The ac57 power adapter allows photographers to use their 580ex ex ii speedlites with an external battery pack while in an ac7 rf shield the ac57 routes the battery pack cable to a position that reduces emi and increases the range and reliability of your 580ex or 580 ex ii ac7 combination simply plug the cord of your canon compact battery pack into the ac57 adapter and then plug the adapter into the side of your speedlite and put your speedlite onto the hotshoe