This paper provides a theoretical analysis on the energy loss of a battery-ultracapacitor hybrid energy storage system based on the equivalent series resistances and a pulsed current load profile. The optimal current distribution that minimizes the overall energy loss is proved to be solely determined by the ratio of internal resistances between battery and ultracapacitor packs. Due to a large difference in the internal resistances, a quasi-optimal current distribution can be considered to let the battery pack provide the average load current and ultracapacitor pack supply the entire dynamic load current. This result clearly demonstrates that the ultracapacitor pack should supply the most of the dynamic load current not only because of battery protection, but also for energy loss minimization. Finally, the theoretical analysis is validated by both simulation and experimental results. Additional discussions, such as sensitivity analysis, the influence of the sizing of ultracapacitors, and a realistic test cycle are also added for reference purposes.