I’ve had this happen to me once before. When I asked about this in the Purism community/general Matrix room, I was told that the battery calibration got skewed over time. Following the calibration reset instructions below seemed to work for me.
Here are the screenshots I took at the time:
Transcript from the Matrix room (with names removed):
[Me] Anyone have any idea why I got this weird power drain on my Librem 5? I literally saw the battery percentage tick down in the span of 30 seconds from around 47% down to 4%. I only had Wi-Fi enabled, and Firefox open with Teddit (Reddit frontend open) as far as open apps go, not including background apps.
[Purism staff] oh 30 seconds? It could be that the battery calibration is off
[Purism staff] power off, remove battery, reinsert battery, and do a full discharge/charge cycle
[Purism staff] over time my battery calibration skewed because I mostly have this phone plugged in at my desk or at a laptop dock, and it only runs for an extended time on battery when I run quick errands, As a result the battery monitor eventually became inaccurate