The only way to check them is to remove all the potting from around the battery. A heat gun, exacto knife and pick make short work of it. Tnen carefully pry up the battery to access the underside so you can test it with your multimeter. Once you are satisfied with the results carefully put the batter back in place and then re-pot with hot glue or quick set epoxy.
Also be advised that even if it has a 3V charge if its an off brand TPMS it likely got an off brand battery in it that might not be as good or have the longevity as name brand battery.
The good news is these chineasuim TPMS sensors are usually about $38 a set and i you have the ability to change yourself are a real money saver even if the lifespan is somewhat shorter. If you go with BMW the sensors are $252.13 each + installation of at least another $200 so either pay the deal around $700 and have a two year warranty. Or pay$38 to do it yourself and have sensors that will likely last at least 2 years.
FWIW it is generally accepted that TPMS sensors have about a 5 year lifespan. Even with that BMW will only warrant the part for 2 years (at least in the US of A) so if it dies on the 731st day you are SOL as far getting the dealer to cover them under warranty.
It really boils down to are the BMW units worth being about 18X as expensive? To me no! I'll run the risk with a off brand and change them if and when they go tits up. FWIW part deux I installed offbrand TPMS sensors on my new to be 2008 R1200GS on 7/26/21 @ 49,272 miles the rear one failed earlier this year and I replace both on 1/25/23 @ 54,688 miles. The rear lasted only 18 months but I think that was more a fluke that the rear failed so prematurely. You pays you money and task your risk....I'm still way ahead of the game. Heck even if I changed TPMS sensors at each tire change I'd be ahead of the game.
Cheers,
P-14
