On Tue, 6 Apr 2021 20:00:59 +0100, rick
wrote:

snip
Over winter I remove and put indoors, and leave it on the charger.
The same charger wired in parallel across a pair of batteries (worked
fine on previous years)

snip

That can be ok and especially if the batteries are of the same voltage
/ charge status (when you hook them together), because ...

You should also connect them via fuses (or at least one fuse in the
case of just two batteries, between the batteries) in case one battery
goes short and tries to discharge the other one ... very quickly. ;-(

Cheers, T i m