We describe the design of an electrical power system prototype for CubeSats and BalloonSats, it uses a lithiumpolymer battery as primary energy source and step-up dc-dc converters to get 12 and 5 volts. It also includes a battery charger circuit and power path management, so, this prototype could be used in terrestrial applications as uninterruptible DC power to 5 and 12 volts. We present efficiency measurements of integrated circuit dc-dc converters for different load conditions. The dc-dc converters show efficiency about 80%. This paper includes measurements of the charge and discharge characteristics of the battery. In addition, it shows the transient response of the 5 volts regulator when the primary source varies due to the external source disconnection. The results presented will contribute to the future development of an efficient energy system with solar cells.