[tgrandahl]

FYI, a word of caution following an unfortunate experience... If you leave the original Chevy volt BMB on the 5kWh block from a Chevy volt pack, and disconnect the bus bar links that connect the 2kWhr modules and the 1kWhr module together to become a 5kWhr assembly. It will discharge the bottom three cells in one of the 2kWhr modules. I just learned this the hard way... On two packs at the same time...

I haven't studied the root cause in detail and don't plan to but it appears to be due to the fact that the 5kWhr BMB splits cell sensing across 4x isolated ASICS that handle sensing for the 3x modules within the 5kWhr assembly. So by electrically isolating those smaller modules I also would have split the connections to at least one of those ASICS. I expect that as a result of doing this the cell balancing circuitry latched on or something else within the BMB started operating with vastly increased current draw.

This only seems to be a problem for 3 of the connected cells, but i didn't catch this until those three cells were all discharged below 3V, one of them just shy of 2V. :-(

 

[notAhoser]

I had some low cells in my Volt pack like you describe, but how can the BMS discharge the cells on its own? I'm not exactly clear about where you are talking about. I had mine sitting for quite some time and slowly charged those cell groups up and they held without discharging for months. I currently have all of the groups in parallel for about 48 volts. The BMS modules are now missing 24 cell voltages (from looking at the CAN data), so I'm looking into that now. Interestingly the cells on the last BMS are all reporting and the 6 at the front of the pack are reporting, but in between all of the 12 cell groups are missing 4 cells each.

 

[piotrsko]

What year are your packs?

 

[notAhoser]

Mine is a 2011.

 

[kennybobby]

On many of the BMBs boards that i have traced out, the board takes it's own power from some of the cells that it is monitoring, e.g. from the bottom 3 cells of a group. It will use this ~12V to generate 5 and 3.3 as needed to power up the processor, communications, etc. Even though it may be a low current, over time it can drain those cells if counter measures are not taken.

 

[notAhoser]

I have taken a couple of the Volt's BMS boards out now, I will check for something like this today. I'm probably not going to be able to use them anyways. They don't seem to work right when the packs are paralleled vs. the original series configuration.

 

[notAhoser]

I took a look at two of the BMS boards that I have taken out of their housings and there's no voltage regulation stuff on those boards, unless the ASIC somehow does this. These are both of the boards from the largest pack that sits at the top of the "T".

At this point I think I'll just make my own BMS.

 

[notAhoser]

FYI I looked through my notes and I think I had the exact same problem a while back. Three cells at the end of the front pack (module 1, which are cells 66,67,68) were discharged and way out of balance. I was able to manually bring each cell back up and they stayed up, but they were quite low, around 3v.

 

[notAhoser]

OK I have to clean up all of my posts. Ignore everything.

I just went through all the cells on the entire pack and I have this issue, and I'm 100% sure it's due to the "BICM" modules on the packs. In pack 1 it's actually 4 cells at the end of the middle pack. On the other packs the same thing happened. I'm balancing them out right now manually. I just got my whole thing installed a couple weeks ago and it's gone through just 2 charge cycles and 1 real discharge cycle. BLAH! So much for using the factory stuff on a Volt pack.

 

[piotrsko]

In my experiences with the pack the cells go whacky like that when you push above 600 amps. You find the weaker cells.

 

[notAhoser]

In my experiences with the pack the cells go whacky like that when you push above 600 amps. You find the weaker cells.

That's not what this is. It's due to the removal of the series-connecting bus bars between the packs. I have never stressed my pack that much, I just have it running a 4400w inverter.