A battery is a chemical power source that converts chemical energy into electrical energy in an electrochemical process of redox. The battery can be divided into primary battery and secondary battery. A primary battery is a battery for one-time use, and a secondary battery is a battery that is used repeatedly, so the secondary here actually means multiple times. Secondary batteries are also called rechargeable batteries or accumulators.
The chemical power supply is mainly composed of positive electrode, negative electrode and electrolyte. When the battery is working, the reactions of the positive electrode and the negative electrode are all reversible reactions. Therefore, after the battery is used, the activity of the two electrodes can be restored to the original state by charging, so that the battery can be reused. An important feature of the battery is that it can be repeatedly charged and discharged. When the battery is charged, the electrical energy is converted into chemical energy and stored in the battery, accompanied by an exothermic process. When the battery discharges, the chemical energy is converted into electrical energy, which supplies power to the load, with an endothermic process. Although the battery reaction process is always accompanied by heat transfer, the heat change is often omitted in the actual battery reaction formula, because we only care about the change of material composition.
Classification of batteries
As far as the mainstream products currently on the market are concerned, there are four types of batteries: lead-acid batteries, nickel-cadmium (NiCd) batteries, nickel-hydrogen (NiMH) batteries and lithium-ion (Liion) batteries. The battery can be used repeatedly, which is in line with the principle of economy and practicality, which is the biggest advantage; the battery also has the advantages of stable voltage, reliable power supply, and convenient movement. Therefore, batteries are widely used in power plants, substations, communication systems, electric vehicles, aerospace and other fields.
There are many performance parameters of the battery, there are four main indicators:
(1) Working voltage, the plateau voltage on the battery discharge curve.
(2) Battery capacity, usually expressed in ampere-hour (A·h) or milliamp-hour (mA·h).
(3) Working temperature zone, the temperature range of normal discharge of the battery.
(4) Cycle life, the number of charge and discharge times for the battery to work normally.
The performance of the battery can be represented by the battery characteristic curve, these working curves are the charge curve, the discharge curve, the charge and discharge cycle curve, the temperature curve and the storage curve. The safety of the battery is assessed by specific safety tests.
Lead-acid batteries have the longest history and are still widely used. The lead-acid battery was invented by Plante in 1859 and has a history of more than 150 years. For more than 100 years, the technology, structure, production performance and application of lead-acid batteries have been developing continuously. The development of science and technology has brought vigorous vitality to the ancient lead-acid batteries.
The discharge working voltage of lead-acid battery is relatively stable, and it can discharge with small current and large current. The lead-acid battery has mature technology, low cost, and good output characteristics following the load. It is its biggest advantage, so it is still an important product in batteries. However, this kind of battery also has obvious shortcomings, such as heavy weight and low mass specific energy. Although the theoretical specific energy of lead-acid battery is 240 W·h/kg, the actual specific energy is only 1050 W·h/kg. This kind of battery needs maintenance and charging speed slow.
Lead-acid batteries have undergone major reforms in modern times, and their performance has made a great leap. The main symbol is the Valve-Regulated Lead Acid Battery (VRLA) battery developed in the 1970s. Gates Energy Products Inc of the United States pioneered the ultra-fine glass fiber absorbing liquid fully sealed technology, thus developing the lead-acid battery. In the past ten years, the bipolar VRLA battery and the horizontal electrode VRLA battery have been further developed. In the bipolar VRLA battery, bipolar electrodes with positive and negative active materials on both sides of the strong sheet are introduced, which greatly reduces the internal resistance, thereby greatly improving the specific energy and charging speed. For the newly developed bipolar VRLA battery and horizontal electrode VRLA battery, the C/3 discharge specific energy is ≥50 W·h/kg, showing excellent performance.
Ordinary lead-acid batteries have problems such as short service life, low efficiency, complicated maintenance, and environmental pollution by the acid mist produced. The VRLA battery adopts a sealed structure as a whole, and there is no gas swell and electrolyte leakage of ordinary lead-acid batteries. Twice), no leakage, safe, non-polluting, recyclable, easy to use, no need to test the electrolyte and adjust acid and add water during normal operation. VRLA batteries have been widely used in many fields such as electric power, post and telecommunications, ship traffic, emergency lighting and so on. The basic structure of VRLA battery is shown in Figure 3-1. It consists of positive and negative plates, separators, electrolyte, safety valve, gas plug, shell and other parts. The active material on the positive plate is lead dioxide (Pb2O), and the active material on the negative plate is sponge-like pure lead (Pb). The electrolyte is prepared by distilled water and pure sulfuric acid in a certain proportion. After the VRLA battery tank is filled with a certain density of electrolyte, an electromotive force of about 2.1V (single VRLA battery) will be generated between the positive and negative plates due to the electrochemical reaction.
The difficulty of sealing lead-acid batteries is the electrolysis of water during charging. When the charging reaches a certain voltage (generally above 2.30 V/cell), oxygen is released on the positive electrode of the lead-acid battery, and hydrogen is released on the negative electrode. On the one hand, the released gas brings out the acid mist to pollute the environment, and on the other hand, the moisture in the electrolyte decreases, so it is necessary to add water for maintenance at intervals. VRLA battery is a product developed to overcome these shortcomings. Its product features are:
（1) The ordinary separator is no longer used between the plates, but the ultra-fine glass fiber is used as the separator. The electrolyte is all adsorbed in the separator and the plates, and there is no free electrolyte inside the VRLA battery; due to the use of multiple high-quality plates The gate alloy increases the overpotential for gas evolution. Ordinary lead-acid battery grid alloy releases gas when it is above 2.30 V/cell (25°C); VRLA battery uses high-quality multi-element alloy, and releases gas when it is above 2.35 V/cell (25°C), thus reducing relatively The amount of gas released.
(2) Let the negative electrode have excess capacity, that is, 10% more capacity than the positive electrode. In the later stage of charging, the oxygen released by the positive electrode contacts the negative electrode, reacts, and regenerates water, that is, O2+2Pb→2PbO+2H2SO4→2H2O+2PbSO4, so that the negative electrode is in an undercharged state due to the action of oxygen, so no hydrogen is produced. The process in which the oxygen in the positive electrode is absorbed by the lead in the negative electrode and further converted into water is called cathode absorption.
(4) The valve-controlled sealing acid filter structure is adopted, the electrolyte will not leak, so that the acid mist cannot escape, so as to achieve the purpose of safety and environmental protection. The VRLA battery can be installed horizontally, which is convenient to use.
(5) There is a safety exhaust valve on the shell, which will automatically open when the internal pressure of the VRLA battery exceeds the threshold value of the safety valve to ensure the safe operation of the VRLA battery.
In the above-mentioned cathode absorption process, since the generated water cannot overflow under the condition of sealing, the VRLA battery can be exempted from supplementary water maintenance, which is also the origin of the VRLA battery called “maintenance-free” battery. However, the meaning of maintenance-free does not mean that no maintenance is done. On the contrary, in order to improve the service life of VRLA batteries, the maintenance of VRLA batteries is the same as that of ordinary lead-acid batteries, except that the supplementary water is exempted. The correct use of VRLA batteries Methods can only be explored in use and maintenance.