DC power source determination circuitry for use with an adapter

An adapter device includes a DC/DC adapter to receive DC power from a DC power source, and output a regulated DC voltage (Vout). A source determination circuitry receives the DC power from the DC power source and compares a magnitude of a voltage of the DC power with a reference magnitude of a reference voltage (Vref). When the magnitude of the voltage of the DC power is greater than the reference magnitude, a data signal (Vdata) having a first value is output. When the magnitude of the voltage of the DC power is less than the reference magnitude, the Vdata signal having a second value is output. The Vdata signal is received by control circuitry of an electronic device. When the Vdata signal has the first value, the electronic device operates in a first mode where battery charging circuitry is disabled. When the Vdata signal has the second value, the battery charging circuitry is enabled.