I started out leaving it on the phone all the time just to see how it works. I didn't notice the phone getting hot at all while the battery pack was on any while it was keeping my phone at 80% charged. I have the Tumi version and on the battery pack it says the output is 5v/1.3A. So I'm thinking its more like a trickle charge. One of the options of the battery back is to have it kick in all the time and keep the phone at 100% or efficiency mode which keeps the phone battery at 80% until the battery pack is discharged to 0%. It says in efficiency mode the mod battery will last longer.
Hope that helps