Backup Battery Modules (also called Cache Backup Battery Modules) are highly recommended to prevent data loss in the event of power failure and should be considered mandatory for business-critical data.
The Backup Battery Modules are redundant hot-swappable modules that provide power to the RAID controller cache for up to 72 hours in the event of an input power failure. When power is restored, Xserve RAID flushes the data protected in the RAID controller cache to the hard drives, preventing loss of the last transaction's data.
Note: Without a UPS connected to at least one of the power supplies, the Cache Backup Battery modules will backup the controller cache, but you could experience data loss if you do not set the Drive Write Cache to Off.