Lead-Acid Storage Batteries
OPERATON AND CONSTRUCTION
volume of electrolyte expands as temperature rises and contracts as temperature drops,
therefore affecting the density or specific gravity reading. As the volume of electrolyte
expands, the readings are lowered and, conversely, specific gravity increases with colder
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 for
certain applications are shown in Table 1.
Table 1. Specific gravities for flooded batteries.
Heavily cycled batteries such as for electric vehicles
UPSStandby with high momentary current discharge
General applications such as power utility and telephone
In the selection of a battery for a given application, some of the effects of high or low specific
gravity to be considered are:
Less space required
More space required
Higher momentary discharge rates
Lower momentary discharge rates
Less adaptable to "floating"
More adaptable to "floating"
More standing loss
Less standing loss
A solution of higher specific gravity is heavier per unit volume than one of lower specific
gravity. Therefore the more concentrated electrolyte created during charging sinks to the
bottom of the battery jar creating a gradient in specific gravity. The gassing that occurs on
overcharge 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 an
equalizing charge and subsequent float charge for at least 72 hours. The reader should seek
additional guidance on specific gravity from ANSI/IEEE Std 450, IEEE Recommended
Practice for Maintenance, Testing, and Replacement of Large Lead Storage Batteries for