Internal power supply paths

[martinjaeger]

There are multiple options for the internal power supply of the BMS IC and the MCU. This issue collects some ideas and describes the current implementation.

The power supply has to fulfill the following features:

1. Not damage any parts of the circuit if the cells are connected before there is a solid GND connection through the BAT- wire.
2. The internal power supply shall not influence the cell voltage measurements (i.e. ideally not go through the balancing wires).
3. The system must go into a defined and safe state if the BAT+ wire is not connected (e.g. because the fuse between the BMS and the top-most cell is blown).

I have re-wired the cell connector such that there is an additional GND wire (also going to negative terminal of the first cell). This wire provides the GND connection for the BMS and is required according to the bq76952 datasheet (point 1 from above).

If the BAT+ pin is not connected, the MCU will currently not be powered and the bq76952 chip should stay in a low power mode. Other options would be to power only the MCU, but put it into a low power (emergency) mode, supplied from Cell 16+ through the balancing wire. There are some stuffing options and 0R resistors on the board for experimentation with different power supply paths.