Odd battery discharge

Hi could anybody help with this strange discharge from my battery it went from 40% to 2% in 30mins with no load?

System :10kw solar and 20kwh battery (2x9.5 givenergy)

R9mey Two batteries is an extra complication I can't help with. But it's a known problem that the GE system isn't very good at tracking the true State-of-Charge of the battery. One problem is the the LFP chemistry has a very flat voltage curve over most of the range, so just the voltage alone is not enough. Particularly when the system is under load (charging or discharging) the voltage is a poor indicator of SoC.

So from time to time, when the battery is idle, the BMS gets a chance to do some evaluation, and may decide that it was wrong, and adjusts the reported charge accordingly. If you search for "sudden" on the forum, you'll find lots of threads about it.

Does your battery routinely visit the extremes of charge (100%, or 4%) ? If not, the reported SoC can drift a long way from the true SoC. I allow my battery to get down under 10% most evenings, and then allow it to sit idle for the first hour of the cheap rate before starting to charge it.

Great reply thanks alot

in the winter the batteries regularly discharge to 10% or 15% and charge to 80% or 90%
but since the summer has arrived I hardley need the batteries so they sit mostly arround 80%
so maybe I let them vary a bit more

Thanks again
Chris