BMS (Battery Management System) - a battery management system that is designed to monitor the status of batteries, control the process of charging / discharging the battery, etc.
Two batteries (18650) can be connected to the module simultaneously. Features of this controller: protection against short circuit (short circuit) and protection against overdischarge (overcharge). Also a feature of the module is to turn off the power supply for the batteries when the maximum capacity of at least one battery is reached. The maximum operating current of this BMS controller is 3 A, and the quiescent current reaches up to 10 μA. The battery charge is switched off when the voltage reaches 4.25 - 4.35 V, and the discharge when the voltage drops to 2.3 - 3.0 V.

The HX-2S-S01 battery charge-discharge controller module has 5 pins:
B +: positive pole of the battery
B-: battery negative pole
BM: mid-point battery assembly
P-: negative charge / discharge terminal
P + positive charge / discharge terminal
If the maximum permissible value of the supply voltage is exceeded, the charge controller activates the "protection" mode. In order to restore the module, it is necessary to re-apply voltage to the input of the BMS board or wait a while until the charge controller automatically disables the protection.
The voltage range of the module is from 8.4 to 9 V. For charging, you can use a step-up voltage converter with a current limiter.