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BOOKS AND ARTICLES Chapter 1 1.1. Types of galvanic cells
Directory / Batteries and accumulators Disposable galvanic current sources are a unified container that contains an electrolyte absorbed by the active material of the separator and electrodes (anode and cathode), therefore they are called dry cells. This term is used for all cells that do not contain a liquid electrolyte. Conventional dry cells include carbon-zinc cells or Leclanchet cells [1]. Dry cells are used for low currents and intermittent operation. Therefore, such elements are widely used in telephones, toys, alarm systems, etc. Since the range of devices that use dry cells is very wide and, in addition, their periodic replacement is required, there are standards for their dimensions [1]. During the discharge process, the dry cell voltage drops from the nominal voltage to the cutoff voltage (the cutoff voltage is the minimum voltage at which the battery is capable of delivering the minimum energy), i.e. typically 1,2V to 0,8V/cell depending on application. In the event of a discharge, when connected to a constant resistance element after the circuit is closed, the voltage at its terminals decreases sharply to a certain value, slightly less than the initial voltage. The current flowing in this case is called the initial discharge current. The functionality of a dry cell depends on current consumption, cutoff voltage and discharge conditions. The efficiency of the cell increases as the discharge current decreases. For dry cells, a continuous discharge of less than 24 hours can be categorized as a high rate discharge. The electrical capacity of a dry cell is negotiated for a discharge through a fixed resistance at a given final voltage in hours, depending on the initial discharge, and is represented by a graph or table. It is advisable to use the manufacturer's chart or table for a specific battery. This is due not only to the need to take into account the characteristics of the product, but also to the fact that each manufacturer gives his recommendations on the best use of his products. The internal resistance of the battery can limit the amount of current needed, such as when used in a flash. The initial steady current that a battery can provide for a short period of time is called flash current. The element type designation contains letters that correspond to the flash currents and the internal resistance of the element, measured at direct and alternating current. The flash current and internal resistance are very difficult to measure, and the cells may have a long shelf life, but the flash current may decrease. 1.1. Types of galvanic cells Carbon-zinc elements Carbon-zinc cells (manganese-zinc) are the most common dry cells. Zinc-carbon cells use a passive (carbon) current collector in contact with a manganese dioxide (MnO2) anode, an ammonium chloride electrolyte, and a zinc cathode. The electrolyte is in a pasty state or impregnates a porous diaphragm. Such an electrolyte is not very mobile and does not spread, therefore the cells are called dry. The nominal voltage of the carbon-zinc cell is 1,5 V. Dry elements can have a cylindrical, disk and rectangular shape. The device of rectangular elements is similar to disk ones. The zinc anode is made in the form of a cylindrical cup, which is also a container. The disc cells consist of a zinc plate, a cardboard diaphragm impregnated with an electrolyte solution, and a pressed positive electrode layer. The disk elements are connected in series with each other, the resulting battery is isolated and packed in a case. Carbon-zinc elements are "recovered" during a break in work. This phenomenon is due to the gradual leveling of local inhomogeneities in the electrolyte composition that arise during the discharge process. As a result of periodic "rest", the service life of the element is extended. This should be taken into account when the cells are used intensively (and use several sets for operation so that one set has a sufficient period of time for recovery. For example, when using the player, it is not recommended to use one set of batteries for more than two hours in a row. When changing two sets, the operating time elements are tripled. The advantage of carbon-zinc elements is their relatively low cost. Significant disadvantages include a significant decrease in voltage during discharge, low specific power (5 ... 10 W / kg) and a short shelf life. Low temperatures reduce the efficiency of the use of galvanic cells, and the internal heating of the battery increases it. An increase in temperature causes chemical corrosion of the zinc electrode by the water contained in the electrolyte and the drying of the electrolyte. These factors can be somewhat compensated by holding the battery at elevated temperature and introducing a saline solution into the cell through a previously made hole. Alkaline elements Like zinc-carbon, alkaline cells use a MnO2 anode and a zinc cathode with a separated electrolyte. The difference between alkaline and carbon-zinc cells lies in the use of an alkaline electrolyte, as a result of which there is virtually no gas evolution during discharge, and they can be sealed, which is very important for a number of their applications. The voltage of alkaline cells is approximately 0,1 V less than carbon-zinc, under the same conditions. Therefore, these elements are interchangeable. The voltage of alkaline electrolyte cells varies much less than that of salt electrolyte cells. Cells with alkaline electrolyte also have higher specific energy (65...90 Wh/kg), specific power (100...150 kWh/m3) and longer shelf life. Charging of manganese-zinc cells and batteries Produced by asymmetric alternating current. You can charge cells with saline or alkaline electrolyte of any concentration, but not too discharged and without damage to the zinc electrodes. Within the expiration date set for a given type of cell or battery, it is possible to perform multiple (6 ... 8 times) restoration of performance [2]. Dry batteries and cells are charged from a special device that allows you to get the charging current of the required form: with a ratio of the charge and discharge components of 10:1 and a ratio of the pulse duration of these components of 1:2. This device allows you to charge watch batteries and activate old small batteries. When charging watch batteries, the charging current should not exceed 2 mA. Charging time is no more than 5 hours. The rechargeable battery is switched on through two parallel-connected chains of diodes with resistors. The asymmetric charge current is obtained as a result of the difference in the resistances of the resistors. The end of the charge is determined by the cessation of the increase in voltage on the battery. The voltage of the secondary winding of the charger transformer is selected so that the output voltage exceeds the rated voltage of the element by 50 ... 60%. The battery charging time using the described device should be about 12 ... 16 hours. The charging capacity should be approximately 50% larger than the rated capacity of the battery. mercury elements Mercury elements are very similar to alkaline elements. They use mercury oxide (HgO). The cathode consists of a mixture of zinc and mercury powder. The anode and cathode are separated by a separator and a diaphragm impregnated with a 40% alkali solution. These elements have long shelf life and higher capacities (for the same volume). The voltage of a mercury cell is approximately 0,15 V lower than that of an alkaline cell. Mercury cells are characterized by high specific energy (90...120 Wh/kg, 300...400 kWh/m3), voltage stability and high mechanical strength. For small-sized devices, modernized elements of the RTs-31S, RTs-33S and RTs-55US types have been created. The specific energy of the elements RTs-31S and RTs-55US is 600 kWh/m3, the elements of RTs-33S are 700 kWh/m3. RC-31S and RC-33S elements are used to power watches and other equipment. RC-55US elements are designed for medical equipment, in particular for implantable medical devices. RC-31S and RC-33C elements operate for 1,5 years at currents of 10 and 18 μA, respectively, and the RC-55US element ensures the operation of implantable medical devices for 5 years. Mercury cells are operable in the temperature range from 0 to +50oC; There are modifications of elements in which alloys of indium and titanium are used instead of zinc powder (negative electrode). Since mercury is scarce and toxic, mercury cells should not be thrown away after they have been used up. They must be recycled. silver elements They have "silver" cathodes made of Ag2O and AgO. Their voltage is 0,2 V higher than that of carbon-zinc under comparable conditions [1]. Lithium cells They use lithium anodes, an organic electrolyte and cathodes made of various materials. They have a very long shelf life, high energy densities, and are operational over a wide temperature range, since they do not contain water. Since lithium has the highest negative potential with respect to all metals, lithium cells are characterized by the highest voltage rating with minimal dimensions. Organic compounds are usually used as solvents in such cells. Solvents can also be inorganic compounds, such as SOCl2, which are also reactive substances. Ionic conductivity is ensured by introducing salts with large anions into solvents, for example: LiAlCl4, LiClO4, LiBFO4. The specific electrical conductivity of non-aqueous electrolyte solutions is 1 ... 2 orders of magnitude lower than the conductivity of aqueous solutions. In addition, cathodic processes in them usually proceed slowly, therefore, in cells with non-aqueous electrolytes, the current densities are low. The disadvantages of lithium cells include their relatively high cost, due to the high price of lithium, special requirements for their production (the need for an inert atmosphere, purification of non-aqueous solvents). It should also be borne in mind that some lithium cells are explosive when opened. Such elements are usually made in push-button designs with a voltage of 1,5 V and 3 V. They successfully provide power to the circuit with a consumption of about 30 μA in constant or 100 μA in intermittent modes. Lithium cells are widely used in backup power supplies for memory circuits, measuring instruments and other high-tech systems. Back (Introduction) Forward (Batteries of leading companies in the world)
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