There isn't a lithium battery in a commercial product anywhere in the world that hasn't got a BMS as part of its charging system, its just too dangerous. You can buy bare lithium cells in electronics shops but they come with a severe warning to only follow the charging guidelines which is best to do with a BMS. If you were just charging it with a power supply or a "dumb" charger you only need to forget about it once and hey presto, thermal runaway creates a small bomb. Most consumer level Lithium cells/batteries even the ones that just look like a larger AA cell have a BMS built in to the case
A BMS has 2 main functions, firstly it measures cell voltage and temperature then adjusts the charging current to suit. At very low voltages and temperatures the current has to be reduced until the SOC reaches about 15 +/- 5% depending on the cell chemistry then it accepts a higher rate of charge until about 80% SOC. The current is then reduced in the saturation phase up to 100% SOC. After that the current has to be completely shut off because trickle charging is a complete no no for all Lithium batteries. If at any time the temperature exceeds about 45 Deg C the charging current is reduced and then stopped and it also will not charge at less than 0 Deg C.
The second function of a BMS is cell balancing, most batteries are made up of a number of groups of cells, each cell in a group is electrically connected so they all are forced to have the same voltage but the different groups of cells need to be balanced with each other, The BMS waits until a high SOC then bleeds off current from the group with the highest voltage until it is reduced to either a reference voltage or the group with the lowest voltage and repeats the process for the next highest group voltage.
