The wireless sensor nodes are most energy-efficient when they operate at the minimal operating voltage. The nominal voltage of the batteries used is often significantly higher. This paper analyzes DC-DC power conversion from the point of view of ultra low-power wireless sensor nodes. Using a modified circuit of a suitable commercial DC-DC converter and a supercapacitor, this paper shows that it is possible to extend battery lifetime of a wireless sensor node for certain spectrum of applications. The efficiency of the DC-DC conversion as well as supercapacitor charge-discharge efficiency have been analyzed and measured. The influence of battery discharge currents and leakage currents is also presented. Furthermore, in this paper, we outline the specifications of a module needed between the battery and the sensor node that will be controlled by the node's MCU and will keep the node operating at minimal operational voltage while at the same time maximizing the energy delivered from the battery to the node. Thus, this paper also aims at setting foundations for future development of an energy-optimization module for ultra low power wireless sensor nodes.