Due to it low cost, hydrometers are often used to measure the specific gravity or the concentration of the sulfuric acid in a battery. The specific gravity measurement reflects the state of charge of a battery. In other words, the higher the charge level of a battery, the greater will be the specific gravity reading. Over time, all acid based batteries will experience a depletion in active material, which is normal. The more pronounced the depletion, the lower the specific gravity will be in a fully charged battery. As batteries age the capacity to hold a charge deteriorates. Therefore, an aged battery even after going through a process of desulfation, will still show a lower specific gravity than a new battery.
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