It'll heat up a good amount when you are charging it from a very low state. I would not be concerned. If I remember correctly the HC has built-in thermal overload protection....if it gets too hot it shuts down.
-Kevin
I think that applies only to the circuitry, not the batteries. There are no sensors I am aware of on the batteries in my HCs. I have charged them from a very low state several times. They do get warm but not hot by any means.
We have a cordless phone handset that the battery gets very hot if you run it way down and then put it on the cradle. So hot you would think it is going to melt the plastic phone. When that occurs I place it on the cradle for a while then let it cool down for some time before returning to the cradle. Batteries getting hot doesn't sit well with me.
I'd keep a watchful eye on your HC. Not leave it unguarded while charging just in case the factory sealed smoke wants out.