Here's how to command your OEM Chevy Volt BMS BICMs (slaves) to bleed charge off of specific cells in order to keep them in balance.
This does not involve the BECM (master). I wasn't able to figure out the command(s) to send the BECM to get it to initiate this work. This is a more manual method that involves sending the BICMs commands indicating which specific cells to bleed. You still have to figure out which cells are out of balance and do the heavy lifting of dropping them to the right level. This would be done by continually sending these commands to the BICMs on a CAN bus with just the BICMs, no BECM.
A little background is useful. In normal operation every 200 milliseconds the BECM sends 3 CAN frames: 0x200, 0x300 and 0x310. If the 0x200 CAN frame contains data content of 0x020000, all connected BICMs send back the voltage and temperatures of their battery modules. That much has been documented.
It turns out that 0x300 and 0x310 are queuing commands for bleeding energy from specific cells. When a specific cell location is sent in either of those commands, the next 0x200#020000 frame causes the BICM to bleed energy from that cell for about 130 milliseconds.
Attached is a scope cap showing that action. This was taken from a BICM PCB across a 1 ohm resistor on the trace that connects the cells to the measuring/bleeding circuits.
The first two blips are the BICM opening a small transistor on the trace to measure the cell voltage. Negative blip is the cell we're watching, positive blip is the next higher cell.
Then since this cell was commanded to bleed, the BICM opens its circuit to bleed across a couple of 15 ohm resistors in series. There are a couple of 1 ohm resistors and a diode in that path so the bleed current ends up being about 100 mA.
If the right 0x300/310 command is sent, this happens every 200 msec.
I've mapped out the right 0x300/310 data bits that correspond to each cell and will drop them in the next post.
