How battery intensive are accelerometers? The last fob I had required a battery change once every year or so already, increasing that frequency could be quite annoying. And I'd rather not have yet another device I need to regularly charge.

Anecdotaly, I have a remote that lights up everytime it's moved (to assist finding it at night as it's a remote for a bed). I've had it for well over a year and haven't changed the batteries yet. Granted, it runs on three AAA batteries. Not entirely sure what tech it uses, but it's not necessarily a full blown accelerometer.

Edit: some / all of the power drain would be offset because the RF transmitter would be off while the sensor is on.

Not bad at all, especially considering you would only need very coarse grained data, and not even that often.

Years ago, McDonalds was handing out pedometers[1] which had a year+ battery life along with a display in a package smaller than most key fobs.

[1] https://i.imgur.com/hU15Q1u.jpg