I have a number of active basses, some of which are used infrequently, so it's nice to know how long the battery has been installed and its date, so if a proactive changing of the battery pre-gig is a good idea. I use my ears to judge when it's change time on my more frequently used instruments, when lower notes are squashed or distorted.
For your consideration: I have been using the little removable 1/2" x 1-3/4" Avery 05422 (or preferably,
Invalid Link Removed) labels to write the date I changed the battery and its expiry date, and stick it to the back of the headstock. Also another label with the last strings used and change date. Being removable there's no damage to the bass. And I use them for lots of other purposes around the house, and also on effects pedals, etc...