Battery life has collapsed on my Bangle.JS2

I was a kickstarter funder of the Bangle.JS2 and have been playing around with the device and other people's developed apps since I received the watch. The first two times I charged the Bangle, the charge lasted close to a month. The third time I noticed that the screen was blank two days after I removed the charger. Since then I've gotten roughly two days of non-use out of the battery before the screen goes blank. If I do anything related to communications, like updating or loading software, the battery life is even shorter.

Is there a warranty on these watches, or can I just buy another one? I'd like to start experimenting with software development but it's frustrating to have to wait for the watch to charge every time I take it out of the drawer.

A screen going blank doesn't necessarily signify a low or 0% battery. What happens if you press and hold the physical button?

First two months, now two days doesn't sound like bad battery but like activated services.
Two months is realistic without sensor, if you use heart rate regular or gps or ... (possibly without knowing), battery gets drawn quickly.
Try disabling stuff in health app, ... and check again.

Could you verify if HRM/GPS/etc. are active?

Another thing to check: I started to store the watch face down when not wearing it some time ago. At least in daytime then the heartrate monitor seems to "see" that its not worn through the light and doesn't use its LEDs anymore. That considerably improved battery time for me.
Of course this is a work-around. Better would be a real recognition (eg. via accelerometer, temperature, ...) and stop all sensors when not worn. Also a stamina mode would be good, stopping sensors below a battery threshold. (so lots of nice projects for software development :-)

The battery is flat. When I plug it in, turn it on and connect it to the computer, it shows 0%. I am not treating the watch any differently than I did at first. I keep it in a drawer after fully charging it, with a watch face that shows battery level. It goes down from 100% within minutes of being unplugged, is at about 40% at the start of day 2, and is completely dead before the beginning of day 3. Pressing the button does nothing unless I connect it to power.

Interesting. Next time I charge it, I will try uninstalling all apps and watchfaces except the one I had running when I got good battery life. I'll also turn off all services I can in the settings, except bluetooth. I'll report back after that.

I know I installed watchfaces that reported heart rate. I don't know if I did anything to activate the GPS, but I will check both. Thanks.

Good news. I uninstalled all apps and watchfaces that used heart rate or GPS, and anything optional that used Bluetooth. I recharged the watch to 100% last night, and it's still reporting 92 percent. So the problem was software, not hardware. I'm slowly reinstalling apps and will try to figure out what caused it, but the app could just as easily have been fixed by now, so I'll proceed as if nothing happened. The Bangle is back!

Great! With this kind of thing I'd always say go to the app loader and choose Install Default Apps - it'll erase everything and will return the Bangle to 'factory' settings, which helps to rule out a bunch of potential issues.

There's now a backup/restore functionality too so you can always back up what's on your Bangle before you do the factory reset.