It's All About the Delta (and Temperature, but the BMS will Protect You) -- Test: Effect of temperature on battery cells life

[Maxx]

The x axis covers 20 cycles of the RPT testing.

How many actual cycles does that translate to? In the graph you posted, that is a sharp drop at 10 cycles. That seems awfully early even at 90% SOC to be regular cycle.

he warned us a few months ago about the the low thermal stability point of the cells shown in one chart. He stated our cells have a thermal stability point that is 95 degrees F lower than what he has seen in similar cells of the same chemistry.

Can you expand on this? What do you mean by lower thermal stability? Does that mean our cells do better with hard acceleration and max regen at lower temperatures but not so much at high temps comparing to other EVs with similar battery chemistry? Does it mean they lose capacity at high DOD and high temp faster than they do at low temp comparing to others. Or it means something else?

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[Maxx]

Does anyone understand what is up with the green line in the left? Why is 30% is dropping faster than 90%?

 

[Scorpio3d]

TLDR: 1. The most important issue and key takeaway is to avoid large temperature swings. The BMS should take care of this, and seems to do a fine job given the battery temp is always in the middle on my dash display. The original leafs with air cooled batteries degraded so quickly because a lack of a BMS and temperature regulation.

2. Avoid large charge/discharge cycles. It seems not as important as 1 according to Ford, since they say you can charge to 90% every day. These batteries love to be closest to 50%. You can take the one way % consumption of your commute and add it to 50%. If you use 15% to get to work, charge to 65%. By the time you get home, you should be at 35%. This keeps you closest to 50%. IMO, I just charge to 80 every night and 100 for roadtrips.

I read this twice as well, even though this is well out of my wheelhouse (so far out that I am in a rowboat and you are in a rocket)and I think you overall nailed it except for maybe this one thing he said
@MickeyAO said:
“On the side, I ran my own test cycling between 80% (actual so 85% SOC on the display) and 50% SOC. Results closely tracked the cycle 30% delta at 25C as I suspected, but I did not graph it.”
so my thinking is that even with a higher state of charge at 77°F it is basically the same??

 

[Grumpy2]

I am not an expert only reading and learning along with everyone else. I just like to know how things work.

He said he only provided "normalized graphs, so I assume this is the end result of a large amount of tables, charts, and graphs. This testing took many months, so you can count on a box full of test results behind this summary lines.

What do you mean by lower thermal stability?

Others have used this term to describe the loss of stability, i.e. thermal runaway... This temperature limit is almost impossible to obtain unless your in an extremely hot environment situation.

As the OP advised, you now have a 7 year warranty that has few limits on how your treat your truck, so just enjoy it. We will probably learn more in the years ahead of how these batteries survive.

 

[Scorpio3d]

How many actual cycles does that translate to? In the graph you posted, that is a sharp drop at 10 cycles. That seems awfully early even at 90% SOC to be regular cycle.



Can you expand on this? What do you mean by lower thermal stability? Does that mean our cells do better with hard acceleration and max regen at lower temperatures but not so much at high temps comparing to other EVs with similar battery chemistry? Does it mean they lose capacity at high DOD and high temp faster than they do at low temp comparing to others. Or it means something else?

I think lower thermal stability comment had more to do with the way our battery packs are made or the difference between them and the battery cells/packs from other manufacturers. I.e. Ford packs versus GM packs or Tesla, etc.

 

[MickeyAO]

Can you expand on this? What do you mean by lower thermal stability? Does that mean our cells do better with hard acceleration and max regen at lower temperatures but not so much at high temps comparing to other EVs with similar battery chemistry? Does it mean they lose capacity at high DOD and high temp faster than they do at low temp comparing to others. Or it means something else?

Thermal Stability is the highest temperature a cell can sit at without an effect to the cell. Once you go above this temperature, the cell will start going through an a thermal reaction and will start increasing the temperature of the cell without other inputs. It will steadily increase in temperature until it hits thermal runaway temperature.

 

[MickeyAO]

Does anyone understand what is up with the green line in the left? Why is 30% is dropping faster than 90%?

The static capacity of that particulate sample is dropping faster than the other sample. I think I called it out this out in my surprises.

 

[MickeyAO]

I read this twice as well, even though this is well out of my wheelhouse (so far out that I am in a rowboat and you are in a rocket)and I think you overall nailed it except for maybe this one thing he said
@MickeyAO said:
“On the side, I ran my own test cycling between 80% (actual so 85% SOC on the display) and 50% SOC. Results closely tracked the cycle 30% delta at 25C as I suspected, but I did not graph it.”
so my thinking is that even with a higher state of charge at 77°F it is basically the same??

Yes. I duplicated my normal drive cycle on the side. 30% delta with the upper limit at 85% SOC (displayed, 80% actual)) closely tracked closely with the 30% delta at 25C at 50% SOC as the center point.

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