Activation of the battery is through the addition of electrolyte. This solution causes the chemical actions to take place between the lead peroxide of the positive plates and the sponge lead of the negative plates. The electrolyte is also the carrier that moves electric current between the positive and negative plates through the separators.
The automotive battery has a fully charged specific gravity of 1.265 corrected to 80°F (27°C). Therefore, a specific gravity of 1.265 for electrolyte means it is 1.265 times heavier than an equal volume of water. As the battery discharges, the specific gravity of the electrolyte decreases because the electrolyte becomes more like water. The specific gravity of a battery can give you an indication of how charged a battery is.
- Fully charged: 1.265 specific gravity
- 75% charged: 1.225 specific gravity
- 50% charged: 1.190 specific gravity
- 25% charged: 1.155 specific gravity
- Discharged: 1.120 or lower specific gravity
These specific gravity values may vary slightly according to the design of the battery. However, regardless of the design, the specific gravity of the electrolyte in all batteries will decrease as the battery discharges. Temperature of the electrolyte will also affect its specific gravity. All specific gravity specifications are based on a standard temperature of 80°F (27°C). When the temperature is above that standard, the specific gravity is lower. When the temperature is below that standard, the specific gravity increases. Therefore, all specific gravity measurements must be corrected for temperature. A general rule to follow is to add 0.004 for every 10°F (5.5°C) above 80°F (27°C) and subtract 0.004 for every 10°F (5.5°C) below 80°F (27°C).
In operation, the battery is being partially discharged and then recharged. This represents an actual reversing of the chemical action that takes place within the battery. The constant cycling of the charge and discharge modes slowly wears away the active materials on the cell plates. This action eventually causes the battery plates to sulfate. The battery must be replaced once the sulfation of the plates has reached the point that there is insufficient active plate area.
In the charged state, the positive plate material is essentially pure lead peroxide, PbO2. The active material of the negative plates is spongy lead, Pb. The electrolyte is a solution of sulfuric acid, H2SO4, and water. The voltage of the cell depends on the chemical difference between the active materials.
FIGURE. Chemical action that occurs inside of the battery during the discharge cycle.
The illustration shows what happens to the plates and electrolyte during discharge. The lead (Pb) from the positive plate combines with sulfate (S04) from the acid, forming lead sulfate (PbSO4). While this is occurring, oxygen (O2) in the active material of the positive plate joins with the hydrogen (H2) from the electrolyte forming water (H2O). This water dilutes the acid concentration.
A similar reaction is occurring in the negative plate. Lead (Pb) is combining with sulfate (SO4), forming lead sulfate (PbSO4). Hie result of discharging is changing the positive plate from lead dioxide into lead sulfate and changing the negative plate into lead sulfate. Discharging a cell makes the positive and negative plates the same. Once they are the same, the cell is discharged.
The charge cycle is exactly the opposite. The lead sulfate (PbSO4) in both plates is split into its original forms of lead (Pb) and sulfate (SO4). The water in the electrolyte splits into hydrogen and oxygen. Hie hydrogen (H2) combines with the sulfate to become sulfuric acid again (H2S04). The oxygen combines with the positive plate to form the lead peroxide. This now puts the plates and the electrolyte back in their original form and the cell is charged.
FIGURE. Chemical action inside of the battery during the charge cycle.