This is the 5A 4S Active Balancer that we use in all our batteries. It’s a great little unit, as it ensures that cells are within a few millivolts of each other. It’s especially useful if you are drawing reasonably high currents as this can create voltage differences in the cells due to minute differences in internal resistance.
It’s much more useful than the 40mA to 150mA passive balancers that a BMS might have inbuilt (a passive balancer is just a resistor that burns off high voltages, as opposed to an active balancer that moves charge from one cell to another).
This active balancer also ensures that you will get the maximum usable capacity out of your cells, even after a decade or more.