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Lead-acid batteries

Lead-acid batteries

The lead-acid battery was invented in 1859 by French physicist Gaston Planté and is the earliest type of rechargeable battery. The most common automotive batteries are lead-acid designs. The wet cell, gel cell, absorbed glass mat (AGM), and valve regulated are versions of the lead acid battery.

Basic Construction:

   A lead-acid battery consists of grids, positive plates, negative plates, separators, elements, electrolyte, a container, cell covers, vent plugs, and cell containers. A grid is a lead alloy frame that supports the active material of each plate. Plates are typically flat, rectangular components that are either positive or negative, depending on the active material they hold.

The positive plate has a grid filled with its active material, lead peroxide. Lead peroxide (PbO2) is a dark brown, crystalline material. The material pasted onto the grids of the negative plates is sponge lead (Pb). Both plates are very porous and allow the liquid electrolyte to penetrate freely.

Each battery contains a number of elements. An element is a group of positive and negative plates. The plates are formed into a plate group, which holds a number of plates of the same polarity. The like-charged plates are welded to a lead alloy or a plate strap. The plate groups are placed alternately within the battery: positive, negative, positive, negative, and so on. There is usually one extra set of negative plates to balance the charge. To prevent the different plate groups from touching each other, separators are inserted between them. Separators are porous plastic sheets that allow for a transfer of ions between plates. When the element is placed into the battery case and immersed in electrolyte, it becomes a cell.

The electrolyte is a solution of sulfuric acid and water. The sulfuric acid supplies sulfate, which chemically reacts with both the lead and PbO2 to release electrical energy. In addition, the sulfuric acid is the carrier for the electrons as they move inside the battery. To cause the required chemical reaction, the electrolyte must be the correct mixture of water and sulfuric acid. At 12.6 volts, the desired solution is 65% water and 35% sulfuric acid. Available voltage decreases when the percentage of acid in the solution decreases.

Casing design:

The container or shell of the battery is usually a one piece, molded assembly of polypropylene, hard rubber, or plastic. The case has a number of individual cell compartments. Cell connectors are used to join all cells of a battery in series.

The top of the battery is encased by a cell cover. The cover may be a one-piece design, or the cells might have their own individual covers. The cover must have vent holes to allow hydrogen and oxygen gases to escape. These gases are formed during charging and discharging. Battery vents can be permanently fixed to the cover or be removable, depending on the design of the battery. Vent plugs or caps are used on some batteries to close the openings in the cell cover and to allow for topping off the cells with electrolyte or water.

 

     Terminals:

   The battery has two external terminals: a positive and a negative. These terminals are two tapered posts “L” terminals, threaded studs on top of the case, or two internally threaded connectors on the side.

 

Discharging and charging:

   The chemical reaction between the active materials on the positive and negative plates and the electrolyte releases electrical energy. When a battery discharges, lead in the lead peroxide of the positive plate combines with the sulfate radical (SO4) to form lead sulfate (PbSO4).

A similar reaction takes place at the negative plate. The lead (Pb) of the negative active material combines with sulfate radical (SO4) to also form lead sulfate (PbSO4), a neutral and inactive material. Therefore, lead sulfate forms at both plates as the battery discharge.

During this chemical reaction, the oxygen from the lead peroxide and the hydrogen from the sulfuric acid combine to form water (H2O). As discharging takes place, the electrolyte becomes weaker and the positive and negative plates become like one another.

The recharging process is the reverse of discharging. Electricity from an outside source, such as the vehicle’s generator or a battery recharger, is forced into the battery. The lead sulfate (PbSO4) on both plates separates into lead (Pb) and sulfate (SO4). As the sulfate (SO4) leaves both plates, it combines with hydrogen in the electrolyte to form sulfuric acid (H2SO4). At the same time, the oxygen (O2) in the electrolyte combines with the lead (Pb) at the positive plate to form lead peroxide (PbO2). As a result, the negative plate returns to its original form of lead (Pb), and the positive plate reverts to lead peroxide (PbO2).

An unsealed battery gradually loses water due to its conversion into hydrogen and oxygen; these gases escape the battery through the vent caps. If the lost water is not replaced, the level of the electrolyte falls below the tops of the plates. This results in a high concentration of sulfuric acid in the electrolyte and permits the uncovered material of the plates to dry and harden. This will reduce the service life of a battery. This is why the electrolyte level in the battery must be frequently checked.

There are many factors affecting the battery life, like:

  • Temperature: Batteries do not work well when they are cold.
  • Corrosion
  • Overcharging
  • Undercharge/Sulfation

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