I’ve had several situation where the low battery warning symbol on my Canon Speedlite 600EX-RT shows when I turn on the flash, even though the batteries are fully charged. The flash refuses to turn on despite trying to Clear settings, removing and installing fresh batteries, etc. This has annoyingly happened on a few shoots.
I discovered by accident that this was due to my leaving the SCH-E1 clip-on colour filter holder on the front when I was turning on the unit.
I suspect this happens when:
- Colour filter is installed on flash
- Flash auto shuts-off (or left off a long time)
- Attempt to turn on flash with colour filter still installed
Simply removing the SCH-E1 colour filter from the flash and turning the flash on seems to do the trick in all cases. After that, I can turn off the unit with the SCH-E1 installed and turn it back on without any problems.
Since this happened to two units that had been used in the same session with the filter installed and left to auto shut-off, I am suspicious that it may be related to the auto shut-off or else the length of time the units were left in the off state.
Here’s the Canon advisory from a couple of years back that I found describing the issue: