I disconnect the battery from my laptop after it is fully charged. My laptop is more or less plug to the A/C outlet 24/7. Keeping the battery in the laptop while connected to the A/C outlet for a prolonged period of time can overcharge the battery and more or less kill it.
This is why some brands (like Samsung I believe) only allows the battery to be charged to 80% unless you override it. I plug in my battery about once every two months to bring it back to a 100% charge because batteries self discharge over time. I do this just in case I actually do have to bring my laptop somewhere so I don't have to plug in the battery for too long to get to a full charge.