A battery-less RFID tag device with an RFID antenna assembly and RFID transmitter, which harvests power from an RF source is disclosed. The RF source can be a mobile phone, and embedded system, and the like. The RFID device can include an energy harvesting component, an energy conversion component, an energy storing component, a non-transitory memory, a controller, and an RFID transmitting component. The energy harvesting component can inductively harvest energy from RF transmissions, which charges a capacitor to power other RFID device components, including the RFID transmitter. In one embodiment, the battery-less RFID tag device can be utilized in conjunction with a mobile phone for vehicle toll payments, which are RFID based.