I ran across a post on Reddit that seems to contradict @propellerh71814's information.
[link|https://www.reddit.com/r/Embermug/comments/tn58dk/looking_to_replace_the_battery_i_took_apart_my/|Looking to replace the battery. I took apart my Ember mug. : r/Embermug|new_window=true]
Their photo shows a completely different battery from the one previously described.
[image|3075030]
That picture shows another Eve battery, but this one is part number P0686-LF and is marked as a 15.2V battery.
Unfortunately, I had just as much luck locating one of those batteries as I did finding a B0664-LF battery; i.e., none whatsoever.