I have a 12V battery, not used on a car but is connected to a 12V
recovery winch in a boat shed.
This is kept between use connected to a CETEK MXS 5 - an 8 step charger
with correct Float charge once up to full charge.

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)
Fitted it on w/end and it was dead ..... So to be sure put it on the
charger again for 24 Hrs
Now it appears to be working fine.

Checking voltage ..... it measures 13.5V after charge - which I can only
think must be a surface charge. (i.e greater than ~12.6)
I used the winch to pull in a 2.5T load, and seemed OK, Voltage after
the recovery was 12.75V

Would this suggest battery is probably OK ?

Perhaps having the charger across 2 batteries, when one of them was new
and higher capacity, may have provided imbalance and it shut down
charging due to back emf of the larger battery and then failed to charge
the winch battery ?

Next year I'll use separate chargers.