This disclosure describes techniques to charge a stylus using the existing near field communication (NFC) integrated circuit and coil on the mobile device. The storage slot for a stylus in a case that holds a mobile device is designed such that upon insertion, the stylus NFC coil automatically aligns with the phone NFC coil, thereby enabling charging during storage. Effectively, the storage slot functions as a charging dock for the stylus. Since the storage slot of the phone case is the normal home for the stylus, charging takes place in the background and becomes a seamless experience for the user. A Hall sensor on the main logic board of the mobile device is used to detect the presence of the stylus; no other hardware changes are required. The device NFC mode is automatically configured to allow other NFC functionality while selectively charging the stylus when the battery level of the stylus falls below a threshold.
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Qian, Chengliang; Chang, Hu-Chi; and Wu, Weiyang, "Charging a Stylus using Mobile Device Near Field Communication (NFC) Coil", Technical Disclosure Commons, (July 18, 2023)