Canon Speedlite 600EX II-RT Manual (en)

Installing the Batteries
Do not use “AA/R6 lithium batteries”.
Note that certain AA/R6 lithium batteries may become extremely hot in rare 
cases during use. Due to safety reasons, do not use “AA/R6 lithium 
When continually firing, do not touch the flash head, batteries, or the 
area near the battery compartment.
When continuous flash or modeling flash is repeatedly fired with short 
intervals, do not touch the flash head, batteries, or the area near the battery 
compartment. The flash head, batteries, and area near the battery 
compartment become hot, resulting in the risk of burn.
Do not use the Speedlite while touching the same part for a long 
period of time.
Even if the product does not feel too hot, prolonged contact with the same 
body part may cause skin redness or blistering due to low-temperature 
contact burns. Using a tripod is recommended for people with circulation 
problems or very sensitive skin, or when using the product in very hot 
Using AA/R6 batteries other than the alkaline type may cause contact 
failure due to the irregular shape of the battery contacts.
For details on precautions when using the Compact Battery Pack CP-
E4N (sold separately), also refer to the CP-E4N Instruction Manual.
When <!> is displayed or the LCD panel display turns off during 
recharging, replace the batteries with new ones.
Use a new set of four batteries of the same brand. When replacing the 
batteries, replace all four at one time.
AA/HR6 Ni-MH batteries can also be used.