Take attention to:
- right polarity, it should be written on the battery otherwise use volt meter
- if you will not be able to insert the battery inside the housing, you can glue it outside the cover. It is good to cover it, to protect against the mechanical stress. Connect external battery contact to internal battery by thin wires and lead it through housing through the holes at top button side.
- batteries can wire parallel only if the its voltage is equal, otherwise high current may flow. I suggest to charge it both to full, to voltage 4,2, or working with open irex until battery voltage becomes equal to external battery voltage. Any other same voltage will be good.
- battery gauge will be fully confused at the begging, after charging cycles it will become smarter.