TMPS sensors use a cheap tabbed CR2450 lithium battery that is permanently soldered to the sensor board. The sensor board itself along with the soldered battery is placed as an assembly within the plastic sensor body. The whole lot is then encapsulated in a silicone potting compound (to protect it from moisture and/or stop people fiddling with it).
As such it's obvious the manufacturers doesn't want you playing with it as they consider the whole TPMS sensor to be a throw away item. Very annoying as the battery itself costs around the $5 mark (consumer physical shop price) so it's much cheaper than the cost of replacing the whole assembly (at around $50 each on eBay). But the battery can be replaced though its a pain in the rear to do. So it's probably not worth it for most when factoring in the PITA value. First you'd have to pry off the plastic case and then scrape away the potting compound. Once done, you can unsolder the old battery and solder a new battery in its place. Finally you can reapply some potting compound ($130/kg) and you're done.
It's rather annoying as it would have been a simple no brainer to design the sensor body to allow for battery replacement during a tyre change. A CR2450 battery compartment costs around $2 at an electronics store and much less in bulk, so in volume, it wouldn't have cost the manufacturer much at all other than the lost opportunity costs to sell us the part again and again (thanks to planned obsolescence).
Luckily there are plenty of YouTube videos showing us how to replace the batteries for the various TPMS sensor around. Just need interest, patience and a little spare time to do it
PS: the TPMS tool is on the shopping list...
PPS: an interesting teardown of a RAV4 TPMS sensor below which seems on of the easier ones to pull apart
