Hello @Skell1000
Hope you are doing well.
According to QN9090 User Manual, this device should integrate a supply monitor that is connected to ADC channel 6: this monitors VBAT.
Please, take a look at the BLE example projects from the SDK (for example heart_rate_sensor); in board.c file you can find the function uint8_t BOARD_GetBatteryLevel(void). Could you please confirm if this is useful for you?
Regards,
Eduardo.