Bad battery choice
Shelly sells a smarthome sensor called Door/Window2. It works well, use wifi, is very configurable and can be used locally, without cloud. Unfortunately they’ve chosen a weird, expensive battery option, CR123A. It can cost around 70 NOK (~10 USD), and does not exist as a rechargable option. And you need two of them! So what do you do if you only have one left?
Use a single, abundant 18650 instead!
It fits almost perfectly, has higher capacity and can be found in any discarded electronics. They are always rechargable, and most have an included protection circuit.
The CR123A is 3 V, so two gives 6 V. A standard lithium-ion is 3.7 V (2.7-4.2 actually), but this still works. The ESP8266, which the sensor is based on, can in theory run on 1.7-3.6V.
Unfortunately the cover is just a bit too narrow for them. May need to design a new one suitable for 3D printing.
Edit: Never mind, someone beat me to it! Shelly DW2 18650 Case