As Rick's tread suggests you can go the whole hog backing up B4 throwing the cabinet breakers - but you dont have to, the battery (as long as it's a Nortel Option 11 specific <ugly battleship grey things>) will have a breaker of their own, of course do a dump/back-up the Mail if you wish then follow the proceedure as specified in :Option 11C Fault clearing Guide Document Number: 553-3011-500, chapter 17: Replacing Equipment, Replacing batteries in the NTAK75/76 Battery Box - The sam also holds for the even uglier/even bigger QBL24's.
Go on, you know you want to!