I've made some changes and corrections to my previous design, and it is now ready for prototype PCBs. It uses discrete PMOSFETs to select one of four series-connected lithium cells, and transfers charge to a capacitor. Then the MOSFETs are turned off and other MOSFETs apply that charge to the ADC input of a PIC through a voltage divider to measure the voltage of that cell. The top three cells use the "flying capacitor", while the bottom cell is read directly.
The 2.5 V power for the PIC is derived from the bottom cell through a Schottky diode and an LDO. The Schottky and a capacitor isolate the power supply during short durations where the cell may drop to as low as 2.5 volts as can happen when drawing high current during acceleration (this is for an EV).
The bottom cell will be subject to greater loss of charge, so there is a buck switching supply that can charge the bottom cell from the whole pack, which will bring cells into balance.
I have also added the ability to shunt balance each cell by turning on opto-MOS SSRs with a 32 ohm resistor to draw about 100 mA. Because I ran out of I/O pins I used one pin to enable the optos to be driven by the same output that selects the cells.
There is also an I2C port for a display and other peripherals, a connector for a Bluetooth serial module for data acquisition and control, and a digital isolator for connection to a master module that can read from and control multiple slave modules. It will send commands with an address corresponding to each slave module, and it will respond via a diode-OR connection.
Here is an image of the schematic:
A PDF:
See any errors? I know there are many BMS chips available for just a few dollars, and this design will be more complex and costly, but it will be customizable and certainly more "fun".
Thanks (and happy Thanksgiving).
Paul