A lead acid battery is an electrochemical device which stores chemical energy. This chemical energy is converted to electrical energy when the battery is connected to an external load such as a vehicle starter. The chemical energy is created by the chemical action between the materials which form the positive and negative plates of the battery, and the electrolyte:

• lead dioxide (Pb O₂), the positive plate material
• sponge lead (Pb), the negative plate material
• sulfuric acid (H₂SO₄), the electrolyte.

A battery relies upon clean plates and strong electrolyte to receive charging current and offer strong discharge current.

When the battery is connected to a load, the sulfate (SO₄) in the electrolyte combines with the active materials of the plates to form lead sulfate (PbSO₄) and release electrical energy. Electrons flow from the negative terminal to the load and back to the positive terminal of the battery.

The Battery Council International recommends that the specific gravity (ie. the unit of measurement of the sulfuric acid content of the electrolyte) of a fully charged 12 volt battery be 1.265 at 80 degrees F. (26.7 degrees C). This means that the sulfuric acid of a fully charges battery is 1.265 times heavier than pure water. As a battery becomes discharged, the strength of the specific gravity decreases because the sulfate is leaving the electrolyte as it forms the lead sulfate which adheres to the battery plates

State of Charge	Specific Gravity	Voltage (12 volt battery)
100%	1.265	12.68
75%	1.225	12.45
50%	1.190	12.24
25%	1.155	12.06
discharged	1.120	11.89

Thus by the time the battery is discharged, the acid becomes dilute as the sulfate has adhered to the plates of the battery as lead sulfate crystals.

When a discharged battery is recharged, the chemical processes within the battery operate in reverse. The majority of the sulfate leaves the plates of the battery and returns to the electrolyte. However, a residue of sulfate remains on the plates of the battery. The quantity of this residue increases with each charge/discharge cycle of the battery. Over time, the battery plates become coated with an insulating layer of sulfate and the electrolyte is weakened because of the loss of the sulfate from the solution. Both these factors serve to inhibit the electron transfers and thus the energy producing function of the battery.

In batteries of some construction, such as those in motive power applications, the sulfation does not permit the battery to be properly recharged, while in batteries of other types of construction, such as automotive batteries, the sulfation actually causes erosion of the plates of the battery.

Over time the sulfate deposits on the plates become hard and crystalline. When in this condition, the plates will not accept a charge under normal conditions, and the accumulation of lead sulfate may cause short circuits during recharging or other mechanical damage to the battery. Often, hairline cracks appear in the plates causing open circuit conditions.