There are videos on how to replace the soldered in battery. I did with both my original keys as well as reflow the bad solder joints. I got working keys out of the process but they didn't work great and after a couple years didn't at all. The weak signal made it seem like the car receiving antenna was failing but when I finally got a new factory key out worked from as block away.

If you have one solid working key by all means look into cutting the non working one open to replace the battery if you have decent soldering equipment and skill.

My plan was to refurbish my failed key once again after I bought a factory key. Car was took out by a deer never got the chance.

__________________
2011 E70 • N55 (me)
2012 E70 • N63 (wife)