A self contained sensor module mounted on a power transmission line. The module is capable of measuring various parameters such as line current, voltage, conductor temperature, and ambient temperature. The parameter values are processed by sensor electronics which digitize and transmit the values to a ground receiver. The sensor module normally derives its power through electromagnetic induction resulting from current flowing through the transmission line. The module contains a power back-up system, including a battery, which allows sensor operation with minimum current and zero current in the transmission line. The battery is float charged while the sensor is mounted on the transmission line using excess energy derived through electromagnetic induction. When line current drops to a predetermined minimum value, float charging is disabled and the battery supplies power to the sensor module. When battery power is depleted to a predetermined level, data processing and transmission are disabled until the battery can be recharged.