Batteries SOC drops to 87%/13.3V without load

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patrick-johannes asked Feb 24, '22

Batteries SOC drops to 87%/13.3V without load

I have a SmartSolar MPPT 100/50 connected to 420W solar panels and two 300Ah LFP batteries in parallel (12V system). It charges during daytime to 100% SOC/14.3V. But when there is zero load on the batteries, the SOC always trickles down to 87%/13.3V by morning. I charged the batteries to 100%/14.3V and disconnected the charge controller, and they hold their charge all night no problem. Why does the charge controller drain power off the batteries by 13% every night, despite there being no other load on the batteries?

Lithium Battery

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wkirby answered · Feb 24, 2022 at 02:51 PM

What battery monitor are you using that says 87%?
If you are just assuming SOC by the Voltage then that is extremely inaccurate especially for Lithium batteries. They have a very steep curve at the top of the charge. Then a very flat curve until about 10%, the Voltage will not change very much here.

The MPPT does use a small amount of energy during the night because the CPU is still running, it does not go completely off. It uses about 10mA IIRC.
This small amount of energy will cause the battery Voltage to drop down by the Voltage that you observe about 0.1Ah during the night.
13.3V or 3.325V per cell is a very nice full battery.

If you want to see real SOC by % then you need a proper battery monitor if you don't have one already. You need one with a shunt which counts the actual energy in and out of the battery. Measuring the Voltage is not going to help you very much. You need to watch the minimum and maximum Voltages of the battery for sure, don't pass those points, but Voltages in between are almost meaningless.

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Answer 1 · 2022-02-24T15:23:14Z

patrick-johannes answered · Feb 24, 2022 at 03:23 PM

Roger that. I am using a DROK 10-100V Digital Capacity/Percentage Level Gauge. So using the voltage reading on my Victron app, the batteries register 14.3V fully charged, and the app says 13.3 in the morning. It sounds like that is reasonable, despite no load.

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