How low voltage is safe under load for lifepo4?

[pekka8]

My battery is running at 12.9V when pretty empty. When at high 2kW load, voltage drops just under 12V. That's a water heater that turns on for couple of minutes every 3-4 hours. Is this safe or good for battery life in long run?

 

[DIYrich]

12v is fine for LiFePO4. That is 3.0v per cell. Make sure you don't draw more current (amps) than the battery is designed for.

 

[BentleyJ]

The minimum voltage for LFP chemistry is considered to be 2.5V and this is a common value used for the Low Voltage Cut Off setting in the BMS. As long as your battery has a BMS with proper settings it is safe to use at 12V.

 

[pekka8]

Thanks for the answers. Battery is 200Ah with built-in BMS and drawing 200 amps occasionally should be safe.

 

[pekka8]

I continued testing the system with low battery, in order to know what to expect in future. Around 11.5V and 300W load battery started to shut down. There was an audible click and voltage dropped to ~1.35V. Only to wake up with charging. Battery specs say "End of discharge voltage 10.0V". Is this normal operation for BMS, so everything works as should?

 

[BentleyJ]

Sounds like the BMS is doing its job and shutting down at LVCO. Keep in mind that once the cells get below 3V (12V) they will drop fast as there isn't much power left. Also the cells are not be perfectly balanced meaning they will not ALL hit 2.5V (10V) at exactly the same time. Under normal operating conditions you would want to stop discharging the battery at around 12V and not let the BMS disconnect. The BMS is really more of a last resort safety item like a resettable fuse and should not be used as part of the charge/discharge control system.

 

[pekka8]

Sounds like the BMS is doing its job and shutting down at LVCO. Keep in mind that once the cells get below 3V (12V) they will drop fast as there isn't much power left. Also the cells are not be perfectly balanced meaning they will not ALL hit 2.5V (10V) at exactly the same time. Under normal operating conditions you would want to stop discharging the battery at around 12V and not let the BMS disconnect. The BMS is really more of a last resort safety item like a resettable fuse and should not be used as part of the charge/discharge control system.

Thanks, that was my thought too. I'll put an alert for SOC for around 12V. With bigger loads voltage drops temporarily to 11.5 so voltge alert won't work.