I've never really investigated cell balancing too much, so I got a two cell protection and balancing board to explore.
The module has three distinct circuitry sections. A two cell protection circuit with charge/discharge MOSFETs to protect against overcharge and over discharge of the cells, where the first cell to reach 4.25V while charging or 2.5V while discharging disconnects the cells from the charger or load.
The other two sections are completely separate and work to bypass low level current across a fully charged cell to allow the other to keep charging.
It's really important to note that the balancing must be done at low current - less than 60mA in this case. A higher current can still overcharge the cells.
The balancing concept is very simple. When a cell reaches 4.2V a MOSFET clamps a resistor across it to prevent the cell voltage from increasing further. That allows current to keep flowing in the circuit to keep topping the other cell up until it also reaches its full charge. The MOSFET turns back off when the voltage drops back down to 4.19V.
The charge current shouldn't exceed the level where the voltage across the bypass resistor can exceed the upper threshold voltage of the cell, as otherwise it will keep charging it until the protection kicks in at 4.25V. It means the charger will have to be limiting the current as the voltage nears roughly 4V per cell.
My thoughts of using an over-simplified three wire charging system would involve the cell protection circuit cutting in when the first cell reached its upper cutoff threshold and then trickling current through the cells directly to allow the balancing circuit to match the voltages. Not ideal, but a lot simpler than a sophisticated charger. The resistor could not be left in circuit for normal use as it bypasses the protection MOSFETs.
A nicer system, and probably the correct one, would just involve the charger putting out marginally above 4.2V per cell, with a series resistor to limit the current progressively as the pack charged to allow the balancing system to do its job.
These are just random thoughts and not intended as a guide to the correct way of using this module.
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