You can write firmware that puts it in a low power state (like the airtags) when it isn't broadcasting. My custom door-unlocker uses bluetooth and wifi, it has

You can write firmware that puts it in a low power state (like the airtags) when it isn't broadcasting. My custom door-unlocker uses bluetooth and wifi, it has a single 18650 battery and lasts about a year (with locking/unlocking daily) before needing to be recharged. I assume if you didn't have a giant servo, you could wire up a modem and it'd easily last years on a single charge, checking in every 10-15 minutes. It wouldn't be as small and inconspicuous as an airtag though.