The fatigue life of ultrasonically welded lithium-ion battery tab joints is studied for electric and hybrid–electric vehicle (EV and HEV) applications. Similar to metallic materials, the electrical resistance of these ultrasonic welds strongly depends on their quality and the crack growth under fatigue loading. A fatigue life model is developed using the continuum damage mechanics (CDM) formulation, where the damage variable is defined using the electrical resistance of ultrasonic welds. Fatigue tests under various loading conditions are conducted with aluminum–copper battery tab joints made under various ultrasonic welding conditions. It is shown that the electrical resistance of ultrasonic welds increases characteristically during the fatigue life test. There is a threshold for the damage variable, after which the ultrasound welds fail rapidly. Due to welding process variation, welds made under the same process settings may have different fatigue performance. This quality difference may be classified using two parameters estimated from the fatigue life model. By monitoring the electrical resistance, it is possible to predict the remaining life of ultrasonically welded battery tab joints using only a portion of the fatigue test data. The prediction is more reliable by incorporating data beyond the half-life of the joints during the fatigue test.