The G5 transmitter runs on 2 Renata 389 batteries wired in series. 1.55v/80ma each. I am still looking for something rechargeable as it is a very interesting proposition. Personally I have used JB Weld 2 part epoxy and Sugru as a sealant. The both can be ground off fairly easily for repeated battery replacements. I generally get 5.5 months per transmitter on XDrip+.
The battery I linked above is 110mAh @ ~3.7v, so maybe similar to 2 389’s? However, looking more closely over its specs, it’s probably a little too long to be able to fit it.
There’s an even smaller one:
This is about the same thickness and width as a 389, and just over the length of two 389, so could fit? (Provided the charging circuit + connector can be kept small enough, and the resin between the two 389’s on the transmitter can be removed.)
But, it’s only 40mAh, so certainly wouldn’t last even close to 2 x 389 button cells. That said, if a charging connection could be made convenient enough (charge while in-use), would it matter if you had to charge it once or twice a month?
The problem then comes down to the charging and regulator circuit that would need to be integrated and epoxied in with the battery. Probably would need to be a custom circuit.
Oh well, I’ll likely just swap out the 389’s with new ones for now…