The HP 656809-B21 is a Flash-Backed Write Cache capacitor pack with a 36-inch cable, designed for HPE Smart Array controllers in ProLiant Gen8 and Gen9 servers. It provides backup power to the FBWC module during unexpected power loss, ensuring cached write data is safely committed to flash memory. Unlike traditional batteries, the supercapacitor design offers a longer operational lifespan with no scheduled replacement cycles and no temperature-related degradation.

This is the long-cable variant (36") suitable for 2U and larger chassis configurations where the controller and capacitor mounting points are farther apart. The short-cable variant is 654873-003 (24"). This is a high-volume replacement part in TPM market as ProLiant Gen8 and Gen9 servers remain widely deployed.