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Hybrid energy storage system (HESS), consisting of battery, ultracapacitor, and the converter interface, is one of the fundamental components in hybrid electric vehicles (HEVs). This paper presents a comparative reliability study between the state-of-art HESS topologies used in HEVs. The operational principles of different HESS topologies are briefly explained, and Markov-based reliability model of each topology is developed. The mean time to failure (MTTF) of each topology is derived in terms of the failure rates of its building blocks. The MTTFs of different topologies are compared and the most reliable topologies are identified. HESS topologies are arranged based on their reliability rankings from the most reliable to the least reliable.