i highly doubt that to be the case, ALTHOUGH, when i was using my esavage dual charger, i noticed it to get very hot when in the car and cool as the air when it was off my power supply. i never did run it directly off my battery though (which is 2-3yrs old)

but, could it be something to do with not being a solid voltage/current, and it ripples and this could damage the charger? i have no idea how they work, but thats why people suggest not using car starter/jumper pack thingys.

I'm quessing the current under those conditions is not the cleanest - spikes and other transients. I don't think an auto lead acid batt can tolerate voltage greater than 15v for long. Charger may have component(s) thats sensitive to transients and can be damaged.