This RAID controller is in a reasonably flash (for it's year) server. The battery shown is not the write cache backup (this is a separate larger battery)... so what is it for, how often should it be replaced and where do I get these?

It's not a 2032, 2450 or other common lithium battery. It's chunkier and has a ridge around the top side.

The battery is 16.3mm x 6.1mm. The server is a Fujitsu Primergy RX300.