This guide worked great thanks. The Left earpiece battery was only lasting about 15 minutes before this repair (the Right earpiece is fine).

- Heating with a hair dryer in a series of 10 - 15 second blasts allowed me to get a very thin screwdriver into the entry point, and I used fingernails to separate the shells

- Like others I didn't have to disconnect the tiny ribbon to be able to remove the battery, but it was very delicate to remove the battery without putting too much pressure on the ribbon

- I used a wooden toothpick to gently pry the battery out from the casing, away from the direction of the ribbon. As a result I managed not to damage the battery contacts, and was able to leave the battery adhesive in place when inserting the new battery.

- I tested the earpiece before resealing

- The replacement batteries I ordered included a glue with precision applicator, so I was able to completely seal the case again

The new battery is working perfectly but I won't replace the other earpiece until there's a real need