A charger and a mobile device exchange messages indicating that high wattage charging is supported over a bidirectional communication channel. After the message exchange, the charger sends a controlled output voltage and current to the mobile device over a cable connected between the charger and the mobile device. The mobile device takes an input voltage measurement that corresponds to the controlled output voltage after being passed through the cable. The charger or mobile device may then calculate a resistance of the cable based on a difference between the controlled output voltage and the input voltage measurement. Based on the resistance of the cable, the controlled voltage and current used to charge a battery of the mobile device is adjusted in order to avoid exceeding capacity of the cable.