- Lead Acid Battery Definition: A lead acid battery is defined as a rechargeable battery that uses lead and sulfuric acid to store and release electrical energy.
- Container Construction: The container is made from acid-resistant materials and includes features to support and separate the plates.
- Plante Plates: These plates are created through electrolysis, forming a PbO2 layer for active material.
- Faure Plates: These plates use mechanically applied active material and are formed with a current to create lead peroxide and sponge lead.
- Battery Assembly: Positive and negative plates are arranged with separators and immersed in sulfuric acid, providing the battery with terminals for electrical connection.
A lead acid battery has two main construction parts: the container and the plates.
Lead Acid Battery Container
The battery holds sulfuric acid, so a lead acid battery container must resist that acid. The material must also be free of impurities that attack the acid. Iron and manganese must not be present.
Glass, lead-lined wood, ebonite, hard rubber or bituminous compound, ceramic and moulded plastics have those properties, so a lead acid battery case is made from one of them. A top cover seals the container.
The cover has three holes: one at each end for the posts and one in the middle for the vent plug. Electrolyte is poured through that vent and gas leaves through it.
Ribs on the floor hold the positive and negative plates. They also keep shed active material from shorting the plates.
Lead Acid Battery Plates
Two methods produce the active material and fix it to the lead plates. They are named after their inventors.
- Plante plates or formed lead acid battery plates.
- Faure plates or pasted lead acid battery plates.
Plante Plate
Plante Process
Two lead sheets stand in dilute H2SO4. An external current then drives electrolysis and evolves hydrogen and oxygen. At the anode, oxygen converts the lead to PbO2. The cathode stays lead, because hydrogen does not form a compound with Pb.
On discharge the peroxide-coated plate becomes the cathode. Hydrogen then forms on it and combines with oxygen from PbO2 to give water:
Oxygen at the same time goes to the lead anode and forms PbO2. That anode then has a thin PbO2 film.
Repeated reversal of the current, or repeated charge and discharge, thickens the PbO2 film and makes polarity reverse more slowly. After hundreds of reversals the two lead plates have a lead-peroxide skin thick enough to give useful capacity. That process of making the positive plates is called formation. Negative plates can be made the same way.
Structure of Plante Plate
The active material on a Plante plate is a thin layer of PbO2 formed on and from the lead surface, so the plate needs a large surface area to hold a useful volume of it. Grooving or laminating increases that area. The figure shows a Plante positive plate: a pure lead grid with finely laminated faces. Many thin vertical laminations are stiffened at intervals by horizontal ribs, which increases the surface area. The aim of this construction of lead acid battery plate is to hold a large volume of PbO2 on the active plate.
Positive plates are usually made by the Plante process and are called Plante plates. Negative plates can be made that way, but the process is not practical for them.
Faure Plate
In the Faure process the active material is pressed on mechanically instead of being grown from the lead plate as in the Plante process. Red lead (Pb3O4) or litharge (PbO), or a mixture of the two, is pressed into the openings of a thin lead grid that also conducts current. After pasting, the plates are dried, hardened and assembled in weak sulfuric acid of specific gravity 1.1 to 1.2 and formed by passing a current between them. Plates that will become negatives are connected as cathodes. Oxygen at the anode converts the lead oxide (Pb3O4) to lead peroxide (PbO2). Hydrogen at the cathode reduces the lead monoxide (PbO) to sponge lead (Pb).
Forming a positive plate converts lead oxide to lead peroxide. A higher oxide such as Pb3O4 is used to save current and time, although a mixture containing Pb3O4 is used in practice. The Faure process suits negative plates better than positive plates.
A large surface must face the electrolyte if the cell is to have high capacity in a small case. One plate has a limited size, so several negative and positive plates are connected in parallel. An insulating barrier or separator stands between neighbouring plates. Separators are plastic, fiberglass, hard rubber or wood. A cell has more negative plates than positive plates so both end plates are negative. That lets every positive plate work from both faces. The assembled plates sit in dilute sulfuric acid in the container. The battery has two terminals: positive and negative.
