FoxESS Community - Owners & Installers Forum

I added 2 new v1 slaves to my existing v1 setup 6 months ago.
Installed as per guides on here, ensuring exisitng stack was 70% soc, got all green lights etc, now have 11.2kwh capacity.
However since day 1 ive experienced sudden unexplained drops in the soc as the soc drops below 40%
I understand the existing batteries were 2 year old and so may not have 100% health, and that it can take weeks for the system to balance.

However 6 months later my system is still dropping between 12 and 20% in one sudden dip as the soc drops below 30%
It's only really become noticable again in recent weeks as less sunlight means more battery use.
For the last week Ive been following guidance from here re cycling the batteries from 10 to 100%, slow charging at 5amps, and letting it sit at 10% for a few hours to bottom balance before charging. But it doesn't seem to be improving.
Ive used a tool found on here to check cell voltages remotely but am not sure if its telling me something useful... cell 18 on all 4 battery units consistently is 0.02v lower than the other 17 cells in each unit... is this normal?

Can I just double check you actually meant 70% soc when you added the new batteries? - as the recommendation is 50% +/- 3% a 20% difference will take time - admittedly 6 months should be plenty but only if you have done a number of regular grid charge cycles to 100% and back down to 10%.

The graph you have posted looks ok, cell 18 usually does that - i’m guessing that is an internal design thing with lead lengths or sensor pickups so that all looks fine, but the best time to run the graph is when the battery stock does (or has done) it’s soc drop as that will show which of the batteries (or cells) is falling off - could you repeat the graph when you have the rapid fall off.

Thank you Dave,

Glad to know the cell 18 voltage is not out of the ordinary.

I found an installation guide on these forums that said to have the battery at 70% and followed that guide to the letter, unfortunately it looks like it got me off to a bad start.

They've had a dozen or so full cycles since being installed, but not very often as the battery has been topped up with solar during the day so hasn't discharged down to 10% much at all over summer.
they're getting fully depleted by 9pm daily from here on, and charged over night.
Ive used a home assistant automation script I think you wrote to reduce charge current once it gets to 93% so will run that for a week or so and see how it goes.
Thank you

ok keep an eye on it, the batteries are designed to maintain small differences but a 20% swing will take it much longer and the slow charge at the top will really help with that.

Sorry I should have said, also make sure your battery firmware is the same on all of your batteries

Hi please Can you help me understand the normal ranges for cell voltages?
I've got one battery with quite a large range within the battery unit which becomes more pronounced the lower the level of charge, and theres also a difference between the newer and older batteries (which I expected given the starting point was 20% off) .
From what I can deduce, 60v (18 cells at 3.33v) indicates the battery is fully charged, and 56.7v ( 18 x 3.15v) indicates the battery is around 10%.
Is this roughly correct?