The requirements of the environment, saving resources, and comfortableness for automobiles are constantly increasing year after year. There is a trend to reduce the fuel cost and exhaust gas by changing the power source to high voltage, shutting the engine off during stopping and replacing the hydraulics parts and compressor, etc. which are driven by the out-put of the engine, with electrically actuated parts. A valve-regulated lead-acid battery is considered to be promising as a power supply suitable for this 42V electrical power system from the viewpoint of reliability and price, and is being actively studied. The 36V-VRLA battery applied to the 42V power supply system of the automobile demands higher out-put characteristics and longer life than a conventional automobile battery. Distances between electrodes are less than in a conventional battery and the pressure to assemble the electrodes will be higher than in the conventional battery. Mechanical short-circuit caused by physical force and chemical short-circuits caused by dendrite growth in this design of the battery could cause problems. We solved the problems by adding organic fibers for preventing the mechanical short-circuit and inorganic filler to restrain the chemical short-circuit to the AGM separator. This new AGM separator has twice the performance preventing mechanical short-circuits and five times for preventing chemical short-circuits in comparison with the separator in the past mixed with the fine glass fibers and coarse glass fibers. This AGM separator can restrain mechanical short-circuits and chemical short-circuits even if the thickness of the separator is decreased. This developed AGM separator should be of wide application because it can be applied to the VRLA battery demanded for the high out-put performance.