If your server storage controllers and adapters have cache batteries installed, then it is most important that they are kept in good condition. Cache improves disk I/O dramatically, but if the power fails before cache data has been physically written to disk, then you have a problem. The battery prevents this. Your server management and diagnostic software will tell you when the cache battery needs replacing.
Bargain Hardware have a huge range of server ancillary items such as batteries, as we regularly decommission large server farms and corporate data centres. We stock batteries from HP Smart Array, Dell PERC and IBM/Lenovo ServeRAID units.