Battery electric vehicles and hybrid electric vehicles require electric energy storage systems that exhibit high energy and power density, as well as good cycle life. Batteries possess good energy density, whereas ultracapacitors possess high power density and cycle life. The complementary features of batteries and ultracapacitors can be advantageously combined to create an integrated system that exhibits high performance with low weight and adequate battery lifetime, at an affordable cost. This paper presents simulation studies on the benefits of adding ultracapacitors to a fuel cell battery hybrid transit bus operating on two standardized driving schedules (Manhattan Bus Cycle and UDDS). Simulations were conducted using our LFM powertrain simulator which was developed in MATLAB/SIMULINK. The energy storage systems considered here include battery only, as well as various combinations of batteries and ultracapacitors. Simulation results show that the addition of ultracapacitors greatly improves performance parameters such as battery C-rates, energy throughput, and energy storage heat generation at comparable cost and weight. (C) 2011 Elsevier B.V. All rights reserved.