20mm Vulcan is one of the representative weapon system that widely operated in army, navy and airforce in Korea. But unstable power which is caused by bad battery cells or insufficiently charged battery box has often caused malfunctions when Vulcan barrel is working. This paper is about improvements of maintenability of APS which is used charging the battery box. The main improvement effects are as follows. First, Break-down of the Power Transistors(PT-8502) inside APS can be prevented by limiting over–current. Second, Automatic charging the battery box is possible by using Micro Controller Unit and SSR. Third, The charging status of battery box can be checked and deficient battery cells recognized by adding LED.