Being that type of guy, I noted each date when my Sonicare toothbrush needs recharging, at least after the battery had declined to about a week between charges, specifically so I could produce this graph:
The peaks include trips where I didn’t use the toothbrush and I’ve certainly blundered a few dates, but you can eyeball a trendline: those cells are kaput!
In round numbers, I bought the thing in early 2010, so the cells lasted maybe 2-½ years. I routinely run the toothbrush until the blinky light indicates that it needs charging, then fill it up overnight, to avoid having the cells fail like the ones in the beard trimmer.
Somehow the notion of discarding the whole toothbrush seems wrong, even though the instruction manual describes how to remove the battery for recycling before you junk the carcass. Talk about planned obsolescence!
Ah, but I know what to do about that…