Ampera CAN bus / where is BMS traffic?

[clanger9]

I have a dumb CAN bus question:

I have connected my Due board to the OBD2 port on my car and I can see the CAN traffic in SavvyCan. Thousands of frames/sec. Yay.

However, I don't see any BMS frames (0x460, 0x470, 0x7E0 etc). Where are they? Do I need to connect to a different CAN bus? I was under the impression that all CAN traffic appears on the OBD2 port.

[arber333]

However, I don't see any BMS frames (0x460, 0x470, 0x7E0 etc). Where are they? Do I need to connect to a different CAN bus? I was under the impression that all CAN traffic appears on the OBD2 port.

Well BMS (BECM master) is located inside battery and communicates with BICMS (slaves) at 125kbaud CAN.
BECM in turn relays its data to HPCM2 module which is a decision maker for BMS, charger and battery heating/cooling. That uses 500kbaud lines. There are 2x 500kbaud lines in Ampera and there are two obd2 ports in Ampera. Try the wiring schematic from GM.

[clanger9]

Thanks - I hadn't appreciated the CAN buses were arranged hierarchically. I have the GM manual here so I will have a bit more of a read...

[clanger9]

I found a bit more info. Apparently, the four 500kbps CAN buses are brought out onto different pins on the two OBD2 connectors:

Driver's side:
6(+) 14(-) onstar, parking brake, fuel pump, brakes, both drive motors, Engine control, HV drive electronics, AC compressor
12(+) 13(-) brakes, both drive motors, Engine control, HV drive electronics, accelerometer, steering

Passenger side:
12(+) 13(-) both drive motors, Engine control, HV drive electronics, 12V inverter
3(+) 11(-) HV battery charger, HV battery electronics

I will try running a capture on some different pins...

[arber333]

Passenger side:
12(+) 13(-) both drive motors, Engine control, HV drive electronics, 12V inverter
3(+) 11(-) HV battery charger, HV battery electronics

There you go. 3 and 11 are then.

[clanger9]

So, another n00b question: do I have to issue CAN bus commands to the OBD2 port to get the BMS to respond for each module?

I see frames on pins 3/11 from BMS module 1 (0x460) every 250ms, but nothing for the other modules (0x470, 0x461 etc.)
I had assumed I'd see traffic from all the modules, not just the first one.

What am I missing? Do I need to sniff the BMS bus directly to see these frames?

[arber333]

That is BECM communicating with HPMS2 module. BICMs are connected on the CAN bus inside battery case. 125kbaud Can does not go out of the battery.

[clanger9]

OK ta, that would explain it. So, follow on question: can I get cell voltages over the OBD2 port?
Do I need to send secret/undocumented CAN messages to say "Get me cell1 voltage" etc?