When you unplug your laptop from the wall and the little box with the computer turned off that normally does not run the battery down. If you are using it running on battery and the battery gets low and needs charging, the computer will beep or tell you some other way that it needs to be charged. You can then plug it in and it will charge. It is best to plug it into the wall outlet first and then the computer, as the transformer in the little box will absorb the surge instead of your computer getting the surge applied to it. Don in AZ