Here's a curiosity I came across.
If you have your battery monitor set to 'show percentage' sometimes the status will be 'charged' and you have the green light on the PSU connector, and sometimes the status is 'charged' (on time view, or a percentage less than 100% and still with the green light, indicating fully charged. Also in this situation the battery symbol will have the 'on mains power' symbol.
This is OSX's algorithm to not overcharge batteries. On a power state change eg connect power, sleep etc.) it checks the battery and if the battery is <>