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oceanwolfe avatar image
oceanwolfe asked

series connecting panels with different wattages

On our sailboat I am in the process of installing three Solbian SP panels, a pair of 118W and a single 144W, each with a separate Victron 75/15 MPPT controller (because of the usual boat shading problems). It occurred to me that on an overcast day, it would be pretty simple to reconnect the panels MC4 connector leads to put them in series to a single controller (one of which I would replace with a larger model able to handle the output of all three panels).

I've read where different power panels could be problematic. How big a difference would cause a problem. Should I keep the idea but limit it to the two identical panels?

Solar Panel
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3 Answers
billknny avatar image
billknny answered ·

Assuming all the panels had specifications for maximum voltage and current flow, it's not likely to be a "problem" but...

You would be compromising output quite a bit. An MPPT controller works to find the combination of voltage and current output from the panels that will maximize their output. This will vary depending on total solar irradiance, shading, temperature, etc. It is VERY unlikely that mismatched panels would have the same optimum point in this equation. By optimizing the mismatched combination, you'll end up with significantly lower output that you would with with the individual panels optimized separately.

As a general rule, the more you divide up your solar bank into smaller blocks, the better the job the MPPT controllers can do.

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oceanwolfe avatar image oceanwolfe commented ·
I do understand that it would be less efficient in sunny weather (and I would have them in parallel then). I was only considering the series connections for overcast/cloudy weather when it looked like I would get nothing from the parallel panels. I measured output last week in bright sunlight (just using a multimeter across the panel leads) and the voltages were all in the low-mid 20s. With thin clouds, they all dropped to ~5.5V.

My understanding is that I need about 19V to charge my 12V LiFePo batteries, hence the idea of reconnecting in series during cloudy weather.

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wkirby avatar image
wkirby answered ·

It's less of a problem with them in series, although not ideal. Even less of a problem under cloudy conditions, so maybe it will work out well for you.

Also, why would you buy a bigger controller for this configuration? If it is cloudy, the reason for series connection, the output is lower, so maybe one of your existing controllers will be just fine?

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oceanwolfe avatar image oceanwolfe commented ·
You are entirely right that I don't expect to need a larger controller, if the cloudy weather will just be cooperatively reliable. The larger controller was just in case of some weather gremlin putting a nice sunny break in the clouds. :-)


The total isc of the panels is about 18A, a bit more than I'm comfortable with for my 15A controllers since the calculation of the panel manufacturer is for a 25A controller for that isc.
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kevgermany avatar image kevgermany ♦♦ oceanwolfe commented ·
Probably better to stay with your original idea. You shouldn't disconnect the panels under load using MC4 connectors. Even on a cloudy day.

One other point. Panels must be producing battery voltage plus 5V before charging starts. Charging will continue as long as panel voltage is 1V higher than battery


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oceanwolfe avatar image oceanwolfe kevgermany ♦♦ commented ·
Ah! Thanks. Hadn't caught that part about charging until 1V higher. That does make it more worthwhile to temporarily series them if there's a sunnier spot to start the charging on a cloudy day.

I was assuming single digit voltages and amps during cloudy weather wouldn't cause arcing problem with the MC-4 plugs. Guess I could add switches to the positive legs, but hate the voltage drop from extra connections it involves.


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phil-garver avatar image
phil-garver answered ·

There actually can be quite an impact depending on the rating, not just watts of the panels. I built a little calculator to help optimize options recently as I was looking to add to my RV. This will allow you to see the best combo of series and parallel. Try it out, see if it helps with your decision. Solar Array Calc

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