I removed an electrical part for cleaning.
A wire sparked and later I noticed my battery monitor was dark. It stayed that way after I cleaned and reconnected the part.
I traced the wire from the back of the monitor in the control panel, through the bulkhead to the engine compartment, and into an electric box. And there it was: a blown fuse.
Fuse replaced (2 for £1.09). Battery monitor works. It says: 14.40.