[Tom Line]

We test our batteries, and have not seen the drop-off that you suggest (20%). In fact, some of our best batteries are not the new ones, but ones that are one or two years old.

If the manufacturer says 20%, they may be hedging their bets in case the battery is used in a critical application, or (more likely) are basing it on statistical sampling and covering the worst-case: for instance 99.999% of the batteries may lose less than 20%.