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 Post subject: Re: Estimating Battery Degradation
PostPosted: Wed May 02, 2018 11:39 pm 
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Joined: Fri Mar 27, 2015 4:13 pm
Posts: 1277
Location: Los Altos, CA
I have a final datapoint from our 2015 e-Golf. We extended the lease to bridge the time until we got our Model 3, but we have it now, so the e-Golf is going back on Monday.

As I indicated before in this thread, I think this method is significantly flawed. However, I tried a couple times to take different data on consecutive days to highlight the variability in the results so people can judge for themselves.

Image

As you can see from the data above, the least squares fit line gives an estimated initial usable capacity of just under 22kWh and a degradation rate of 2.09Wh/day with a final capacity at the end of the trend line of 19.46kWh. However, the last actual data point was calculated as 20.0kWh. The calculated degradation slope correlates to 3.5% degradation per year.

Image

This scatter chart shows how the amount of battery used in the calculation contributes to the resulting value. The yellow and red lines represent data that is time and mileage adjacent, so there's definitely a correlation there.

Image

This last chart shows how the first data point from October 2015 skews the results. When that point is removed and least squares fit is performed on the remaining data, the slope of the degradation line is reduced to 1.13Wh/day and the estimated original usable capacity is lowered to 21.01kWh. That comes to less than 2% degradation per year.

So, that's my data for from our car. It would be much better if we could get more accurate capacity data from the car's computers, but this methodology is the best I could come up with in its absence. The VIN ends in 907055 if anyone cares to look for it later. It's a Limestone Gray 2015 e-Golf LE.

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 Post subject: Re: Estimating Battery Degradation
PostPosted: Thu May 03, 2018 2:06 am 
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Joined: Thu Jan 07, 2016 1:09 am
Posts: 30
Congratulations on the Model 3 finally arriving, miimura! I'll be interested in hearing your thoughts on how you like it and how it behaves, etc.

The Ioniq Electric is super, super efficient—my husband has one, and often has miles-per-kWh readings in the 5s. The guessometer shows estimated ranges on a full charge in the 140-mile range. I realize some 2017 e-Golf drivers can see those ranges, too, but all that range on a 28 kWh battery pack? This is one slippery car!

Still haven't gotten my e-Golf yet, but I'm hoping the 2018s are just around the corner. I'm sort of curious to see what Hyundai will do with the Kona Electric this fall (a 250-mile range is enticing), but I still prefer VW's styling and handling over Hyundai's. That being said, I do appreciate the effort Hyundai put into their first EV.


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 Post subject: Re: Estimating Battery Degradation
PostPosted: Fri Apr 12, 2019 9:34 am 
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Joined: Wed Mar 27, 2019 1:37 pm
Posts: 14
I'm going to throw this out there as the math nerd in me can't help himself. The method you all use for calculating battery degradation is, as some have pointed out, limited by the data that VW gives you. Every single datum you're using to calculate usable battery capacity only has two significant figures, and following sig fig rules, your answer will only have two significant figures. This is particularly relevant because we don't know how VW is arriving at these figures. For example, are they rounding up or simply cutting off the display after the second sig fig? If your average consumption figure is 4.3, does it mean the real figure is in the range of 4.25 to 4.34? Or is the real figure in the range of 4.30 to 4.39?

I just started my own data collection on my 2019, and the data points on the graph will be two significant figures. In my case, my first three y-values are 31, 32, and 30.

That's not to say that you eventually won't get a usable graph out of it. With enough data points, you *will* see a trend, and that trend's slope is going to give you a decent approximation of the degradation per 1,000 miles or per year, depending on how you graph. (Per 1,000 miles is a better way to do it.)

Just throwin' it out there.

--Chris

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