Lead-Acid Storage BatteriesDOE-HDBK-1084-95OPERATON AND CONSTRUCTIONRev. 0Page 19Batteriesvolume of electrolyte expands as temperature rises and contracts as temperature drops,therefore affecting the density or specific gravity reading. As the volume of electrolyteexpands, the readings are lowered and, conversely, specific gravity increases with coldertemperatures.The specific gravity for a given battery is determined by the application it will be used in,taking into account operating temperature and battery life. Typical specific gravities forcertain applications are shown in Table 1. Table 1.Specific gravities for flooded batteries.Specific gravitiesApplication1.300Heavily cycled batteries such as for electric vehicles(traction)1.260Automotive (SLI)1.250UPS—Standby with high momentary current dischargerequirement1.215General applications such as power utility and telephoneapplicationsIn the selection of a battery for a given application, some of the effects of high or low specificgravity to be considered are:Higher gravityLower gravityMore capacityLess capacityShorter lifeLonger lifeLess space requiredMore space requiredHigher momentary discharge ratesLower momentary discharge ratesLess adaptable to "floating"More adaptable to "floating"operationoperationMore standing lossLess standing lossA solution of higher specific gravity is heavier per unit volume than one of lower specificgravity. Therefore the more concentrated electrolyte created during charging sinks to thebottom of the battery jar creating a gradient in specific gravity. The gassing that occurs onovercharge serves as a "mixer" and makes the specific gravity uniform throughout the cell. To avoid erroneous readings, specific gravity measurements should only be taken after anequalizing charge and subsequent float charge for at least 72 hours. The reader should seekadditional guidance on specific gravity from ANSI/IEEE Std 450, IEEE RecommendedPractice for Maintenance, Testing, and Replacement of Large Lead Storage Batteries for
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