This thesis presents the steps taken to design a charger and tester instrument for Li-ion batteries used on picosatellites. The paper discusses the most widespread battery types for small satellite missions. It gives a detailed description of the characteristics and the charging method of the Li-ion cell. The thesis lists the most important measurable parameters of a battery and their measuring principles. It introduces different battery equivalent circuits to define the meaning of internal resistance. The paper covers the chemical background of the elements of the chosen equivalent circuit. It presents an algorithm capable of determining the values of the elements of said model. It lists the technological requirements of the charger and tester device, then discusses the function of the different components of the designed circuit. The thesis also describes the operation of the embedded software created for the instruments. It analyzes the structure of the battery simulator designed for the authentication of the testing device. The measurements used for authentication are also presented. Finally, the results of battery tests on different temperatures are prested and compared.