Ok completed the following test.Once you isolate the -ve terminal battery connection from 12V to car's chassis, the 12V is in effect 100% isolated from the car, and the car's own charging whatevers cannot affect it at all. You could equally leave the -ve connected to the chassis, and lift the +ve terminal stuff away. Again that will isolate the 12V 100% from the car.
I use a Ctek MXS 5.0 to desulphate & recondition mine every few months, and simply connecting the 2 leads from that to the battery's terminals does the job. It couldn't care less whether one or other lead post (but NOT both!) happens to be connected to something else.
1) Disconnected negative car cable from the battery terminal.
2) Measured a voltage of around 12.2v
3) Connected the Optimate 6 battery charger to desulphate/charge battery
-- Charger did not detect sulphation, and after about a few hours showed the battery in good health and was in float maintenance mode (~13.6v for 30 minutes on and off)
4) After about 8 hours, removed charger, and let battery sit isolated for about a day (~18 hours)
-- measured charge of about 12.96 v (which is mainly surface charge)
5) Reconnected negative car cable back to negative terminal on battery
6) Measured voltage on battery, and voltage dropped to 12.4, 12.38, etc
7)
-- Perhaps there was a large draw when I first connected the cable, but to go from 100% to 75%??
Sounds like the battery can't hold its charge above 75%? But then why did the Optimate 6 indicate a good battery? The Optimate is designed to also measure the health of the battery and indicated excellent health at 100%.
I guess I need to do a load test first with the battery isolated to see if it keeps its charge?