This study addresses pyroelectric power generation through an original electrothermodynamic cycle for environmentally friendly automotive applications. Relaxor-based ternary Pb(In1/2Nb1/2)O3-Pb(Mb1/3Nb2/3)O3-PbTiO3 (PIN-PMN-PT) ceramics near a morphotropic phase boundary were applied as pyroelectric materials, and their electrical properties were investigated at different temperatures. Structural disordering, depending on the PIN content, influenced the diffuse phase transition between the tetragonal and cubic structures and contributed to the power-generating behavior. The net power-generating energies (Pnet) were 2.43–3.01 mW/cm3 at temperatures above 200 °C and were maintained at above 1 mW/cm3 over a temperature range of approximately 100 °C. In the PIN-PMN-PT system, the temperature dependence of the power-generating performance was improved, and disordering of the perovskite crystal structure can be controlled to achieve pyroelectric energy conversion by the electrothermodynamic cycle over a wider range of usage temperatures.