There is an easier way to fix this device without destroying it completely:
- buy spare battery from AliExpress (e. g. this one)

- remove back cover as shown in step 4

- cut off the wires from the original battery and remove it

- cut off the connector from the spare battery and solder its wires the original ones

If you need more space inside the device, you can remove the original battery first (!) and then use a "Dremel" device to gently remove the small piece of plastic between the original battery and its connector (see picture here).