richard said:
I recommend reading the key conclusions on page 11 at the very least.
That survey contains no data which can shed light on the question of whether capacity degradation slows down as the battery degrades. On top of that, the results were written in December 2012 when we had even less data on how es battery degrades.
Let's go through this again: The BEST way to determine whether capacity loss slows, accelerates or stays constant, is to track capacity losses for INDIVIDUAL VEHICLES. But capacity loss varies dramatically with temperature, so we need to be able to capture data which is recorded under similar temperature conditions. Unfortunately, this eliminates nearly ALL of the data we have available to us.
But we do have one data point which sheds like on this question: cyellen's LEAF bar losses
Bar 1: June 8, 2012: 10200 miles
Bar 2: August 20, 2012: 11558 miles
Bar 3: July 31, 2013: 17500 miles
Bar 4: October 4, 2013: 18942 miles
The hottest month in Phoenix is July and 2012 was hotter than 2013.
Bar 1 to Bar 2: 72 days and ~1350 miles
Bar 3 to Bar 4: 65 days and ~1450 miles
So, for this particular Nissan LEAF, the fourth capacity bar was lost seven days FASTER than the second bar. This is true, even though the temperatures were cooler on 2013 when the fourth bar was lost. OTOH this LEAF was driven 7.5% farther between the bars 3 and 4 than between bars 1 and 2.
No, this data is not perfect and it would be great to have more points. But it IS real data and it directly contradicts unsupported statements such as this one:
WetEV said:
Two bar loss or 80% means that your battery is about half way in time and cycles to 70%, a normal EOL percentage.
This statement implies that capacity loss between 80% and 70% occurs at HALF the rate that it did between 100% and 80%. But the data we have for this car indicates the following:
100% to 78.75%: 16.5 months and 11558 miles = 1.28%/month and 1.84%/1000 miles
78.75% to 66.25%: 12.5 months and 7384 miles = 1%/month and 1.69%/1000 miles
This latest calculation is not as interesting as the earlier one showing no slowing of degradation since this result contains seasonality which affects short-lived LEAF batteries in Pheonix much more significantly than it affects those of us who go through many seasons before losing our first bar. Still, even this distorted result indicates that hoping for capacity loss rates to drop by half after losing the first 20% will result in severe disappointment. Most likely, the time and miles between the losses of bars 3 and 4 will closely match those between bars 1 and 2, assuming similar temperature and driving profiles.
Feel free to dig up more high-quality data like Stoaty has collected from cyellen with bars two and four lost under very similar conditions. More is certainly better. But don't expect those results to be much different than this first data point.