question

victoroni avatar image

Why is Smartsolar current low with 41V input?

I've got a 160W portable solar panel feeding a Victron SmartSolar 75/15, which charges a 12V LifePO4 lithium battery (with its own battery management system).

I've rewired the solar panel in series to reduce voltage drop over a long extension, so that it now has an open circuit voltage of 44V, and an operating output of 36V and 4.4A. The panels have bypass diodes so I believe that should help with shading. If I shade one panel then it should have an output of 18V and 4.4A.

It works fine sometimes. But other times if I just partly shade one panel the SmartSolar app shows its input is around 40V, but the current is only 0.1A. I thought maybe the SmartSolar has a delay in making a change, for some reason, but I've watched it for over a minute and it doesn't improve.

Can anyone explain what's going on here?

smartsolar-screenshot.png

victron-solar-panel-shading.png

MPPT - Solar Charge Controllersolar
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3 Answers
victoroni avatar image
victoroni answered ·

@Stefanie

The battery's BMS has bluetooth connectivity and an associated app. It shows the charge level is 89%. Once I remove the partial shading from the panels it works fine again.

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The charger works independently of the BMS (unless the BMS controls the charger).

So maybe show us your charger settings before we continue guessing.

You're correct, the BMS is independent, though does limit overcharging etc.

Settings screenshot below (not sure if there's a better way to show the settings).

"Load Output Operation Mode" is set to "Battery Life".

Ambient temperature is a comfortable 25 deg C :-)

settings-battery.png

settings-battery.png (133.7 KiB)

What happens if you change float voltage to let's say 13.70V and then shade one of the panels? More power coming in or is it the same?

Thanks, good suggestion. It's dark here now but I'll try that tomorrow

@Stefanie I haven't been able to test this properly yet but I think your suggestion is probably correct. I've increased the float voltage on the MPPT's settings and hopefully that will solve the problem

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Stefanie answered ·

Hi @Victoroni,

what are the charger settings for your battery?

It would behave like that if battery is fully charged or floating.

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Kevin Windrem avatar image
Kevin Windrem answered ·

A lot depends on how much of the panel is shaded. Cells are arranged in one or more series-connected strings. The strings are then connected in parallel through isolation diodes. Shade one cell in the string and it shuts down the entire string. That reduces the current of the array to what ever the remaining strings can supply. So in a 2-string panel, you cut the total array output in half by shading one cell of one panel. If you shade two cells and they happen to be in different strings of a 2-string panel, you shut down the entire array!

Even partial shade on a single cell can have a huge impact on the output of that string.

Most people suggest that parallel connected panels suffer less since all the strings in all panels are in parallel so shutting down one string in one panel does not affect other panels. E.g., two 2-string panels in parallel reduce to 3/4 of the total output with one cell of one panel shaded rather than the theoretical 1/2 for the same panels in series.

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Hi @Kevin Thanks for the detailed explanation. It confirms the way I was beginning to understand that it works.
In my diagram above, the solar panels (numbered 1 and 2) each have two strings of cells, and each string has its own bypass diodes though I've just simplified it and shown one bypass diode for the whole panel. In my tests, I was shading 1 or 2 strings in one of the panels, but the other panel (consisting of its own two strings) was fully in the sun. Sometimes the resultant power was 1/2 the maximum as I expected, but other times it reduced to almost nothing.
I'll keep in mind what you suggest about parallel vs series and will rewire them back to parallel if the shading proves to be a bigger problem than voltage-drop over a long extension lead.