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Bad performance af Bluesolar MPPT 75/10 with 40W of panels

I am struggling to find the cause of very bad performance of my sailboat 40W solar panel installation.

The controller is Bluesolar MPPT 75/10. There are 2 horizontal panels of 20W each, connected in parallel. The panels are Sunbeam Tough Flush T20F. According to data they are supposed to have Vmp=20.88V, Imp=0.99A, Voc=24.48V and Isc=1.19A each.

My gel battery is not full and needs charge. The last weeks there has been plenty of sun. I am in south of Sweden. However the max power seems to be only mediocre 4W and a typical daily yield is only 30-50 Wh. The panel current sum normally is 0.3A in full sun when the panels are not shaded.

The cable is 2.5 mm2 from the panels. I have checked all connections. I even tested another temporary cable just to be sure.

According to nearby weather stations solar radiation intensity is typically 960 W/m2 at this time of the year, which is not that far away from the test conditions 1000 W/m2.

According to the Victron calculator the panels should produce almost 150 Wh/day or a max charge current of around 3A. But my poor 30 Wh/day or max current 0.3A is simply so much lower that I am now frustrated. I was hoping for at least 100 Wh/d or perhaps even more. A max power of 25W or current of 2A was also in my expectations from this 40W installation.

With a Fluke 179 multimeter I have measured the sum from the two parallelled horizontal panels as Voc=21.9V and Isc=0.95A when in max sun at 13:00. That is much lower than the panel data, isn't it? As there are two panels Isc should be 2*1.19A=2.4A at perfect conditions.

My tests also reveal that one of the panels seems weak, as it only gives half the current than the other does. And its voltage seems lower than the other.

The solar panel producer Sunbeam says that the Victron controller is not good enough and wants me to buy theirs instead. They say that their controller starts at a much lower voltage difference and consumes much less power, They say it is better than the Victron, in what ways I don't know.

I have read various posts here on the subject, but it seems as everyone is getting much more out of their panels than I do.

Should I claim warranty on one or both of the panels?
Or am I simply expecting too much?
Or could there be a problem with the Victron Bluesolar MPPT 75/10 controller itself?

Solar Panel
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seb71 avatar image seb71 commented ·

The panels are installed horizontal?

How often they are shaded? Any shade will get you a significant power drop (and you have tiny panels to start with).

For testing, try connecting them in series and see if you get better results (and in this case place them in the same direction and with the same elevation, if they are not already). Voc=24.48V x 2 should be safe for a 75/10 charger in summer.

How long are those 2.5mm^2 wires?

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johwin61 answered ·

It turns out that my both panels were bad. I have now installed 2 new and fresh panels of the same model. Preliminary reading is Pmax=30W from installed power of 42W, which I think is quite ok. That is 10 times more than from the previous panels.

This time I connected the panels in serial and added bypass diodes.

So far, my hypothesis is that my use of the original panels in parallel without blocking diodes, has reduced my panels efficiency to almost nothing. From the beginning they produced as expected, but after a while the output started to decrease.

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mort avatar image mort commented ·
Any reason why with a 75/10 why you wouldn't use the panels in series? Voc would be well under 75V, and my limited understanding would imply that the controller would be more efficient then. Happy to be corrected - here to learn more!
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johwin61 avatar image johwin61 mort commented ·
I think you are right; series should be better. The reason i first used parallel is that the PV manufacturer recommended that.


I have now put the new panels in series with added bypass diodes.

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Mike Dorsett answered ·

I would test the Isc of EACH panel under the full light conditions. The low current could be due to a faulty connection. Once checked, I would operate the two modules in series - this will give a higher voltage on the pv side, and lower current. The MPPT start sooner this way, and also be more efficient.

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johwin61 answered ·

The 2.5 mm2 connection from the PV to the controller is 3 m long. The only shading is from the boats tiller. The panels are horizontal.

I have tried serial connection. Then the charging time becomes a bit longer. But the current is only half compared to parallel. The controller starts earlier, but the current is so low that the yield is negligible. The panel manufacturer also claims parallel connection is the preferred, as I otherwise should add bypass diodes.

The boat is small, so there is simply not room for more panels.

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johwin61 answered ·

I actually may have caused this problem myself. I connected the two panels in parallel. I didn't put any diode in the circuit. There where no instructions with the panels when I got them.

After a bit of reading I now realize that each panel probably should have had an additional stop diode in series to prevent current from flowing backwards into one shaded panel when the other panel has sun. These thin flush panels don't seem to have diodes built in.

I think here's how to do it:

  • Parallel panels; serial stop diode at each panel.
  • Serial panels; parallel by-pass diode at each panel.

Could this lack of stop diode have destroyed one or perhaps even both of my panels?

Is a stop diode of the same type as a by-pass diode?

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