Why would the battery be heated if it's not charging? You can set a gap - e.g. 80-100% where the battery won't start charging until it drops below 80%. Look at the documentation for your relevant power-management module.
Put the back of your hand (or your bare leg skin) against a laptop that's been in use. My Macbook is noticeably warm (maybe 15*F delta) in normal, non-charging, operation. That's with the benefit of conduction to the free air; I assume the inside delta-T is higher.
That's not answering my skepticism to GGP's post - what he is saying is: a non-charging plugged in device has a significantly hotter battery than a non-plugged in device because of thermal-conduction across a non-charging wire?
