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More reason to minimize charging beyond 88% / Hilltop Reserve Mode.
 

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Sorry. Look again. The 1.6% SoC delta is 0.110 mV. As for 4.180V being too high. It is the result of a cheap BMS, which apparently GM engineers figure is good enough to keep them within budget, and the 60% degradation warranty.
You are right. The delta is 110mV. I mistook one of the numbers.

Perhaps the load balancing has moved charge from the strong cell to weak cell to keep it from shutting down the pack, hence the delta widens again. If you data between 50% and 2%, I bet you will see the difference will continue to shrink.

Actually my data matches yours between 100% to 50%. I just keep going down to 30%, where I see the difference crosses zero into negative. The point; the cell with highest voltage at 100% is the weakest cell. It has the least capacity, and its voltage changes at the highest rate. Load balancing tries slow down its voltage change by injecting / extracting to / from the weakest cell.

I have admit I don't have the courage to drive the soc down to 2%. I simply don't want to get stranded.

-TL

Sent from my SM-N960U using Tapatalk
 

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You are right. The delta is 110mV. I mistook one of the numbers.

Perhaps the load balancing has moved charge from the strong cell to weak cell to keep it from shutting down the pack, hence the delta widens again. If you data between 50% and 2%, I bet you will see the difference will continue to shrink.

Actually my data matches yours between 100% to 50%. I just keep going down to 30%, where I see the difference crosses zero into negative. The point; the cell with highest voltage at 100% is the weakest cell. It has the least capacity, and its voltage changes at the highest rate. Load balancing tries slow down its voltage change by injecting / extracting to / from the weakest cell.

I have admit I don't have the courage to drive the soc down to 2%. I simply don't want to get stranded.

-TL

Sent from my SM-N960U using Tapatalk
No OEM I know of is using active cell balancing as you describe. The difference in voltage gets more dramatic the closer to the bottom I get. You are obviously very enthusiastic about your theories. But they don't comport with anything I have read, or seen with any of my battery packs, be it lawnmower, bicycle, car, etc.

 

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Isn't the linked article describing the same thing? They aren't my theories.

-TL

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The linked article describes all the know methods of balancing cells. The one you imagine you have is called active cell balancing. No OEM that I know of is using this system, because it is the most expensive. Car manufacturers are looking for every way possible to lower the cost of battery packs. Tesla battery packs have been shown to have incredibly tight cell voltage spreads at all states of charge. If anybody is using active cell management it is Tesla. But it appears the actual reason for their small voltage variation is the consistency of their cells, as seen even in packs removed from wrecks and sitting around for months.
 

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I see your point. No, I don't know for sure bolt has active load balancing. I was trying to understand the wider positive voltage spread you observed at 2%, so I said may be.

However, I still think bolt is having some sort of load balancing during discharge. I see that by comparing voltage data between moving and stationary. Professor Kelly also hinted that in his battery pack disassembly video. The low voltage power supply to the BECM needs to be disconnected first to disable the unit before unplugging any other connectors. Otherwise the BECM may detect cell imbalance and will try to do the balancing, and may destroy itself doing so. So I guess I may not be the only person imaging the same thing.

I will put up plots when they are ready. I want to go through a few more cycles to be sure.

-TL

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However, I still think bolt is having some sort of load balancing during discharge.
Well if they spent the extra money for an act balancing system, they did a pitiful job at it. The voltage spread is no tighter than it is on my electric bicycle with a $50 Chinese BMS.
 

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In light of the battery recall, bringing this thread back to life for recommendations. Seems some say cell balancing does not happen unless you charge to 100%... Hm...
 
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