Estimating Battery Degradation

[mfennell]

Thanks for posting these updates. My daily charge is 80% - 90% in winter - and I don't drive a lot of miles (14k in 2 years) so I don't really see degradation.

 

[f1geek]

I found some interesting information:

https://avt.inl.gov/sites/default/files/pdf/fsev/2015eGolfDCFCAtTempBOT.pdf

According to this test lab, the battery has 75 Ah total capacity, and I found in the 2015 e-Golf battery warranty, VW says the initial net capacity is 64 Ah, so maybe I was wrong about the initial net capacity being 21.76 kWh (about 90% of gross). If we assume that the initial net capacity is 64/75=0.853 (85.3% of gross capacity), that equates to 20.65 kWh net usable battery capacity. If this is true, then if my car is currently at 18.4 kWh net capacity, then it has only lost 11% of its initial capacity. If I assume a linear rate of capacity loss, then it may still be at 70% net capacity in 8 years, but then again, it may not. There's also the possibility that the rate of degradation will either increase or decrease with time and not follow a linear relationship.

 

[miimura]

I looked at the BEV tests that they did across several vehicles and was disappointed to find that they don't run the same tests on all the cars. For the e-Golf they focused on fast charge performance at different temperatures, but for other vehicles they focused on other aspects of the vehicle performance. It would have been nice to see the fast charge performance of other vehicles on the same tests that they ran on the e-Golf, especially vehicles with battery pack active thermal management.

 

[JoulesThief]

Ran my car down to 5 miles left on the Guess-O-Meter. Last full fillup showed 6 .0 miles per kwh. Recharged fully with e-motorwerks 40 Pro JuiceBox. JuiceNet log shows 20.56 KW used to recharge. Efficiency is about 91%, So I added 18.71 kW to the battery on that recharge. 5 miles left would indicate .83 usable kW still in the battery. About 19.54 kwh current useable battery capacity at 6.0 miles per Kwh = 117 miles of "range". Exactly what my guess o meter is currently showing on a full recharge. So, that's what my current usable battery capacity is after 14500 miles and 26 months of ownership. 19.54kWh. Car had about 21.3 kWh when brand new.

 

[f1geek]

Anybody find a way to use an OBD device, such as OBDEleven, to get a read on the battery health (capacity)?

 

[user 703]

Another data point:

[*]23 months, 14,000 miles, estimated degradation 6-8% based on charging/range info.
[*]e-Golf is only being charged with L2. It typically is being recharged to 100%, and sometime reaches 5-10 miles range before being charged again (function of commuting).
[*]Regen is B or D3, if used.

Side note on regen impact: It would be interesting to understand if using max regen (B) really impairs the battery. Theoretically, this makes sense. But would VW not limit potential damage/aging to the battery by capping regen based on whatever appropriate metric (temperature, cycles, etc.)? One would think that VW would not be interested in a wave of claims regarding prematurely aging/failing batteries.

With the warranty at 70% for 100,000 miles/8 years, it is conceivable that some of e-Golfs will reach the 70% threshold before the distance/age. And would this be 70% of 24.2 kWh (=16.94 kWh)? Also interesting - would a dealer be able to update the battery management to open the discussed "reserve", by this ensuring a 70% plus capacity level?

 

[f1geek]

I vaguely remember reading that max regen on the e-Golf is around 40 kW. While the high regen found in B mode will send more 40 kW spikes into the pack, I don't believe it would generate the same amount of heat from DCFC charging where 43 kW (at a 125 amp DCFC station) is sustained continuously for 20 to 30 minutes. Heat is what speeds up the pack degradation.

VW warrants the NET capacity. Based on a European e-Golf manual I got my hands on, it says the starting net capacity is 64 Amp hours. AT 323 V nominal pack voltage, that translates into 20.7 kWh starting net capacity. Therefore, 70% of 20.7 is 14.5 kWh. But I could be wrong about the correct starting net capacity.

I suspect that the "reserve" capacity is automatically activated by the battery management software once the health of the battery degrades to a certain level. Of course, once that happens, the battery health will then decline even faster than before.

Based on my net capacity measurements, my car has about 19 kWh of usable capacity (about 10% loss), after 24,000 miles and almost three years. I'm not sure the warranty will be triggered but since it's getting hotter in the SF Bay Area, I think the battery will be taking a beating over the next several summers, and the capacity may drop faster than it already has.

 

[f1geek]

I just got OBDEleven Pro. I found that at "Full" on the battery gauge, actual battery SOC is 97.2%, according to the car computer. Also, the battery voltage is 357 Volts (each of the 88 cells was at 4.06 V). If only there were a way to find out the total amp hours of the pack... Anyone know if this live data can be found somewhere?

 

[JoulesThief]

I just got OBDEleven Pro. I found that at "Full" on the battery gauge, actual battery SOC is 97.2%, according to the car computer. Also, the battery voltage is 357 Volts (each of the 88 cells was at 4.06 V). If only there were a way to find out the total amp hours of the pack... Anyone know if this live data can be found somewhere?

Full charge should be 4.20V per cell. 4.06 is a good compromise, and adds considerably to the lifetime of the battery.

I just try to recharge it at an hour or more in time, for the 2015 to 2016 24.3 kwh batteries, the Charge Point 100 DC level 2 chargers are just fine rated at 24 kWh. The highest I have seen a Charge Point DC 100+ charge my e-Golf is 21.2 kWh rate. The 2017 e-Golf can take probably 33 to 35 Kwh charge on board probably pretty comfortably.

Any Lithium Ion battery that stays cool should be fine with a recharge to 90% SOC in a 1 hour time limit window. Key is staying cool while recharging, and only a 90% SOC when finished.

 

[f1geek]

According to the Idaho National Laboratory, the maximum voltage of the e-Golf battery pack is 4.10 volts, not 4.2 volts. The minimum voltage is 3.00 volts.

 

[JoulesThief]

According to the Idaho National Laboratory, the maximum voltage of the e-Golf battery pack is 4.10 volts, not 4.2 volts. The minimum voltage is 3.00 volts.

The 4.10V at charging from the charger pack probably yields a finished charge level of 4.05 to 4.06V per cell. The actual chemistry is probably beyond that and at 4.20V per cell, which tends to shorten the life of the cells if recharged to 100%. Go to Battery University to learn more about battery chemistries and charging limitiations based on particular battery chemistry.

Volkswagen went with a battery chemistry known for durability, large capacity, pretty high internal resistance inside the cells, and slow discharge rate, which limits acceleration performance in favor of turtle like performance for long, steady, slow, economical discharge rates and increased range per recharge. 0-100kph rates in seconds is not it's strong suite. 6 to 7 miles per kWh of discharge IS it's long suite, in city BLVD and residential road traffic conditions.

 

[miimura]

According to the Idaho National Laboratory, the maximum voltage of the e-Golf battery pack is 4.10 volts, not 4.2 volts. The minimum voltage is 3.00 volts.

The 4.10V at charging from the charger pack probably yields a finished charge level of 4.05 to 4.06V per cell. The actual chemistry is probably beyond that and at 4.20V per cell, which tends to shorten the life of the cells if recharged to 100%. Go to Battery University to learn more about battery chemistries and charging limitiations based on particular battery chemistry.

Volkswagen went with a battery chemistry known for durability, large capacity, pretty high internal resistance inside the cells, and slow discharge rate, which limits acceleration performance in favor of turtle like performance for long, steady, slow, economical discharge rates and increased range per recharge. 0-100kph rates in seconds is not it's strong suite. 6 to 7 miles per kWh of discharge IS it's long suite, in city BLVD and residential road traffic conditions.

You stated it backwards - the cells were selected for low internal resistance and therefore low internal heat generation. This is the primary reason that they decided to forego the active thermal management of the pack - they felt the internal heating was low enough that it wasn't needed. IMHO, they were wrong. Active thermal management (heating and cooling) may not be needed very often, but when it is needed, it's vitally important.
Any EV battery will perform better under low discharge rates like driving slower and more efficiently. The relatively small capacity of 24kWh (2015 & 2016 cars) and the relatively poor aerodynamics of the Golf body just highlight the fact that the car works better at low speeds.

 

[f1geek]

miiumra, you are correct in everything you stated. I agree that VW should have added active thermal management, as it would have enabled a much longer battery life (just read an article at insideevs.com about how BMW expects the i3 pack to last at least 15 years and still have 70% net capacity, probably large due to active temp control) and they wouldn't need to discourage people from back to back DC charging sessions due to heat generation at high charge rates.

 

[JoulesThief]

According to the Idaho National Laboratory, the maximum voltage of the e-Golf battery pack is 4.10 volts, not 4.2 volts. The minimum voltage is 3.00 volts.

The 4.10V at charging from the charger pack probably yields a finished charge level of 4.05 to 4.06V per cell. The actual chemistry is probably beyond that and at 4.20V per cell, which tends to shorten the life of the cells if recharged to 100%. Go to Battery University to learn more about battery chemistries and charging limitiations based on particular battery chemistry.

Volkswagen went with a battery chemistry known for durability, large capacity, pretty high internal resistance inside the cells, and slow discharge rate, which limits acceleration performance in favor of turtle like performance for long, steady, slow, economical discharge rates and increased range per recharge. 0-100kph rates in seconds is not it's strong suite. 6 to 7 miles per kWh of discharge IS it's long suite, in city BLVD and residential road traffic conditions.

You stated it backwards - the cells were selected for low internal resistance and therefore low internal heat generation. This is the primary reason that they decided to forego the active thermal management of the pack - they felt the internal heating was low enough that it wasn't needed. IMHO, they were wrong. Active thermal management (heating and cooling) may not be needed very often, but when it is needed, it's vitally important.
Any EV battery will perform better under low discharge rates like driving slower and more efficiently. The relatively small capacity of 24kWh (2015 & 2016 cars) and the relatively poor aerodynamics of the Golf body just highlight the fact that the car works better at low speeds.

Can you name an under $37k electric car with a cD lower than the e-Golf? Just curious, at that price point, what you consider good aerodynamics, because you can look at an XL1 model and see what VW can do with coefficiency of drag.

 

[f1geek]

The Hyundai Ioniq BEV has a CD of 0.24, I believe, and with a 28 kWh (net capacity, I think), gets the same range as the e-Golf with the 35.8 kWh (gross) battery pack. The base Ioniq BEV costs near $30K, I think. And it charges DCFC up to 100 kW and I believe it has battery thermal management, though it may be only air cooled, it's still better than nothing.

 

[miimura]

^^^^ This. Ioniq is way more efficient than the e-Golf, mostly due to aerodynamics. And it doesn't look weird like the 1st gen Leaf and the i3.

 

[JoulesThief]

^^^^ This. Ioniq is way more efficient than the e-Golf, mostly due to aerodynamics. And it doesn't look weird like the 1st gen Leaf and the i3.

It's $36,885 by the time you get HID headlights.

 

[miimura]

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.

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.

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.

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.

 

[currentlydriving]

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.

 

[Chris]

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