BMS and the Knees

BMS stands for Battery Management System.  It provides two things, Protection and Balancing.  If it doesn't do those two things, you don't want it.

Lead batteries have a linear charge/discharge profile. The percentage of charge is directly related to the voltage.  12.8 volts is fully charged, 12 volts is 50% and 10 volts is pretty much dead.

The Lithium profile is quite different.  You could be 90% charged at 13.3 volts and have 20% remaining at 13.1 volts. 

Because of such a narrow voltage range, we can't use voltage tell the charge of a Lithium battery.  A device needs to keep track of how much power goes in and out and use that as the State of Charge (SOC).

The big problems happen where the curve goes from basically flat and around the corners (The Knees) during the final stages of charge or discharge.  At the Knees, a little bit of charge goes a long way with regards to the voltage.  

The battery could be charging nicely at 13.3 volts and be 90% charged, It could have even taken all day to get from 13.1 volts (20% SOC) to this point.  Then in a matter of minutes the voltage could go to 14 volts and Thermal Runaway a few minutes later. 

The BMS is what provides protection at the Knees.  A good BMS is imperative in any installation.