Had the same problem last night shooting a wedding and wedding reception. No luxury there of giving my Canon 580 ExII a rest. Having read a few posts re batteries, two of us, each with different Speedlites, (the other is Nikon, but not ExII) chose alkaline high performance batteries. Mine still heated up, while my fellow photographer's did not. My solution that I stumbled upon, was having two sets of batteries. When one set heated up (so hot I couldn't hold them), I switched to the other set that had cooled down. I won't wait until they're that hot in the future. Can't be good for the camera. The second set of batteries, once cool, worked fine. So there may be something with the EXII that heats up batteries more than other Speedlite models. Curious about the 600 series, if they solved this problem.