The BBU will hold the data in cache for a period of time (up to 72 hours). If power is resumed within that period data will be writted back to the disk. For RAID 5 configuration the recommanded Write Cache Policy is Write Back with BBU. The RAID controller will use Write Back policy when BBU is present to improve write performance, and switch automatically to Write Through to protect data when BBU is bad or missing, or in a re-charge cycle.
Upgrading RAID controller firmware will not erase your data or RAID configuration. However, doing a full backup before upgrading is always recommended.