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Specific Gravity

One of the key parameters of battery operation is the specific gravity of the electrolyte. Specific gravity is the ratio of the weight of a solution (sulfuric acid in this case) to the weight of an equal volume of water at a specified temperature. This measurement is usually measured using a Hydrometer. The specific gravity of a fully charged GB Industrial Battery is the industry standard of 1.285.

Specific gravity is used as an indicator of the state of charge of a cell or battery. However, specific gravity measurements cannot determine a battery's capacity. During discharge, the specific gravity decreases linearly with the ampere-hours discharged as indicated in the illustration below.

Changes in voltage and specific gravity during charge and discharge.

Therefore, during fully charged steady-state operation and on discharge, measurement of the specific gravity of the electrolyte provides an approximate indication of the state of charge of the cell. The downward sloping line for the specific gravity during discharge is approximated by the equation below:

Specific gravity = single-cell open-circuit voltage - 0.845 (example: 2.13v 0.845 = 1.285)

Or

Single-cell open circuit voltage = specific gravity + 0.845.

The above equations permit electrical monitoring of approximate specific gravity on an occasional basis. As mentioned earlier, specific gravity measurements cannot be taken on sealed lead-acid batteries. Measurement of the cell open-circuit voltage has been used as an indicator of the state of charge of a sealed battery. More reliable methods for determining the state of charge of sealed batteries are under development.

The specific gravity decreases during the discharging of a battery to a value near that of pure water and it increases during a recharge. The battery is considered fully charged when specific gravity reaches its highest possible value.

Specific gravity varies with temperature and the quantity of electrolyte in a cell. When the electrolyte is near the low-level mark, the specific gravity is higher than nominal and drops as water is added to the cell to bring the electrolyte to the full level. The 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 temperatures.

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.

Specific Gravity

Applications

1.285

Heavily cycled batteries such as for forklifts (traction).

1.260

Automotive (SLI)

1.250

UPS Standby with high momentary discharge current requirement.

1.215

General applications such as power utility and telephone.

Table 1

As mentioned earlier, the specific gravity (spgr.) of a fully charged industrial battery, or traction battery, is generally 1.285, depending on the manufacturer and type. Some manufacturers use specific gravities as high as 1.320 in an attempt to gain additional Ah capacity, but at the cost of a shorter cycle life.

Represented in Table 2 (below), the electrolyte in a fully charged battery is still 62.48% water. Higher gravity acid, such 1.600 spgr, can be used to adjust the gravity of batteries that have been diluted due to repeated overflow caused over-filling. Note: Acid adjustments should only be performed by factory-trained technicians in a controlled environment.

% Sulfuric Acid

% Water

Specific Gravity (68F)

37.52

62.48

1.285

48

52

1.380

50

50

1.400

60

40

1.500

68.74

31.26

1.600

70

30

1.616

77.67

22.33

1.705

93

7

1.835

Table 2

In the selection of a battery for a given application, some of the effects of high or low specific gravity to be considered:

Higher Gravity =

Lower Gravity =

More capacity

Less capacity

Shorter life

Longer life

Higher momentary discharge rates

Lower momentary discharge rates

Less adaptable to "floating: operation

More adaptable to "floating" operation

More standing loss

Less standing loss

Table 3

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. 

 

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Last Updated: Sunday, April 08, 2007 - 6:22 PM Eastern Time.