The asset monitoring system and associated method includes an asset monitor for providing a remotely located central station with information relating to a container, such as a trailer, both during tethered periods in which the energy storage reservoir of the asset monitor is electrically connected to an external power source, such as the electrical system of a tractor or truck, and during untethered periods in which the energy storage reservoir of the asset monitor is electrically untethered or disconnected from the external power source. As a result, the energy storage reservoir is recharged by energy received from the external power source during tethered periods and supplies energy to the asset monitor during both tethered and untethered periods. The asset monitor also includes a controller for controlling its operations. The controller includes tether status determining means for separately identifying tethered periods and untethered periods. The controller also includes power management means for placing the asset monitor in an active mode during tethered periods and in an energy conservation mode during untethered periods. Once placed in an energy conservation mode, the asset monitor can continue to draw power from the energy storage reservoir so as to perform selected ones of its operations. Typically, however, the asset monitor continues operations at a reduced rate or frequency relative to the rate of operations performed by the asset monitor during the active mode in order to conserve the energy stored by the energy storage reservoir.