Keep in mind that, *in the very specific case of the 0% report*, there's a bug/race in which reading the cover's battery gauge will fail (as in, the actual `read` call fails) for a short while after plugging in the cover (and possibly on resume after suspend). This is fairly badly handled in Nickel as reporting 0%, instead of displaying the last-known-good value.
If it doesn't actually display something meaningful after a few minutes, though, then never mind me, it actually did drain the battery somehow ;o).
Everything isn't entirely kosher with the cover, though, it also has a tendency to trigger spurious USB events (both plug & host), and will trigger an USB reset when it crosses a charging threshold, which may be highly problematic if that happens during an USBMS transfer
.