Wanted to post an update here since it looks like h1791story hasn’t yet.
I was having a problem with my unit showing that it was charged to 100%, but once disconnected, it would not turn on. When I would plug it in again, the charging indicator showed 0% and the process would reatart
I opened my Theragun Elite and tested the 18650 batteries with a multimeter - read as 0.3 V each, so this confirmed that there was a problem with the battery to the point where it wasn’t able to be recharged anymore. It seems the internals are not truly in sync with the battery, so it would read as charged to 100% even though the battery was not accepting charge at this low voltage.
I am a beginner at spot welding and handling 18650 batteries, but it was fairly easy to disconnect the 3 native batteries from the circuit and spot weld 3 new batteries (I purchased 2500 mAh 20A from bulkbattery) that were in series. I wasn’t able to record, but there are tons of guides on YouTube on how to spot weld 18650s, so I would reference any of those.
Once connected, the unit works again and also has an accurate charging indicator. Hope this helps someone - I am an absolute beginner with DIY projects but this was very manageable and fun to learn about the capabilities of 18650 li-ion batteries. Happy to answer any questions!