Canon Speedlite 600EX II-RT Manual (en)

f Flash Firing Restriction due to Temperature Increase
Do not open or close the battery compartment cover while flash firing is 
being restricted. Doing so is very dangerous since the flash firing 
restriction is canceled.
Even when level 1 warning is not displayed, the firing interval will be 
extended as the flash head begins to heat up.
If level 2 warning is displayed, allow a rest time for at least 40 min.
Even if you stop flash firing after level 1 warning is displayed, level 2 
warning may be displayed.
If flash photography is performed in high temperatures, the firing 
restrictions may be activated before the number of flashes listed in the 
table on the preceding page is fired.
For cautions on the number of flash firings, see page 17 (continuous 
flashes), page 42 (stroboscopic flash), or page 47 (modeling flash).
The Speedlite may not fire in rare cases due to environmental factors 
such as temperature rise.
The number of continuous flashes until the warning is displayed on the 
preceding page is the number when the 600EX II-RT is used by itself and 
when the Compact Battery Pack CP-E4N (sold separately) is used 
together with the 600EX II-RT. When an external power source other 
than the CP-E4N is used, the number of continuous flashes until the 
warning (level 1) is displayed becomes fewer.
When the bounce adapter is used, when the color filter is used, and 
when both the bounce adapter and color filter are used together, the 
number of continuous flashes until the warning is displayed becomes 
slightly fewer.
When C.Fn-20-0 is set (p.116), the warning beep does not sound even 
when flash firing is restricted.
When C.Fn-22-1 is set (p.117), the warning with red illumination of the 
LCD panel will not be displayed even if the temperature of the flash head 
When using Compact Battery Pack CP-E4N (sold separately), also refer 
to the CP-E4N Instruction Manual.