Upload & Sell: On
The cp-e4 is awesome for wedding receptions and the like, but there is one setting that you might consider changing. I don't remember the custom function, but I set the 580 to use only the cpe4 batts when it is plugged in. This saves the flashes' internal batts for keeping it alive. I actually had an issue one time where the flash batts were dead even though the cpe4 had power, so the flash was useless. Unfortunately it happened during the cake cutting :-(.
Doing it the other way, you can just unplug the cpe4 when it dies
Excellent tip, Tim! That could save somebody's bacon...