The VCC_BACKUP pin of the Apalis modules is used to power the module's internal RTC. The VCC_BACKUP pin is only required to be connected if the RTC is required to keep time while the module is powered off.
Note: This is in contrast to the Colibri modules which require power to be applied on the VCC_BATT pin even if the RTC is not required.
Below is an example of how to supply the VCC_BACKUP pin with either a battery or a primary 3.3V source:
Some modules provide battery charging functionality. Therefore, when using a single non-rechargeable voltage source, the inline diode and resistor will still be required to protect the supply from reverse current. A FET may also be used for reverse current protection. Take care to ensure the circuit's reverse leakage current is low enough for safe operation with the chosen battery/power supply. High quality and/or redundant diodes should be used to safeguard against component failure which could result in battery fire/explosion.
More information regarding the use of a backup battery is provided at the following page: Backup battery.