Green transportation is playing a key role in driving society toward clean energy and reliable infrastructure. A major barrier to accomplishing such an objective is the creation of reliable energy storage systems. The battery model is the most viable option to meet the objective of green energy. To assess the efficiency of the battery model, its parameters signify an important role and hence need to be estimated accurately. This chapter delves into the method for estimating the internal parameters of lithium-ion batteries to varying environmental and load conditions in the context of an equivalent circuit model (ECM). The estimating procedure carries with pulse charge and discharge test data and addresses the battery parameter variation with state of charge. The transient voltage response occurs in the relaxation period of the pulse that is used to calculate the battery parameters. A hybrid pulse power characteristic test is conducted to record the real-time data for the validation of the ECM accurately. The battery parameters are achieved with simulation and experimental data, both at an acceptable degree of accuracy. The pulse test is conducted with the variable C rates under the laboratory conditions keeping the temperature parameter constant throughout the whole experiment.