG_2484.JPG

IMG_2481.JPG

So it appears I have a panel with lots of hot spots due to bad cells and junctions. It's going back. Interesting that thermography was the only way to tell I had a problem other than the reduced power output.

[TechnicalLee]

Also, I tested the panel wattage again after draining the battery down to 11.9V. Hopefully we can all agree it would be in bulk mode at that point, but the wattage was about the same (2.3 amp x 13.0V = 30W). Panel voltage of 13V is still really low IMO. FYI the genasun is aways in bulk mode if the battery voltage is below the temperature compensated float voltage of about 13.2V.

[SunEagle]

IMO I wouldn't waste any more time with those cheap 50watt panels and go get a single 200watt or at least something that will provide the output you need.

[Sunking]

Also, I tested the panel wattage again after draining the battery down to 11.9V. Hopefully we can all agree it would be in bulk mode at that point, but the wattage was about the same (2.3 amp x 13.0V = 30W). Panel voltage of 13V is still really low IMO. FYI the genasun is aways in bulk mode if the battery voltage is below the temperature compensated float voltage of about 13.2V.

Incorrect. Bulk has no voltage associated with it except a set point to turn it off and switches to CV mode of Absorb or Float. Bulk set point for Genasun is is 14.6 volts for FLA and to 14.3 volts for AGM. Float in Genasun is either is 13.5 volts for FLA or 13.7 for AGM.

[Sunking]

Also, I tested the panel wattage again after draining the battery down to 11.9V. Hopefully we can all agree it would be in bulk mode at that point, but the wattage was about the same (2.3 amp x 13.0V = 30W). Panel voltage of 13V is still really low IMO.

Then you have a defective controller stuck in PWM mode. Contact Genasun for replacement. Make sure you specify the correct voltages for your battery..

Extremely easy to tel if you are in PWM mode or MPPT. All you have to do is measure Input and output current. If they are equal you are in PWM mode period. If output current is higher than input current, then you are in MPPT mode. It is that simple. It is still possible for the controller to be in MPPT mode while in Absorb or Float Mode if the battery or equipment demands current beyond PWM mode.

FWIW is input current is greater than output current, you have a paperweight, a very warm paperweight.

[TechnicalLee]

Just now:

Input: 1.67A @ 14.00V 23.4W
Output: 1.57 @ 13.55V 21.3W

I conclude the controller is in MPPT mode and difference in power is due to efficiency losses related to the small differences in voltages. Based on what I've seen with the currents it stays in MPPT bulk mode until it gets up to bulk set point or the panel voltage drops too low to do anything more.

[Sunking]

Just now:

Input: 1.67A @ 14.00V 23.4W
Output: 1.57 @ 13.55V 21.3W

I conclude the controller is in MPPT mode and difference in power is due to efficiency losses related to the small differences in voltages. Based on what I've seen with the currents it stays in MPPT bulk mode until it gets up to bulk set point or the panel voltage drops too low to do anything more.

Wrong conclusion, you are in PWM mode. Your Input Current roughly equals Output Current minus 2 watts the controller is using.

If you were in MPPT Mode Input would be roughly 18-20 volts @ 1.15-1.29 amps, and Output would be 13.55 volts @ 1.57 amps. This tells me you are in Float PWM Mode with a Genasun Controller made for FLA batteries..

The numbers do not lie and are telling you exactly what is going on if you know how to read them.

[TechnicalLee]

Alright. I don't believe the controller is in float mode simply because the LED is blinking. When it gets to CV mode the LED goes solid, and it does do that eventually.

Before we call the controller bad, I'd like to pose a scenario. Let's say I have a screwed up solar panel whose maximum power point is really at about 12V instead of 18V. I don't know if that is possible or not but let's just say it is. The controller might be tracking the power point, but it still has to keep the PV voltage a little bit above the battery voltage get any charging (I'm guessing it's only a buck converter instead of a buck/boost). Let me know what you think about that situation because that's my belief right now.