Dear 100 Hour Board,
With lithium ion batteries like for a laptop computer, is it harmful to leave the adapter always plugged in? If I should unplug it after the battery is fully charged, when should I plug it back in?
According to this website, it's generally okay to leave the adapter plugged in. The computer stops charging when it reaches capacity. What you have to watch out for is heat, since that can damage your battery. If heat is an issue, it might be better to remove the battery rather than to unplug the laptop. That will reduce heat production and save you some charge cycles on your battery.
There's a lot of myths out there about rechargeable batteries that come from old technologies and don't apply anymore. For instance, you don't ever need to fully discharge a lithium ion battery like you had to for NiMH batteries. However, you still don't want to store a lithium ion battery fully charged for too long. It might be wise to make sure your laptop discharges a bit every now and then (once a month or so).