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Near Field Communication (NFC) shows potential in multiple areas like payment, identification, transport, etc. To enable these features to a larger group of users, NFC-capability is nowadays integrated in mobile devices like smart phones. This integration unfortunately leads to an increase of the device's battery drain because the transponder is powered by the provided magnetic field of the mobile device. To decrease this drain, power-management techniques like magnetic field strength scaling are used. Through this scaling the power transfer can be reduced to the transponder's required level. The challenge of this technique is to dynamically adapt the magnetic field strength to physical relation changes of the transponder even during communication. Without this adaption, scaling down the field can lead to the transponder's undersupply or energy is wasted through oversupply. This paper proposes a method, named NFC-DynFS, to realize this adaption and to proper scale the magnetic field strength. In a case study a system, to read digital business cards using NFC-DynFS, is simulated and implemented on real hardware. The power consumption results are evaluated and compared to implementations without NFC-DynFS. Furthermore, possible undersupplies of the transponder are investigated. It can be shown that, compared to implementations without field strength scaling, approximately 26% of the energy can be saved and an undersupply of the transponder can be avoided, until the reader's power transmission limit is reached.