Canon 600EXII-RT Instruction Manual page 19

CAUTION
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
batteries".
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
places.
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.
Installing the Batteries
19