EOL is an arbitrary number that is spec'd differently from manufacturer to manufacturer. Batteries are in a constant state of decline that is fairly linear. The 80% capacity cutoff is what Apple chose as their EOL. In other industries, 70% and 60% can be common EOL parameters. Other companies use the internal resistance of the battery to determine EOL as this is what affects the voltage drop under heavy loads.
I've done hybrid battery testing at work and race scale electric vehicles in my free time. It's rare I go more than a week without testing some sort of battery to see how healthy it is. Trust me. Lithium batteries don't all of a sudden lose their ability to hold their voltage under load once they lose 20% of their original capacity. It's a gradual decline. The point is that 80% capacity retention isn't a magic number.
This is what makes my blood boil: That Apple is writing this off as a battery age issue and they were willing to offer us a "fix" by throttling the processor. No. Either they made an engineering mistake in sizing the battery and the selection of the chemistry or they had a huge quality control issue. Either way, the customer shouldn't have to pay to get it fixed.
