Hi,
Here’s the Ifixit [guide|133888] guide which shows how to open the ear buds.
The guide was only to show the internal components and is not intended as a repair guide because of the damaging way in which the earbuds have to be opened.
The battery is a [https://www.texim-europe.com/product/63124101511-VAR|Varta CP1240 A3] (supplier example only) if you think that you want to try and replace them