4S 40A Battery Balancer.

A Battery Management System, or BMS for short, is used to protect your battery during charging and discharging. During charging, the BMS will monitor the voltage of all of your cells and balance the cell groups to ensure they are charged equally.

The 40A 4-cell Battery Balancer is used for 4-cell 18650 rechargeable batteries. The circuit has a separate charge function for each cell, automatic shutdown when full, short-circuit protection, over-voltage, and under-voltage.

4S 40A Battery Balancer Specification:

Product Size: 46 * 60 * 3.4mm (balanced version)
Charging voltage: 16.8V - 18.1V
Continuous discharge current (upper limit): 40A (if the cooling environment is not good, please  reduce the load current use)
Continuous charge current (upper limit): 20A