Abstract: The batteries are important part of 220 V DC system in Longkou Hydropower Station.After repeated check discharge tests, the author found that the first group of battery capacity did not meet 80% of the rated capacity, so its renovation and capacity test were carried out.Carry out three capacity tests, record each test data and calculate the battery discharge, discharge capacity could reach 100% of the rated capacity, according to the provisions of capacity test, the group of battery capacity test is qualified.