This is a bit different than MTBF like on harddrives. Batteries are usually warrantied to 80% capacity because it’s a wear thing, not a random chance of complete failure.
A battery isn’t going to last twice as long as another one by chance, this is all about determining the average or worst-case operating range the battery will be in and using that to figure out a warranty period where they think all cars will fall within.
This is a bit different than MTBF like on harddrives. Batteries are usually warrantied to 80% capacity because it’s a wear thing, not a random chance of complete failure. A battery isn’t going to last twice as long as another one by chance, this is all about determining the average or worst-case operating range the battery will be in and using that to figure out a warranty period where they think all cars will fall within.