I agree with Paul. My battery charger showed fully charged on my S2, but when I tried to start it after a few months, I got the dreaded 'loud clicking' to indicate the battery hadn't got enough woomph.
I'm not convinced about leaving trickle chargers on the car all the time and once our chargers show 4 out of 4 bars, I disconnect them for a week, then reconnect them. My thinking is that our chargers should display 'CHA' once the battery is fully charged, but they never do, (they display 4/4 bars), presumably cos the cars are drawing charge constantly for the alarms and ECUs (so will never be fully charged) and keeping a battery constantly charging is not good for it, so by disconnecting for a while and then reconnecting once they are down to 2 bars, you are as near as possible to simulating normal driving conditions.
Please feel free to shoot my theory down in flames 😀.