I just bought a Gen 1 yesterday (because why the hell not), used. I've noticed that whenever I actually USE the watch, the battery drops pretty significantly, like, 3% for 30 seconds. The battery forecast says it'll last for 24 hours operating the way it is now, which is really just for checking the time. Brightness is set to 1 and I have Google Fit and Moto Body disabled. Yesterday the battery life was less than half what it says I'll get now. ADB over Bluetooth is also disabled. I've already ordered a replacement battery as the phone was bought new in 2014 to be on the safe side, but I'm still curious.