Cell Balancing post Recall 196..

Thx, we go in for recall 196 on the 23rd

 

Great information. Unusual balancing is done at 80%. I wonder how effective this will be? If the charging continues and the internal resistance of a cell is much different, then the cell would get more charge thus becoming out of balance again.

 

I think we've been assuming it's "balancing" based on what we think the problem is. It very well may be but noting that in this case they're already balanced within the granularity of the TP voltage measurement reading and very likely what the BMS actually sees. But the BMS could instead be checking for internal leakage by looking for an unusual drop in voltage over the 10 minutes.

Having the Bolt join us in the fun we having, the idea that faulty cells are the root cause has gained traction.

80% could have been chosen because it's a compromise between safety and the sensitivity of detecting a deteriorating cell within each group (of three cells wired in parallel).

 

After looking through some of the spreadsheets on JejuSoul's github site, the omission of maximum deterioration appears to be common throughout. Minimum deterioration is expressed as a percent (from 100).
I would venture a guess that maximum deterioration is actually the same as the SoH.

 

My thought also - take a look at the equation structure for each of these two pids...

 

I'm still wondering what, if any, the "balancing" mechanism actually is. Has any evidence of
passive or active energy-transfer circuitry been brought to light at all? Or is this just a "pause
and hope for the best" algorithm that still depends on a good prior match?

_H*

 

I've seen no direct evidence other than voltage and current sensing. But while pursuing through what docs I have I did come across this little gem, which has a familiar ring to it despite being two years old:

The context of "first" and "second" is not entirely clear, whether a one time only thing on a new car or with every rapid charge.

 

The simplest method would be to shunt higher cells through a load resistor and drain them down to match the rest of the pack.

They might also use a silicon based solution to create a bypass that clamps max cell voltage, but they can't take cells out of the loop completely since there are no switching elements on each cell which would allow that. Bypassing the cells might also need thicker wire than what looks to be used for the CMU since it would need to handle the full charging current. (I'm basing this off the Chevy Bolt pack teardown by Weber Auto, which is probably close enough)

Does anyone know if the CMUs are liquid cooled? Knowing if they are able to dissipate a lot of power might be a clue.

How do other EVs manage charge balancing?

 

From observation of the video, the current draw continuously is about 1.6 to 1.7 amps. Not a large amount to require liquid cooling. I'm curious if the balancing is a software routine that is triggered by telling the CMU that it is at 100% SOC. May be the same software routine that is used at the normal 100% SOC balancing. It does look like simple shunt balancing. I've seen several claims to balance pushing energy from one cell to another but haven't seen it done at this scale and configuration.
It looks like the larger battery pack is a big disadvantage over the small Tesla battery configuration. If a Tesla battery shorts, it burns off the bridge and takes it out of the circuit, reducing slightly the overall energy of the pack. No need to worry about fire since the energy in a single cell is not likely enough to ignite especially if it exceeds the amperage to burn out the bridge and take it out of the circuit. The Kona, having such big cells does need to take extra precautions as a short has a much higher potential to heat. I'm betting a short could quickly exceed the capability of the liquid cooling to dissipate the heat.

 

The service info shows interconnections at a subsystem level only, I haven't seen any detailed electronics schematics or any hints about how the CMS units work.

 

Balancing will be done in the same unit that monitors cell voltages, as it has connections to each cell.
Bear in mind that balancing will be making small changes to adjust cell voltage, and can do this fairly slowly, so the power dissipation isn't likely to be very high. There are some pics of some BMS modules on the openinverter forum - looks like a bunch of transistors and resistors, e.g.
Kia PHEV : https://openinverter.org/forum/viewtopic.php?f=13&t=1179
Zoe : https://openinverter.org/forum/viewtopic.php?f=13&t=1179

 

Cell balancing would be very, very slow if triggered for the reasons inferred above. If there's no pause then either it's not applicable under DC charging (as I assume you're suggesting) or perhaps it's only because your cells are already sufficiently balanced.
Based on a very rough calculation, the energy required to shift one cell group by 0.02 V is about 11 Wh. So, assuming a resistive balancing strategy, a resistor dissipating 10 watts local to the CMS would need just over an hour to make that adjustment on one cell group. If they use another methods that could be less but I'd be surprised if Hyundai broke any technological hurdles in the Kona's BMS.