I got curious and started doing a little research and a little math. An automotive grade Li-Ion battery is supposed to have a lifetime of 1500 cycles. End of life is defined as 70% of original capacity. That 1500 cycles seems to be based on charging to 90% and discharging to 20%, Charging to 100% shortens the battery's life particularly if held there for long periods. Discharging below 20% will shorten the life to some extent as well. Shallower discharges (80%-40%) will increase the battery life somewhat at the expense of range. For now, I'll limit the discussion to 90%-20% discharge cycles.
If you use a Leaf as a base, 3.8mi/KWH and 30 KWH battery, Range starts at 114 mi maximum with a usable range of 80 mi after charging. At 30% degradation, the maximum range is 80 miles and the usable range is 56 miles. Assuming linear degradation, you could drive 102,000 miles. That's probably close to a best case scenario. In real life, you probably won't do that well.
In my case, the first battery failed after 874 cycles at 45,000 miles. That's about 52 miles to a charge. The second battery has about 24,000 miles and 450 cycles with 53 miles per charge. The new battery is down 10% at this point and I'll likely see 1300-1400 cycles and get about 70,000 miles out of it. A lot better than the original but not good. I've always needed more than half the reported capacity for my commute and always charged as soon as I got home in case I needed to make a second trip. A larger battery would have solved a lot of problems.
A 30 KWH battery isn't large enough to last the life of the car but if you double the battery size things start to look a lot better. I'd only need to charge ever other day and could do it overnight so the battery would not sit at full charge for more than a couple of hours. If Nissan ever includes a 90% charge option I could use that as well. Assuming a 90%-20% discharge cycle and 1500 cycles, you could drive 200,000 miles on the original battery. At 12,000 mi/yr for the average driver, that's almost 17 years. Considering that the average service life for a car is 12 years, the battery would likely last longer than the car.
It looks like a 50-60 KWH battery is large enough to last the life of almost any car. With proper temperature management and charge control to limit charging to 80-90%, batteries might last far longer. Tesla S models seem to be able to go 150,000 miles with 10% degradation on average. The key here is a larger battery and proper temperature management. Higher miles/cycle, fewer cycles annually, and less stress on the battery during discharge. If the discharge rate is 15KW per hour (60mph) then a 30 KWH battery is discharging at a ½ C rate while the 60 KWH battery discharges at ¼ C rate. The lower the discharge rate, the higher the total discharge value ( you can pull more energy out of a battery if you do it slowly) and the lower the stress on the battery. If the battery is built with multiple cells in parallel and more cells are added to increase the KWH then the current per cell drops and internal heating drops as well.
If we look at a Tesla Model 3 Long Range and use Tesla's current recommendations then we can use a 90%-10% discharge cycle and 1500 cycles as our reference. Maximum range is 325 mi, usable range is 80% or 260 miles. Range at 70% would be 228 miles and the usable range would be 182 miles. Assuming linear degradation over the life of the battery, you could drive 330,000 miles on that battery. At 12,000 mi/yr, that's 27 years of driving. While the Model 3 is too new to have much data on it's pack and that pack uses the newer 21700 battery, if we go by the Model S data then 450,000 miles on the battery is conceivable. That's 37 years of driving. Subtract 25% for the Model 3 Standard+.
I can't imagine driving a car for 20 years but I can imagine it going though 3-4 owners in 20 years. If electric cars prove to be as maintenance free as expected, then the auto market undergoes an upheaval as people hold onto their cars longer and used cars end up as much more reliable transportation.
2016 SV, New battery at 45K mi.
San Diego East County