It is great to read all the responses made to this thread.
I have managed to use the Canon Battery Pack CP-E4 only for a short shoot out for a swimming event in a public pool in the practice hour before the start of the major event. I set my cn function on the 580 to external battery source. After 30 to 40 shots with intervals, the recycling time slowed and then the flash did not respond.
Tried several methods but found that disengaging the cn fn to off for external flash, it worked perfectly well.
Well, it is great walking around with this pack hanging from the belt and the curly wire attached to flash which somehow caught the eyes of many camera enthusiasts.
Will be trying out again this weekend in a banquet room, which would mean firing up the ceiling for bounce flash effect.
Of course I have tested the pack off and on but then the duration is too short to determine its advantages - though I have now found out that I have 3 pack of batteries to charge and when you use the pack for short durations - a new problem arises when you pick up the pack again for another session with the question hanging in your head - should I charge the batteries again or carry spares - just in case.