Battery Design. Causes of Battery Failure

The word "accumulator" originates from the Latin word accumulo, meaning to collect or accumulate. A rechargeable battery is a chemical source of electric current that is capable of storing (accumulating) electrical energy and, when necessary, delivering it to an external circuit. It was observed long ago that if two insulated plates are immersed in an acidic or alkaline solution, a potential difference, or voltage, will arise between them. The earliest prototype of the modern rechargeable battery consisted of two plates - copper and zinc - immersed in an electrolyte.

At present, there are many different types of rechargeable batteries that differ in design and operating principle. The most common types include:
lead-acid;
alkaline nickel-iron;
silver-zinc;
nickel-cadmium;
lithium-ion

Of the types listed above, lead-acid batteries have become the most widely used. Batteries containing nickel and iron are the second most common. Silver-zinc devices, by contrast, are used only rarely. The reason is their high cost and relatively short service life. Owing to their high specific capacity, nickel-cadmium and lithium-ion batteries are widely used in portable devices

Lead-Acid Battery Design

The lead-acid battery was invented by French physicist Gaston Plante in 1859. In 1878, Camille Faure improved its design by coating the battery plates with red lead. To this day, over the course of approximately a century and a half, this design has undergone virtually no fundamental changes. A lead-acid battery consists of a container filled with electrolyte, into which lead electrodes are immersed. The electrolyte is a solution of sulfuric acid and distilled water. The electrodes are made in the form of plates, one of which is manufactured from spongy lead Pb, while the other is made from lead dioxide PbO:. As a result of the interaction between the electrolyte and the electrodes, a potential difference arises across them.

The battery housing contains plate packs consisting of several plates connected in parallel, with alternating polarity; these assemblies are also referred to as galvanic cells. This configuration makes it possible to increase the capacity of the rechargeable battery, because the total contact surface area is increased. Increasing the contact surface area also reduces internal resistance, which contributes to an increase in the maximum current that the battery can deliver.

Battery Operating Principle

When a load is connected, a discharge current begins to flow in the battery. In this process, sulfate ions SO. combine with the lead of the electrodes and form lead sulfate PbS04 on them, while hydrogen ions combine with oxygen and are released at the positive plate in the form of water. As a result, the electrodes become coated with lead sulfate, while the sulfuric acid is diluted by the water that is formed; in other words, the electrolyte density decreases as the battery discharges. Therefore, the state of discharge of the rechargeable battery can be determined from the electrolyte density.

When electric current passes through the rechargeable battery, the reverse electrochemical processes take place. Hydrogen ions formed as a result of the decomposition of water interact with the lead sulfate on the electrodes. Hydrogen combines with the sulfur-containing deposit to form sulfuric acid, while spongy lead is restored on the electrodes. Oxygen released from the water combines with the lead of the positive plate, forming lead peroxide; the water content in the electrolyte decreases and the acid content increases, as a result of which the electrolyte density rises. When restoration of lead on the electrodes ceases, the battery charging process is complete. If electric current continues to flow, electrolysis (decomposition) of water begins, the battery starts to "boil," and an explosive mixture of gaseous hydrogen and oxygen is formed.

Causes of Battery Failure

The principal cause of battery failure lies in the physical nature of the electrochemical charge and discharge process. As the plates with active material applied to them receive and deliver current, they expand and contract. This process gradually destroys the plate structure. The active material begins to shed and accumulates at the bottom, which eventually causes a short circuit between the positive and negative plates inside the battery even before the plates become completely exposed and cease to retain charge. This problem was addressed by providing additional compartments at the bottom of the battery housing, separated by ribs, in which the so-called sludge is intended to collect. In addition, the positive plates inside the battery began to be placed in special envelope separators, which are packages made of flexible porous material that readily permits the electrolyte to pass through while retaining the sludge. These design improvements made it possible to reduce battery dimensions and increase resistance to mechanical loads.

Additional information:
Internal resistance of a rechargeable battery, Battery capacity, WET rechargeable batteries, AGM batteries, EFB battery systems, GEL batteries