Supongo ya lo has solucionado. Por si acaso
según la hoja de datos:
VBackup
Connection for a Secondary Power Supply. Supply voltage must be held between 1.3V and 5.5V for proper
operation. This pin can be connected to a primary cell, such as a lithium button cell. Additionally, this pin
can be connected to a rechargeable cell or a super cap when used with the trickle-charge feature. If not
used, this pin must be connected to ground. UL recognized to ensure against reverse charging current
when used with a lithium battery
