My 15" MBP will turn 2 in October.
I recently updated it to the new mountain lion.

When it is charged to 100%, I continue to leave it plugged in.
When I'm home, I usually use the power source. Only when I take it out the house that I use the battery which is about 3 days a week. I usually drain the battery once a month. The cycle count is 255.

The problem is, after I updated the OS, when its fully charged at 100%, it will drop to 99% after a day and by the next day its 98%. It hasn't gone to 97% yet. The old OS used to display "Charged".

What I always do at the end of the day is shut down and switch off the power. The next time I use it, I turn the power back on. Does this have an effect on the battery?

So the percentage going down is not a worry? The battery is fully charged but the display shows 98% (it was 99% yesterday).

thanks a lot.