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rival asked

Adding 2 new solar panels to existing system... advice needed

I have an off-grid system set up with 4 x Trinasolar 405W – 11.77A (Imp) – 41.4V (Voc) Panels wired as 2 x Series strings. Each string connected to a Victron BlueSolar MPPT 60A/150V charge controller MC4. Both controllers feeding four Rolls 6v S6-L16 Batteries (24v series) going to Victron Multiplus 24/3000/70-50. The system has worked fine for over a year, but I need a bit more ‘sun-power’ to charge the batteries this winter, and I managed to get hold of 2 x Eurener 410W – 17.4A (Imp) – 28.15V (Voc) Panels.

My intention is to connect each of the new panels to the spare MC4 inputs on both of my charge controllers… I think the math works out in terms of max volts and amps.

My question is ‘Would this intended upgrade work OK?’ I’m a bit concerned about feeding the controllers with power from slightly dissimilar panels!

Any help and advice would be appreciated.

Solar Panelsystem designsystem integration
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2 Answers
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pwfarnell answered ·

No you can not do this. The paired MC4 connectors are common. You have 2 existing panels in series producing Vmp = 2 x 34.4V = 68.8V. One new panel has a Voc of 28.2V. Therefore, if the MPPT runs at 68.8V you will get no power out of the new panel.

If you are combining panels of different types then the rule is for parallel connection the Vmp needs to be the same, for series connection the Imp needs to be the same. In your case neither the Vmp nor the Imp match.

The only way that I can see you getting this to work with the existing controllers is remove the existing panel string from one MPPT and add this to the other MPPT so now you have 1620W on one MPPT with Vmp = 68.8V and Voc well below the 150V limit. At a charging voltage of 28.8V, 1620W will generate a charging current of 56A, just below the 60A rating so all is OK.

The 2 new panels are then connected as a series string on the now unused controller. The two different types of panels are therefore kept entirely separate so they can both work independently to their optimum. If you want to add another 800W in the future make sure to buy panels that have a close match in Vmp and Imp to your Eurener panels.

You could always buy a new MPPT for the new panels.

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rival avatar image rival commented ·
Thank you very much for your advice... it all makes perfect sense!
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gazza avatar image
gazza answered ·

No, I am sorry this would not work well. It is never a good idea to mix different brands of panels. However, if you do..... then the specifications have to be very similar to allow the MPPT to do it's job. (it cannot find the max power point on different spec panels at the same time)

The existing panels Vmp 34.4V, Imp11.77A, which you are running as a series pair so Vmp circa 68.8V and current Imp 11.77A. However, the intended extra panels have Vmp 23.46V and Imp 17.48A.... very different.
I would suggest keeping the new panels on a separate MPPT wired as a series pair, either the 100/50 (if you are happy with screw terminals) or another 150/60 if you want MC4 terminals on the charger.

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rival avatar image rival commented ·

Thank you very much for your advice... again, it's easy to understand and much appreciated.

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