Background: The paper deals with improvement of electric vehicles' performances via optimization of their energy storage. Galvanic batteries are known to suffer from current surges and the charging currents during braking should be less than nominal discharge currents. Ultracapacitors can mitigate these problems, but to apply them it is necessary to choose the optimal ratio of capacities. Material and methods: Different configurations of hybrid (combined) energy storage systems are examined and specific features of their operation are outlined. In order to find optimal ratio of battery and ultracapacitor's capacitances, all possible combinations are explored with limitations taken into account. Results: It is shown that the best topology is "battery - double buck DC/DC converter - ultracapacitor". The optimal point (capacities of the components) always lies at the border of the limiting curve. Concrete results for various driving conditions and vehicles are given. Conclusions: The schematic solution for power electronic converter, optimization approach and obtained results may be useful for engineers who design powertrains for electric vehicles.