Two cell lithium protection and balancing module (Patreon)

Lithium cell balancer reverse engineering with schematic

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 over charge and discharge of the cells, where the first cell to reach an upper or lower voltage threshold during use 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 reach its full charge. 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 the full charge voltage 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 charge current cannot exceed the level where the voltage across the bypass resistor can exceed the upper threshold voltage of the cell, as otherwise it will stress the resistor and potentially overcharge the cell. It means the charger will have to switch to a lower 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 would involve feedback from the balancing circuits to indicate when the first cell had reached near full charge and switch to the low current balancing charge. These are just random thoughts and not intended as a guide to the correct way of using this module. Supporting the channel with a dollar or two on Patreon helps keep it independent of YouTube's quirks, avoids intrusive mid-video adverts, gives early access, bonus footage and regular quiet Patreon live streams. https://www.patreon.com/bigclive #ElectronicsCreators