I'm sure everyone has experienced their laptop or phone power gauge going from 100% to 0% in the blink of an eye after the battery reaches a certain age.
What you will probably see is that aircraft batteries will have a sort of TBO number, either expressed in years, or in load/unload cycles. After that TBO number has passed, you are required to exchange them for new (or maybe have them thoroughly tested every x cycles so you essentially use them "on condition").