I just had a similar experience with my Libra. I opened it to start reading, and the battery was at 30%, which is about where it was yesterday. After a few minutes of reading, I got the "low battery" alert, and the meter said 8%. I plugged it in, and after 20 minutes of charging, it's magically back to 46%. The battery meter is just picking random values to display, I guess.
It doesn't seem to have any idea how battery voltage correlates to remaining capacity.