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niive avatar image
niive asked

MPPT 75/15 - not charging at high battery voltage

Hi all - happy new year.

I have a setup using the MPPT 75/15 to charge a LiFePO4 (12,8V) battery. My load is a low power LED panel for my garden.

Unfortunately I don't see the charger actually charging the battery even though pv voltage increase to Vbatt+5v. See attached photo. Would you recommend a panel configuration with higher voltage potential? In my area the light these days is very low. Would a panel of same size but with a higher voltage potential (i guess more panels in series) allow me to start charging even though we have these low light conditions?

I also had three similar setups with different loads. The one with the highest consumption started charging before the others. I believe this could be due to the lowered Vbatt of this one? They were all in the same area.

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Thank you :-)

MPPT SmartSolarsolar sizing
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2 Answers
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kevgermany answered ·

Sorry, looks like the panel isn't up to it. There is some charge shown, but only 1W. If it is winter where you are, then there's a chance if it being enough. But looks as if the panel is too small.

Panel voltage is not an indication of the power, as soon as you draw power, the voltage drops, so the MPPT only draws the max amount of power the panel can supply, this means it has to stop the panel voltage from dropping too far.

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niive avatar image niive commented ·

Hi - thanks.

In my design I cannot physically fit larger panels. What would you suggest then? Any charge controllers capable of handling these very low power inputs better?

And would it also explain why a lower charged battery (lower voltage) does charge some and higher charged batteries dont?

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kevgermany answered ·

The controllers are about as efficient as it gets. If you have multiple identical panels, you can get a little more out by wiring in series, this will mean they charge for longer and cut out less with low light. But be careful not to exceed 75V at your coldest temperature.

Just a hint on the readings. Pmax is the highest power output recorded. But it's clearly not for long as there isn't even a Watt hour generated.

What battery charge parameters are set in the controller?

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niive avatar image niive commented ·

Okay I will try putting two panels in series to see the effect in these conditions.

Yes I noticed that as well and came to the same conclusion. I also see the lowest reading in the app is 10Whrs (with 10 Whrs step size) which I guess means at least 10 Whrs has to be generated to have an indication on the bars.

My parameters can be found here:

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