We're building a BMS that has a micro on each cell in the pack. I know this isn't standard and the issues that come with it, however the pack isn't a standard layout, and the benefits out weigh the issues (if I can figure out solutions ;) )
The main issue I'm stuck on is connecting micros at different voltage levels to the same I2C bus (actually SMBus) at a reasonable cost (aiming 50c, any lower is a bonus)
Ideas looked at:
Opto/capacitive isolation for each channel, while this is by far simplest I believe this is too expensive $2 per channel. (Happy to be corrected)
(very useful resource for those interested AN10364: Opto-electrical isolation of the I²C bus www.nxp.com/documents/application_note/AN10364.pdf)
The obvious first place to look is at gnd:3.3V -> gnd:5V level shifters that are used in i2c networks, there are two ways I can see these be used, but with their own issues.
If you level shift between each cell, you get low voltage shifts (making it lower cost) and infinitely scale able (in theory) in terms of cells in series.
However, you don't have common grounds, So while it works in one direction (3.3v signal pulls the 5v signal low) it doesn't in the other (5v can't pull the 3.3v signal low)
With this design, it effectively a direct replacement for the opto/cap isolation. It goes from what ever the voltage is at the cell down to 3.3V on the i2c bus. This should fit closer to the traditional level shifter, but when the signal drops to digital O on the high side, it should only drop to the cell voltage,
Intrigued to know if
a) I've totally missed an easier way of doing this,
b) If anyone has any helpful pointers to anyone who's done this before,
c) I'm a bubbling buffoon and it'll never work.