The RAID card battery has various error codes that indicate it's condition, which appear in RAID Utility when there's a problem.

The error codes follow the format "Battery failed (code 0x#0 = XX)", where 'XX' can be a number, usually 17, 18 or 19. Good luck finding out what any of those mean... You may also see "Battery failed (other than expired)".

In most cases this guide will prove successful regardless of the error code.