Hybrid-electric and pure electric vehicles include a traction battery. During vehicle operation, impedance parameters of the traction battery may be estimated. To ensure accurate estimation results, certain persistent excitation criteria may be met. These conditions may not always be met, in which case active excitation of the traction battery power demand may be initiated. During periods of generally constant battery power demand in which a predetermined range of frequency component amplitudes of battery power demand is less than a predetermined magnitude, active excitation may be desired. A controller may cause at least one of the frequency component amplitudes to exceed the predetermined magnitude without affecting acceleration of the vehicle. Battery power demand may be affected by operation of an electric machine and an electrical load. An engine and a wheel brake may be operated to offset changes in battery power demand such that driver wheel output power is not affected.