Discharge protection

[FJimmy]

Hi guys,
I've had my 100ah battery wired into my car for about 6 months. 4s configuration, 120ah JBD BMS. It mainly runs my fridge and some 12v accessories. It is charged by a 20a lifepo4 DCDC charger.
When charging, I've noticed that one cell tends to shoot up past 3.65 up to about 3.8 while the other cells are ~3.4. Charge protection will then kick in, and the cell will slowly drop to 3.4V. When discharging, some cells would drop to ~2.8. These BMS settings are the standard "LiFePo4 Normal" settings. I thought I would use the Overkill Solar standard 4s BMS settings, which keeps cell voltages between 3.0V and 3.4V. I saved these settings to the BMS, and carried on with my day.
Now I've noticed that the BMS is preventing cell discharge; if I tap the button it will momentarily change to enabled, then flick back to disabled. When charging, the discharge button will move to enabled, but current won't flow i.e. my fridge won't turn on.
I've disassembled the battery and checked each cell with a multimeter and cell voltages are as measured by the BMS. The screenshots below are of the cells and BMS only, no charger or loads attached.

Any ideas?

Thank you

 

[FJimmy]

I bought a USB cable and hope that resetting the BMS will solve the issue. I really can't see what's stopping the battery from discharging.

 

[mikefitz]

Use the Overkill app.

Settings
Function configuration
SW switch circuit enable set to off
BMS write

You should then be able to toggle the discharge switch.

You have odd settings in the BMS protection.
Balance volts and low temperature protection and capacity perameters need changing.

What settings did you change for expected 3.0 to 3.4 ? there are no such settings.

Balance start at 3.4 volts, low temp protection, trigger at say 3 deg C, release at 5 deg C.
Capacity perameters set to default, the BMS uses these to guess at SOC.

 

[FJimmy]

Use the Overkill app.

Settings
Function configuration
SW switch circuit enable set to off
BMS write

You should then be able to toggle the discharge switch.

You have odd settings in the BMS protection.
Balance volts and low temperature protection and capacity perameters need changing.

What settings did you change for expected 3.0 to 3.4 ? there are no such settings.

Balance start at 3.4 volts, low temp protection, trigger at say 3 deg C, release at 5 deg C.
Capacity perameters set to default, the BMS uses these to guess at SOC.

I'm kinda annoyed that this was so easy to fix. Thank you mikefitz!


What is SW switch anyway? Is this something the xiaoxiang app doesn't allow the user to modify? That would explain why reloading the xiaoxiang BMS settings didn't fix the problem.

Thanks again

 

[mikefitz]

It's in the xiaoxiang app hidden on the function setting page.
On the BMS board there are connections for a switch option, a mechanical switch, that disables output .
If this switch is not fitted or the board is not populated to allow it to be fitted, the softwere still needs to be told to disable that function.
Glad I could help, it took me some time to solve the same issue.