Battery design. Causes of battery failure

The word "battery" is derived from the Latin word accumulo – to gather, to accumulate. A battery is defined as a chemical current source capable of storing (accumulating) electrical energy and releasing it into an external circuit as needed. It was long observed that if two insulated plates are immersed in an acidic or alkaline solution, a potential difference or voltage arises on them. The very first prototype of the modern battery consisted of two plates – copper and zinc – immersed in an electrolyte.

Currently, there are many types of batteries differing in design and operating principle. The most common devices include:
lead-acid;
alkaline iron-nickel;
silver-zinc;
nickel-cadmium;
lithium-ion

Of the types listed above, lead-acid batteries are the most widely used. In second place by popularity are batteries with nickel and iron electrodes. As for silver-zinc devices, they are hardly used due to their high cost and short service life. Thanks to their high specific capacity, nickel-cadmium and lithium-ion batteries have become widely used in portable devices.

Design of a lead-acid battery

The lead-acid battery was invented by the French physicist Gaston Plante in 1859. In 1878, Camille Faure improved its design by coating the battery plates with lead oxide. To this day, for more than one and a half centuries, this design has undergone virtually no changes. The construction of a lead-acid battery represents a vessel filled with an electrolyte in 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 composed of spongy lead (Pb), and the other of lead dioxide (PbOx). As a result of the interaction of the electrolyte with the electrodes, a potential difference arises between them.

Inside the battery case, sets of plates are installed, consisting of several parallel-connected plates with alternating polarity, also referred to as galvanic cells. This design makes it possible to increase the battery capacity, since the overall contact surface is enlarged. Increasing the contact surface also reduces internal resistance, which contributes to an increase in the maximum current deliverable by the battery.

Operating principle of a battery

When a load is connected, a discharge current arises in the battery. The sulfate ions (SOx) combine with the lead of the electrodes, forming lead sulfate (PbSOx) on them, while the hydrogen ions combine with oxygen, releasing water on the positive plate. As a result, the electrodes become coated with lead sulfate, and the sulfuric acid is diluted with the formed water, i.e., during discharge, the electrolyte density decreases. Therefore, the degree of battery discharge can be determined by measuring the density of the electrolyte.

When an electric current flows through the storage battery, reverse electrochemical processes occur. Hydrogen ions formed during water decomposition interact with the lead sulfate of the electrodes. Hydrogen, combining with the sulfate deposit, forms sulfuric acid, while spongy lead is restored on the electrodes. Oxygen released from water combines with the lead of the positive plate, forming lead peroxide; the water content in the electrolyte decreases while the acid content increases, resulting in a rise in electrolyte density. When the restoration of lead on the electrodes ceases, the charging process ends. If the electric current continues to flow, the electrolysis of water begins, causing the battery to "boil" and generating an explosive mixture of hydrogen and oxygen gases.

Causes of battery failure

The main cause of battery failure lies in the physics of the electrochemical charge-discharge process. While receiving and delivering current, the plates coated with active material expand and contract. This process leads to the destruction of plate structure. The active material gradually crumbles and accumulates at the bottom, causing a short circuit between the positive and negative plates inside the battery before they are fully exposed and cease to accumulate charge. This issue was resolved by equipping the bottom of the battery case with additional compartments separated by ribs, into which the so-called sludge should settle. Furthermore, the positive plates inside the battery began to be placed in special envelope separators, which are packets made of flexible porous material, easily passing the electrolyte while retaining the sludge. These design improvements led to smaller battery sizes and increased resistance to mechanical stress.

Additional information:
Battery internal resistance, Battery capacity, WET batteries, AGM batteries, EFB batteries, GEL batteries