The external Canon Compact Battery Pack CP-E4 improves flash recycle times significantly and increases the number of firings before batteries need to be replaced.
The pack takes the optional CPM-E4 rapid-change Battery Magazine that serves as a replacement or backup battery. It can be loaded with 8 AA-size batteries - alkaline, lithium, or Ni-MH - prior to a shoot to ensure fast and smooth transitions from one battery set to the next - important in critical shooting situations or to keep up momentum in the studio.
Updated from the earlier version, the CP-E3, the CP-E4 has been improved with better dust and moisture sealing at the contact points, to get it to match the weather-resistance characteristics of the 580EX II Speedlite.
CP-E4 Compact Battery Pack