Added a panel now I have less power! What's up?

[tvtruth]

Hope someone can help me. I have 2x175w panels on my RV running into a combiner box and then connected to a TriStar-45 MPPT charge controller. On a typical sunny day I can get about 230 watts at peak hours out of the two panels (they are laying flat). I added a third 85w panel into the combiner and now my wattage has dropped dramatically. I'm only getting 98-100 watts at peak hours. I've double and triple checked all of the connections. The panels test fine individually but when I add them all together in the combiner ( a quality unit which was working fine until I added the 3rd panel) the combined voltage drops. What gives? Could something be wrong wight the TriStar that is back feeding the panels? FYI - all 10 gauge wire from the panels to the combiner and 4 gauge from there to the TriStar and 4 gauge to the batteries. I've tried equalizing and pulling all of the panels off of the combiner and making fresh connections. I've also tried going back to the original setup without the 3rd panel. No change. HELP!

[Mike90250]

If the battery is full, the panels and controller won't feed them much. Try testing at solar noon, with a beefy load on the the batteries (turn on the microwave) and see what happens. It may be the batteries get charged up 9am - noon and they are full when the sun is overhead.

If you don't have good sun, you also can't get good power either.

[SunEagle]

As Mike suggested, your battery needs to be in a state of requiring charging. Put some load on it and then try connecting one panel at a time to see what type of out put you get. If all three panels perform per spec then go back to using the 2 x 175 watt together and measure again.

I am a little concerned with adding the third panel to your system. It is half the wattage of the first two. You really shouldn't combine panels with that much of a difference of wattage rating to the same system. I believe it may cause an issue with the charge controller operating properly.

[inetdog]

I am a little concerned with adding the third panel to your system. It is half the wattage of the first two. You really shouldn't combine panels with that much of a difference of wattage rating to the same system. I believe it may cause an issue with the charge controller operating properly.

+1 and -1
It is not the wattage match that is important, it is the Voc/Vmp match when going in parallel and the Imp match when putting in series.

With a PWM controller, adding additional panels of at least the necessary voltage will always increase the power output. Not necessarily by the amount of power that the added panel could produce by itself, but at least some gain. (And when using a PWM controller you are likely to have all of the panels having the same nominal voltage. If not, then you have even more problems.)

But with an MPPT controller you need to match the voltages of the panels that you put in parallel to be sure of getting any increase at all. If the Voc values are within 5% of each other, you will get very close to the sum of the individual panel power ratings.
If the Voc values are different enough, then the MPPT algorithm of the CC could end up at an incorrect power point where the power is at a local maximum, but there is another local maximum that is higher total power.

In your case, if the Vmp of the third panel is low enough it could cause the other two panels to also be operated at 1/2 their Vmp or even lower.
Other MPPT controllers will do a wide sweep and find the true maximum. That may end up being exactly the same power that the two panels provided on their own, since the third panel will be above its Voc and will be contributing nothing.
Or if the lower voltage panel is actually breaking down when exposed to the higher voltage, your MPPT point for any design CC will be with the original panels working at that lower voltage and the new panel contributing nothing.

Note: Once you add the third panel in parallel, you also need to put individual fuses in series with each panel.

[SunEagle]

+1 and -1
It is not the wattage match that is important, it is the Voc/Vmp match when going in parallel and the Imp match when putting in series.

With a PWM controller, adding additional panels of at least the necessary voltage will always increase the power output. Not necessarily by the amount of power that the added panel could produce by itself, but at least some gain. (And when using a PWM controller you are likely to have all of the panels having the same nominal voltage. If not, then you have even more problems.)

But with an MPPT controller you need to match the voltages of the panels that you put in parallel to be sure of getting any increase at all. If the Voc values are within 5% of each other, you will get very close to the sum of the individual panel power ratings.
If the Voc values are different enough, then the MPPT algorithm of the CC could end up at an incorrect power point where the power is at a local maximum, but there is another local maximum that is higher total power.

In your case, if the Vmp of the third panel is low enough it could cause the other two panels to also be operated at 1/2 their Vmp or even lower.
Other MPPT controllers will do a wide sweep and find the true maximum. That may end up being exactly the same power that the two panels provided on their own, since the third panel will be above its Voc and will be contributing nothing.
Or if the lower voltage panel is actually breaking down when exposed to the higher voltage, your MPPT point for any design CC will be with the original panels working at that lower voltage and the new panel contributing nothing.

Note: Once you add the third panel in parallel, you also need to put individual fuses in series with each panel.

Thanks for the clarification. You are correct that it is the Voc not the Wattage that needs to be close between panels.

What bothered me was the way the wattage output dropped when he added the 3rd panel. I figured the circuit had changed somehow to cause the CC to drops its output but more than likely it had to do with the battery being almost charged or the lack of sunlight hitting the flat panels.

[inetdog]

Thanks for the clarification. You are correct that it is the Voc not the Wattage that needs to be close between panels.

What bothered me was the way the wattage output dropped when he added the 3rd panel. I figured the circuit had changed somehow to cause the CC to drops its output but more than likely it had to do with the battery being almost charged or the lack of sunlight hitting the flat panels.

The battery voltage level or light level could certainly explain that, but as I indicated there are several ways that adding a grossly mismatched panel in parallel in an MPPT configuration could actually decrease the output too.

[SunEagle]

The battery voltage level or light level could certainly explain that, but as I indicated there are several ways that adding a grossly mismatched panel in parallel in an MPPT configuration could actually decrease the output too.

Yep it could very well be how he wired that third panel. It is hard to diagnose a problem without having all of the facts on how the system is wired and how much sun the panels are really being exposed to.

[inetdog]

Yep it could very well be how he wired that third panel. It is hard to diagnose a problem without having all of the facts on how the system is wired and how much sun the panels are really being exposed to.

I would much prefer knowing the actual specifications of the two types of panels, instead of just their wattage.

[tvtruth]

Fuse ratings

+1 and -1
It is not the wattage match that is important, it is the Voc/Vmp match when going in parallel and the Imp match when putting in series.

With a PWM controller, adding additional panels of at least the necessary voltage will always increase the power output. Not necessarily by the amount of power that the added panel could produce by itself, but at least some gain. (And when using a PWM controller you are likely to have all of the panels having the same nominal voltage. If not, then you have even more problems.)

But with an MPPT controller you need to match the voltages of the panels that you put in parallel to be sure of getting any increase at all. If the Voc values are within 5% of each other, you will get very close to the sum of the individual panel power ratings.
If the Voc values are different enough, then the MPPT algorithm of the CC could end up at an incorrect power point where the power is at a local maximum, but there is another local maximum that is higher total power.

In your case, if the Vmp of the third panel is low enough it could cause the other two panels to also be operated at 1/2 their Vmp or even lower.
Other MPPT controllers will do a wide sweep and find the true maximum. That may end up being exactly the same power that the two panels provided on their own, since the third panel will be above its Voc and will be contributing nothing.
Or if the lower voltage panel is actually breaking down when exposed to the higher voltage, your MPPT point for any design CC will be with the original panels working at that lower voltage and the new panel contributing nothing.

Note: Once you add the third panel in parallel, you also need to put individual fuses in series with each panel.

I am going to replace this smaller panel with another 175w panel. You mentioned putting fuses inline. What size fuse for each panel and should I locate them inline just before the combiner?

[inetdog]

I am going to replace this smaller panel with another 175w panel. You mentioned putting fuses inline. What size fuse for each panel and should I locate them inline just before the combiner?

Inside the combiner box is common, but inline anywhere is good. If you put the inline fuses next to the combiner it is easier to check them all.
A typical modern panel specification sheet (or label) will list the maximum size inline fuse. Anywhere from that value down to Imp should be fine.

[tvtruth]

Thanks very much. I have the same BPsolar 175B panels that you have listed on your avatar

[inetdog]

Thanks very much. I have the same BPsolar 175B panels that you have listed on your avatar

You are welcome.
How to fish:
1. Google "BP Solar 175B"
2. Go to http://www.solarpanelsaustralia.com....lar_bp4175.pdf and look at page 2.
3. Isc is ~5.5A, maximum series fuse is 15A.
I would use either 10A or 15A fuses depending on what I have readily available with at least a 48 volt DC rating. If I had a free choice, I would use 10.

[tvtruth]

all fixed

Thanks for your help with this. I added the 3rd 175watt panel and fused all three at the combiner box. I reset the TriStar MPPT controller and everything is happy again. I was getting nearly 20 amps from the flat panels and the low winter sun yesterday!

[inetdog]

Thanks for your help with this. I added the 3rd 175watt panel and fused all three at the combiner box. I reset the TriStar MPPT controller and everything is happy again. I was getting nearly 20 amps from the flat panels and the low winter sun yesterday!

Congratulations, I am glad you were able to get it sorted out!

Please stick around and keep participating in the Forum.