The Battery Backup Unit (BBU) is an add-on card that can be attached to a 3ware 9000 RAID controller to supply power from a battery pack in the event of a system power loss. This allows the controller to use write-caching for optimal performance and to preserve data in the event of a system power failure. When fully charged, the battery preserves the contents of the cache memory for up to 72 hours. When power is restored, the cached write data is written to the disks.