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Automotive lead acid batteries are made up of stacked lead and lead oxide plates. The plates are surrounded by an electrolyte solution. The make-up of the solution is 35 percent sulfuric acid and 65 percent water.== Testing Lead Acid Battery with Hydrometer ==Most modern car batteries are sealed, however, many motorcycle, ATV, and other utility batteries allow access to the solution inside. With the battery full charged take readings from each cell. Insert the hydrometer into each of the openings and take measurements. The reading on the hydrometer will tell you the specific gravity of each cell. There should be no more than .05 differences between the cells. The specific gravity should be above 1.225 or the battery is defective or near end of life.