Ampera CAN bus / where is BMS traffic?

Discussion about components from Ampera/Bolt and the PSA group which owns Opel these days
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clanger9
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Ampera CAN bus / where is BMS traffic?

Post by 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. :?
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Re: Ampera CAN bus / where is BMS traffic?

Post by arber333 »

clanger9 wrote: Sat Nov 14, 2020 1:06 pm 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.
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Re: Ampera CAN bus / where is BMS traffic?

Post by 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...
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Re: Ampera CAN bus / where is BMS traffic?

Post by 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...
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Re: Ampera CAN bus / where is BMS traffic?

Post by arber333 »

clanger9 wrote: Sat Nov 14, 2020 6:08 pm 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.
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Re: Ampera CAN bus / where is BMS traffic?

Post by 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?
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Re: Ampera CAN bus / where is BMS traffic?

Post by 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.
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Re: Ampera CAN bus / where is BMS traffic?

Post by 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?
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