A RAID controller battery is used to protect the data in the cache memory of the controller in case of a power failure. The cache memory allows the controller to improve the write performance by buffering the data and writing it to the disk in an optimized way. However, if the battery fails, the controller will switch to write-through mode, which means it will write the data directly to the disk without caching. This will reduce the write performance and increase the latency of the system