RSC said:

May 2022 readings

Doesn't look like I'll qualify for warranty replacement. The warranty period for me ends on 4/19/2023. My warranty booklet (2015) says up to 35% degradation. If I assume that my readings are correct and that the initial capacity is 19 kWh, I would have to get down to 12.35 kWh.

There are ways you can accelerate or reverse your HV lithium pack's capacity. However, and this is my 2 cents on battery replacement, there isn't a chance the Spark will see out-of-warranty replacement packs bought by owners at a price even 1:1 with the blue book value of the car. OEM is around $1200/kwh after all. So the battery replacement warranty is a pretty important milestone for each Spark if these cars are going to remain on the road and in circulation in the used market for the next decade until capacity drops below that fuzzy real-world threshold of feasible range. Also, hey things are going to get a lot more expensive by the end of this year for a long time. It might just come down to necessity.

Speaking of necessity, if any of you had the choice, would you augment your original battery pack's capacity if you could? If so, would you change your behavior to reach the battery warranty in time? Or would you instead try to rejuvenate cells and reverse the degradation?

Anyway, onto charts:

Last time I provided an exponential trendline using both your individual degradation data because it had the highest R^2. In other words, it fit the data points best. However, I was skeptical of this extrapolation, so I scouted for some li-ion degradation papers and found this one regarding calendar aging.

https://www.semanticscholar.org/paper/Modeling-of-Lithium-Ion-Battery-Degradation-for-Xu-Oudalov/09b0c41c2f131e1acebd65297799990368cc3cab

If you look at some of the image previews you can see that there is initially an exponential trend in new packs, but the rest of its lifecycle is linear.

Also, Kermit, I wasn't sure what your warranty date was so I just used RSC's 2015 Spark warranty and advanced it by a year since you have a 2016. I can edit this later if you share it.

Also, I removed mileage from both charts. Partly because it didn't seem to be visually demonstrating much while cluttering the plot area, but mostly because Microsoft Excel is limited to a primary and secondary series set of axes and it's impossible to get a scatter plot with X and Y error bars while also retaining a horizontal date range. Excel's charting rules are actually really nuanced and annoying when you start getting into complex charting.

Because RSC's R^2 is only a 75% fit, there's a chance we could see another big dip in capacity, like in mid-2020 / '21 and the Spark will make it in time for its replacement warranty with your current behavior, but it would have to be JUST in time, and that's without any lead time for drawn-out correspondence with dealers and GM.

Last point, to use the trendline equation, substitute your battery capacity in kWh for y, and solve for x, the number of days after your first data entry. So if you wanted to know what your capacity will be like in 2030, or when you hit 10 kWh, you could do that.