In message , Brian Gaff
writes

Brian, anything is possible. All I will say is that I remove the
battery fairly regularly, and the terminals are smeared with Vaseline,
which normally keeps them clean.

The only other thing is a high resistance somewhere which is intermittent
making the battery not have much guts on a high current. Could you, by
disconnecting the earth have managed to clean the connection perhaps?

--
Graeme