@omghax.eth
I've been planning to replace my OP-1's battery since I bought it as the original owner informed me that they believed it was probably due to be changed as they only ever used the device plugged in then it sat in their closet.
When I got it I read in the manual a way to recalibrate the battery indicator which worked for a week or so but the battery has actually held up pretty well so I wasn't as worried about changing it anytime soon.
I did notice that when I turn on the device it almost warms up with the volume starting low and then slowly increasing to its normal volume which I attributed to the bad battery until recently finding out it's a failing capacitor on the connector board!
I remembered seeing one on Reverb for like $200 and sadly iFixit hasn't had it in stock for years. I reached out to teenage engineering support in hopes they might have some left as they did have my OP-Z pitch bend + microphone part (even though their help docs say don't ask them for parts) and to my surprise they did! 🙌