When an encryption key is stored in the FPGA battery-backed RAM (BBRAM), the encryption key memory cells are volatile and must receive continuous power to retain their contents. During normal operation, these memory cells are powered by the auxiliary voltage input (VCCAUX), although a separate VBATT power input is provided for retaining the key when VCCAUX is removed. Because VBATT draws very little current (on the order of nanoamperes), a small watch battery is suitable for this supply. To estimate the battery life, refer to VBATT DC Characteristics in the respective data sheet ( Kintex UltraScale FPGAs Data Sheet: DC and AC Switching Characteristics (DS892) or Virtex UltraScale FPGAs Data Sheet: DC and AC Switching Characteristics (DS893)) and the battery specifications.
VBATT does not draw any current and can be removed while VCCAUX is applied. VBATT cannot be used for any purpose other than retaining the encryption keys when VCCAUX is removed.