The Canon CP-E4 Compact Battery Pack is for use with the Canon 540EZ, 550EX, 580EX, 580EX II Speedlites offering a portable & powerful power solution. Batteries in your Speedlite 580EX II, MT-24EX & MR-14EX flash units will usually provide more than enough power for most uses but sometimes you want to take lots of pictures with flash, the flash needs to recycle really quickly, you might want to use a high power burst of flash frequently. The Canon CP-E4 battery pack provides an extra power source to help you to do this. Any photographer making use of flash extensively should consider the CP-E4. With eight AA batteries (not included), it will help the flash to recharge even quicker & will give you more flashes between battery changes.