The Lead-Acid Storage Battery

Author: E.E. Kimberly

The lead-acid battery consists of one or more cells in which one or more "positive" plates of lead peroxide on a lead frame are interleaved with two or more "negative" plates of spongy lead and the whole is immersed in a solution of sulfuric acid and water. See Fig. 22-1.

When the battery is discharging, the lead peroxide is combining with the sulfate (SO₄) of the electrolyte to form lead sulfate (PbSO₄). The lead sulfate collects on the positive plates. Lead sulfate is formed on the negative plates and collects on them. This may be written as follows:

Fig. 22-1. Lead-Acid Storage Battery

Fig. 22-2, Hydrometer Used in Testing Lead-Acid Batteries

Thus, when the battery is fully charged, the SO₄ is in the electrolyte; and, when the battery is discharged, the SO₄ is on the plates in the form of lead sulfate and the electrolyte is greatly diluted with water. Hence, the condition of charge may be determined by measuring the specific gravity of the electrolyte. Fig. 22-2 shows a hydrometer reading of 1.280 indicating full charge and one reading 1.150 indicating complete practical discharge.

Water lost by evaporation and electrolysis must be replaced frequently by distilled water or other water of satisfactory purity. Batteries designed for portable service and for high momentary power output such as is required in automotive service usually have a specific gravity of 1.280 when fully charged. Batteries designed for stationary service and continuous duty usually have a specific gravity of about 1.220. It is therefore necessary to know the specific gravity for which the battery was designed before a hydrometer can be used intelligently to determine the condition of charge.

The automotive type of battery has the Fauré type of plate, which consists of active material pasted into recesses or pockets of a lead frame. This type of plate affords a maximum energy capacity per pound of weight and per cubic foot of space. The life of the Fauré plate seldom exceeds 2 or 3 years in active service, largely because of shedding from the positive plates.

The stationary type of battery has the Planté type of plate, which consists of active material formed electrolytically on the lead frame. This type of plate affords a maximum useful life, but is too heavy and bulky for portable use.

The normal open-circuit voltage of a lead-acid battery is approximately 2 volts per cell. The battery should not be discharged further after its terminal discharge voltage falls to 1.75 volts per cell. Over a discharge period of 6 to 8 hours, the average voltage is roughly 1.95 volts per cell.

The maximum charging voltage required is about 2.6 volts per cell.