The WLR089 Xplained Pro Evaluation board has a power supply domain selection header (J302). Place a jumper cap between pins 1 and 2 of the header (J302) to connect with the battery to test the Battery Backup mode using the PB03 supply controller pin (SUPC/VBAT). Place the jumper cap between pins 2 and 3 to connect PB03 with the 3.3V module supply (VCC_MCU).