Foxess 4800 L9 Battery or BMS issue
Posted: Wed Oct 07, 2026 3:21 am
I have a FoxESS EQ4800 L9 battery with 98% SOH, a reported maximum capacity of 41.09 kWh, and a KH10 inverter.
I’m seeing what appears to be an 18.8% energy loss when discharging the battery from 100% down to 20%.
Discharge figures:
This means I’m seeing a difference of approximately 6.17 kWh, which is around 18.8% of the expected usable energy.
When charging to 100% auto gain ~4 kWh:
I’ve also noticed something similar when charging.
When charging from 20% SOC (8.22 kWh) to 100% (41.09 kWh), I would expect approximately 32.87 kWh of energy to be required.
However, the system only records around 28.80 kWh of energy used for charging to 100%.
Based on this, I suspect that around 2.2 kWh could be normal charging/discharging efficiency losses, while the remaining ~4 kWh may be related to the BMS, SOC calibration, or potentially a battery fault.
I have already tried discharging the battery down to 10% SOC and then slowly charging it above 80% at around 1–2 kW, but this didn’t resolve the issue.
I’ve been noticing similar figures for the past few weeks.
Has anyone with a FoxESS EQ4800 L9 experienced similar behaviour? Could this be an SOC calibration/BMS issue, or should I be concerned that there is a problem with the battery?
Any advice or suggestions on how to diagnose this would be appreciated.
Thanks!
I’m seeing what appears to be an 18.8% energy loss when discharging the battery from 100% down to 20%.
Discharge figures:
- Battery capacity: 41.09 kWh
Energy remaining at 20% SOC: 8.22 kWh
Expected energy discharged: 41.09 − 8.22 = 32.87 kWh
Actual energy delivered: 26.70 kWh
Energy difference/loss: 32.87 − 26.70 = 6.17 kWh
This means I’m seeing a difference of approximately 6.17 kWh, which is around 18.8% of the expected usable energy.
When charging to 100% auto gain ~4 kWh:
I’ve also noticed something similar when charging.
When charging from 20% SOC (8.22 kWh) to 100% (41.09 kWh), I would expect approximately 32.87 kWh of energy to be required.
However, the system only records around 28.80 kWh of energy used for charging to 100%.
Based on this, I suspect that around 2.2 kWh could be normal charging/discharging efficiency losses, while the remaining ~4 kWh may be related to the BMS, SOC calibration, or potentially a battery fault.
I have already tried discharging the battery down to 10% SOC and then slowly charging it above 80% at around 1–2 kW, but this didn’t resolve the issue.
I’ve been noticing similar figures for the past few weeks.
Has anyone with a FoxESS EQ4800 L9 experienced similar behaviour? Could this be an SOC calibration/BMS issue, or should I be concerned that there is a problem with the battery?
Any advice or suggestions on how to diagnose this would be appreciated.
Thanks!