Canon SPEEDLITE EL-1 Advanced User's Manual page 218

Caution
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 1 warning is displayed, allow a rest time for at least 50 min.
Even if you stop flash firing after level 1 warning is displayed, level 2 warning may
be displayed.
When taking flash photography in < > flash mode or at high temperatures,
the number of flashes may be restricted to fewer than those listed in the table.
For cautions on the number of flash firings, see "Continuous Flashes",
"Stroboscopic Flash" or "Modeling Flash".
The flash may not fire in rare cases due to environmental factors such as
temperature rise.
The number of continuous flashes until the warning (Level 1) is displayed is the
number of times the EL-1 is used on its own and when it is used in combination
with the compact battery pack CP-E4N (sold separately). 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 P.Fn-06-1 is set ( ), the warning beep does not sound even when flash
firing is restricted.
When C.Fn-22-1 is set ( ), the warning by the LCD panel illumination will not be
displayed even if the temperature of the flash head rises.
When using Compact Battery Pack CP-E4N (sold separately), also refer to the
CP-E4N Instruction Manual.
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