For the types of batteries, as far as the mainstream products on the market are concerned, there are four types of batteries: lead-acid batteries, nickel-cadmium (NiCd) batteries, nickel metal hydride (NiMH) batteries and lithium ion (Liion) batteries. The battery can be used repeatedly, which is in line with the principle of economy and practicality. This is the biggest advantage. The battery has the advantages of stable voltage, reliable power supply, and convenient movement. It is widely used in power plants, substations, communication systems, electric vehicles, aerospace, photovoltaic power generation and other fields. . There are many performance parameters of the battery, mainly including the following four indicators:
(1) Working voltage, the plateau voltage on the battery discharge curve.
(2) Battery capacity, usually expressed in ampere-hour (Ah) or milli-ampere-hour (mAh).
(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 include charge curve, discharge curve, charge-discharge cycle curve, temperature curve and storage curve. The safety of the battery is evaluated by a specific safety testing method.
Lead-acid batteries have a long history and are widely used. Lead-acid batteries were invented by Plante in 1859 and have a history of more than 150 years. For more than 100 years, the process, 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 mass specific energy of lead-acid battery is 240W·h/kg, the actual mass specific energy is only 10~50W·h/kg. This kind of battery needs maintenance and charging. slow.
Lead-acid batteries have undergone major reforms in modern times, and their performance has made great leaps. The main sign is the Valve-Regulated Lead Acid (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 performance of bipolar VRLA batteries and horizontal electrode VRLA batteries has been further improved. Bipolar electrodes with positive and negative active materials on both sides of the strong sheet are introduced into the bipolar VRLA battery, which greatly reduces the internal resistance, thereby greatly improving the specific energy and charging speed. This VRLA battery has high energy, low cost and long life. (10 years), large capacity (twice that of ordinary lead-acid batteries), non-leakage, safe, non-polluting, recyclable, maintenance-free, and easy to use. For the newly developed bipolar and horizontal VRLA batteries, the C/3 discharge specific energy is ≥50Wh/kg, showing excellent performance.
Due to the problems of short service life, low efficiency, complicated maintenance, and environmental pollution by acid mist, ordinary lead-acid batteries have a limited range of use and have been gradually eliminated. The VRLA battery adopts a sealed structure as a whole, and there is no gas expansion and electrolyte leakage of ordinary lead-acid batteries. It is safe and reliable to use and has a long service life. During normal operation, there is no need to test the electrolyte, adjust acid and add water. Maintenance” battery. It has been widely used in electric power, communication, ship transportation, photovoltaic power generation, emergency lighting and other fields. The basic structure of VRLA battery is shown in Figure 1. It consists of positive/negative plate, separator, electrolyte, safety valve, gas plug, shell and other parts. The active material on the positive plate is lead dioxide (PbO2), 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 a certain density of electrolyte is loaded into the VRLA battery tank, 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 2.30V/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 Grid alloy to increase the overpotential for gas evolution. Ordinary lead-acid battery grid alloy releases gas when it is above 2.30V/cell (25°C). After using high-quality multi-component alloys, the gas is released only when the temperature is above 2.35V/monomer (25°C), which relatively reduces 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, 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 oxygen of the positive electrode is absorbed by the lead of the negative electrode, and then further converted into water, which is the so-called cathode absorption principle.
(3) In order to allow the oxygen released by the positive electrode to flow to the negative electrode as soon as possible, a new type of ultra-fine glass fiber separator that is different from the microporous rubber separator used in ordinary lead-acid batteries must be used. The porosity is increased from 50% of the rubber separator to more than 90%, so that the oxygen can easily flow to the negative electrode and be converted into water. In addition, the ultra-fine glass fiber separator has the function of adsorbing sulfuric acid electrolyte, so even if the VRLA battery is toppled, there is no electrolyte overflow. Due to the special structural design, the production of gas is controlled. In normal use, the VRLA battery does not generate hydrogen, but only a small amount of oxygen, and the generated oxygen can be recombined in the VRLA battery and absorbed by the electrolyte.
(4) The valve-controlled sealing acid filter structure is adopted, the electrolyte will not leak, and 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) A safety exhaust valve is installed on the casing of the VRLA battery. When the internal pressure of the VRLA battery exceeds the valve opening threshold of the safety valve, the safety valve will automatically open 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 method can only be explored in the use and maintenance.