When a lot of PV systems connect to utility grid, it is very important to control huge reversed power from PV to keep a quality of electric power. In order to control output power of PV systems, use of battery is the one of the solution. Ni-MH battery is the most proper battery from the energy performance and safety. However, since Ni-MH battery is expensive more than lead-acid battery, it is very important to clear the optimum capacity. To decide the optimum capacity from the standpoint of power quality and economics, it is necessary to simulate Ni-MH battery combined to PV system with high accuracy. To simulate with high accuracy, we have to make a good equivalent cuircuit of Ni-MH battery. To make the equivalent circuit, internal impeadance changes for a long time after charge or discharge. We studied the proper timing to measure internal impeadance of Ni-MH battery by charge or discharge test.