Lithium batteries have its safe discharge charging and overcurrent restrictions.
The protection board is added so that the value does not exceed the safe range when using a lithium battery.
For example, a lithium battery safety voltage limit is 4.3V, if you do not add a board, then it will always rush down when charging, just like a person is full, you still ask him to eat, and finally to die. The lithium battery won't die, but he will flatten.
The role of the protection board is to stop the power when he is full.
The principle of discharge and overcurrent detection is the same.