This product is either discontinued or we cannot get that from suppliers.
Qnap CTL-ES1640DC-V2-48G Controller, Version 96GB, Including FAN and BBU, Controller FRU for ES1640dc v2 96GB, Dual active controllers, dual mini-SAS channel backup, 40GbE-ready, SAS 12Gb/s-enabled.
The QNAP Field-Replaceable Controller Unit is a spare controller for the 96GB version of their ES1640dc v2 16-Bay NAS Enclosure. It includes a fan and a backup battery unit.