The process of transferring data to and from disk storage includes storing the data temporarily in cache memory located on the RAID array controller that is managing the data transfer. Write cache can improve the performance of any RAID level by combining two or more adjacent write requests into a single request for a disk drive. Once a data write is completed, the cache must be able to protect this data in the event of a power loss before the write data is moved to the drives. If external power to the system is interrupted, if the host computer fails, or if the RAID controller fails, battery power maintains data in the cache long enough for the user to recover the data and preserve the integrity of business-critical information. Depending on the capacity of the batteries when external power is lost, battery-backed cache is typically capable of retaining the integrity of data in the cache for several days.