Number of Cells in Series or Array Voltage. Which is more important?

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coghst asked May 28, '20

Number of Cells in Series or Array Voltage. Which is more important?

I have reviewed manuals for several SmartSolar MPPT charge controllers. Section 3.3 "PV Configuration" lays out the guidelines for the solar array. It gives minimum, recommended and maximum number of individual solar cells in series, i.e. " Minimum number of cells in series: 72 (2x 12V panel in series or one 24V panel)".

Is the number of cells important or is this a generalization to get the user in the correct operating voltage range? That voltage range being Vbat+5 to 10-15% below the voltage limit.

For example, charging a 12v battery bank with a controller that has a recommended series cell count of 72. Using a single 24v panel, would it matter if the panel had 72 cells or 60 cells as long as the voltage is within the parameters?

I am using the Victron Energy MPPT Calculator Excel spreadsheet to determine voltage range vs temperature for individual panels.

Thanks

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Guy Stewart (Victron Community Manager) answered · May 29, 2020 at 01:26 PM

The operating voltage range is what is important (within the minimum and maximum parameters that you specified).

The number of cells is a shorthand to assist with sizing, and reality checking.

A 12V battery would be fine to charge from either a correctly installed 60 or 72 cell panel.

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coghst commented · May 30, 2020 at 02:58 PM

Excellent. Thanks for clarifying Guy!

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Answer 1 · 2020-09-15T05:23:33Z

childcarepro answered · Sep 15, 2020 at 05:23 AM

@COghst, what an eagle eye you have! I bought 24V nominal panels for my 24V battery bank and Victron SS 100/50. I just learned about MPPT SCCs requiring 36/72 cells (not 30/60 cells) to function well for 12/24V nominal panels. And there it is, right in the manual!

For your 12V battery bank, 60 or 72 would work equally well. But for my 24V LiFePO4 battery bank and my 60 cell 24V panels, not so much. My options are:

1. Series connect my panels (for terrible partial shade performance);

2. Live with suboptimal performance as my battery approaches full charge;

3. or consider direct connecting to my panels using something like Electrodacus's solution.

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