I've heard about the technical on how these puppies collect battery power by building up power within it's wiring so it has plenty of power to release within the flash. It used to be common practice (whether you use high power strobes to on/off camera flashes) to "dump" the power by firing the flash right before you turn it off/disconnect power and I still do that to this day. On the old flashes you could audibly hear the power within the flash by giving off a slight hum but you dont get that anymore with the modern flashes. I would still recommend dumping power by pressing the test/flash button to fire the strobe then turn it off immediately. If this doesn't help your battery situation, take the batteries out after usage or send it to canon to look at. Has your flash always done this or just recently? Perhaps after a long time of power build-up the flash got some damage within the circuit controller that is suppose to tell the flash to stop drawing power? This is assuming the flash is completely "off" when you shut it down rather than in hibernation/sleep mode due to inactivity?