An innovative method is developed to measure specific surface areas (SSAs) of a battery’s cathode active material from its particle size distribution and XRD measurements and its applications to observe and monitor the rate-capability behavior of the lithium ion battery. This method is applied on Li-ion batteries of spinel cathode material and its validity is discussed based on the measured capacities of the fabricated batteries and comparison of the calculated SSAs with those measured experimentally. The present method predicts the processing effect on battery performance and is thought to be time-saving in optimizing the processing parameters of the cathode material. It can also be extended to other systems of spinel structures thus assisting, in general, the development of lithium ion batteries.