A first device, such as a charger, includes electrical contacts to supply power to a second device, such as a wearable device. The first device includes a detection circuit that measures a sense voltage proportional to an effective resistance, which includes resistances of the contacts, an internal resistance of the second device, and any contaminants between the contacts. When the sense voltage is within a range, the detection circuit determines the second device is compatible and correctly oriented and determines there are insufficient contaminants to alter the effective resistance. The first device de-energizes the contacts while the sense voltage is outside of a voltage range and provides a power supply to one or more of the contacts when the sense voltage is within the voltage range. In some aspects, the detection circuit may delay the power supply for a period of time to avoid false detection events.