FoxESS Community - Owners & Installers Forum

Foxcube batteries discharging below minimum SOC level, they are in the attic which is a fairly stable 12-13 degrees. The discharging beyond minimum SoC is not every night, they get charged off peak 00.30 until 07.30 on Octopus eco7. It doesn't seem to follow any pattern, has anyone got any ideas?

I have cube batteries in the utility room, the temperature around the batteries is around 13 deg.C on cold-ish days and nights, so similar to yours.

Minimum SoC is currently set to 20% as the batteries rarely get to 100% charge from solar at this time of year. I notice that the batteries do drop below 20%, usually in the early hours, dropping as far as 16% sometimes. The system tops them up from the grid, presumably to stop them over discharging. It seems to do this in bursts of some 500 W for 40 mins or so.

So why does the battery charge drop, even though no power is being taken from it? I suspect it is because it is actually difficult to determine battery charge state with any accuracy. On smaller products with rechargeable lithium batteries, the main way of doing this is with a monitoring system that measures charge in and charge out, and also battery voltage. Integrated circuits that do this are known as gas gauges.

The Fox system monitors battery temperature too.

My guess is that the battery gets used during the evening (discharging), which keeps it a little warmer than the surroundings. Once it has reached minimum SoC and stopped doing anything, its temperature drops, especially if this coincides with a night time drop in the environment temperature. This may cause the apparent SoC to drop, as seen by the monitoring system, so it feels the need to add some charge. As I say, just a guess, this is outside my area of expertise.

I did try reducing the Min SoC to 10% for a time, while leaving the Min SoC on grid at 20%. As we never lost grid during that time the minimum SoC was effectively always 20%. Curiously, the system seemed to charge the battery from the grid with rather more 40 min bursts than it does with both minimum SoCs set to 20%. No idea why.

I intend to reduce minimum SoC to 10% to give more usable battery capacity as the summer approaches. It will be interesting to see what happens then, also when the weather is warmer.