AT&T Galaxy S4: How hot is acceptable when wireless charging?

[PaulQ]

I was all excited to set up wireless charging with my GS4 and it works... However, the phone seems to get pretty hot... when I take it off the charging plate, the back of it is quite hot. Not to the point where you can't touch it, but I would say very warm (this is all relative, I have no way of measuring it in degrees).

**EDIT: I figured out how to get at the phone's temperature reading sensor... it is heading up towards 39C degrees (102F)

Can anyone give me a sense of how hot is too hot? I love this whole concept of wireless charging but not if it destroys my phone.

(using RAVpower charging plate, seems like a very solid, quality plate)

Thanks all!