After only 1.5 years, my X5III has a bulge on the back cover. I assume this is the battery that has now generated a gas pocket.

That's disappointing and scary after only 1.5 years, with maybe 50 days "in the field", so it has 50 charge cycles. I have only used a slow charge at 500mA. It still seems to carry a decent charge, but a puffed up LiPo is an explosion and fire liability.

Therefor I'd be grateful for any battery replacement tips.

Where to source a good battery and how to replace it? There seems to be some micro torx screws at one end of the chassis - maybe those are the ones to undo to get the back cover off? I can solder if needed, no problem.

I'll mention that I've had a look at AliExpress which is what comes up in a search, but it looks like a rag-tag selection, and I'm aware that you risk getting used and/or under-capacity batteries with faux lables applied.

TIA!