Jan 11, 2019 Pageview：162
The battery is supplied with direct current. All batteries are supplied with direct current. For example, lithium batteries, lead-acid batteries, manganese-zinc dry batteries, solar cells, etc.
Direct current (direct current)
Don't change over time of current magnitude and direction. It is also known as a constant current. Through circuit dc, is composed of dc power and resistance closed conductive circuits. Formed in the circuit, a constant electric field, outside the power supply, positive charge by resistance from high potential to low potential area, within the power supply, by the power of the role of the electrostatic force, to overcome the electrostatic force, and then reaches high potential from the low electric potential, so cycle, constitute a closed current line. So, in the dc, the role of the power supply is to provide constant electromotive force does not change with time, as on the resistance of joule heat consumption.
In the relatively simple dc power source electromotive force, resistance, current and the relationship between the voltage between any two points according to ohm's law and the definition of electromotive force. Complex DC networks can be solved according to the G.R. Kirchhoff equations. It consists of two parts, the node voltage equation and loop currents equation, points out that the former for any node (more than 3 or 3 branch node), flow in and out of the node of the current algebra and zero, this is a requirement of constant conditions, the latter pointed out that for any closed loop (mesh), the algebraic sum of each part of the voltage drop to zero, this is the result of the electrostatic field loop theorem, both constitute the complete system of equations.
Measure dc current, voltage, resistance, power source electromotive force in the physical properties of the instrument is called direct current meter. Common galvanometer, ammeter, voltmeter, bridge and potential difference meter, etc.
Dc power supply with chemical batteries, fuel cells, thermoelectric battery, solar cells, dc generators, etc. Direct current (dc) is mainly used in various electronic instruments, electrolysis, electroplating, dc electric drive, etc.
On the power transmission, since the 1880 s, due to the low dc voltage is not convenient to reach the high voltage for long-distance transmission, dc transmission have made in ac power. Since the 1960 s, due to high voltage, high power converter will direct current into alternating current, direct current transmission system is taken seriously again and get a new development.
Alternating current (ac)
Alternating current is referred to as "communication." Generally refers to a voltage or current whose magnitude and direction change periodically with time. Its most basic form is sinusoidal current. The standard frequency of AC power supply in China is specified as 50 Hz. The form of alternating current over time can vary. Different forms of alternating current have different applications and effects. Sinusoidal alternating current is the most widely used, and other non-sinusoidal alternating currents can be mathematically processed to become superposition of sinusoidal alternating current. Sinusoidal current (also known as harmonic current) is a simple harmonic function of time
I = Imsin (omega t plus phi 0)
When the coil rotates at a constant speed in the magnetic field, an alternating current whose magnitude and direction are periodically changed is generated in the coil.
The alternating current used now generally has a change in direction and intensity of 52 Hz per second.
The electricity we use for electric lights, motors, etc. is all AC. In practice, the alternating current is indicated by the symbol "~".
Current (I) changing with the time, thus: sinusoidal alternating current needs, peak frequency and phase three physical parameters to describe. Is the basic problem of alternating current (ac) will be discussing the relationship between current, voltage circuit and power allocation problem (or energy).Due to the alternating current (ac) has the characteristics of the change over time, thus produced a series of different from the characteristics of dc. Used in ac circuit element not only has resistance, and the cell capacity and inductance components, the use of components, complex phenomenon and law.
The battery basically has a charge (which is also called a wet battery) and cannot be charged (also called a dry battery). Among the batteries that cannot be charged, the No. 5 battery is mainly used, which is called an alkaline battery. It is better to have a lithium-ion battery, and the endurance is basically about 5 times that of an alkaline battery, but the price is also 5 times. At present, the lithium ion No. 5 battery of Panasonic and Jinba is the best non-rechargeable battery. The rechargeable battery is divided into a nickel-cadmium battery, a nickel-hydrogen battery, and a lithium ion rechargeable battery. Among them, lithium ion rechargeable batteries are the best. Nickel-cadmium batteries are usually the size of the 5th battery, which is older and has been eliminated, but it is sold outside. NiMH batteries are usually the size of No. 5, and now they are the mainstream No. 5 rechargeable batteries, with 2300mAh to 2700mAh as the mainstream. Lithium-ion rechargeable batteries are usually the size of the manufacturer's own design. As for the battery life of rechargeable batteries, lithium-ion rechargeable batteries are best, then nickel-hydrogen, then nickel-cadmium. The advantage of lithium ion is that the power can be kept above 90%, until the last 5% of the electricity, and then suddenly there is no electricity. The nickel-metal hydride battery is going all the way, indicating that it is 90% at the beginning, then 80%, then 70%. The battery's battery life cannot meet the high-end electronic products that consume more power. Especially when the digital camera needs to use the flash, it takes a long time to take another picture, and the lithium-ion rechargeable battery does not have this problem. So if the camera is not using the No. 5 battery, it will basically be a lithium-ion rechargeable battery designed by the manufacturer. This is the first choice. If it is a No. 5 battery, you can buy a Ni-MH rechargeable battery yourself, and then buy a better charger. It is better to discharge it first, which will prolong the battery life. Nickel-metal hydride rechargeable batteries may now be better for GP, Jinba, Sony, etc. Domestically produced products are also good. However, the charger is preferred by Sony because of the discharge function.
A "lithium battery" is a type of battery using a lithium metal or a lithium alloy as a negative electrode material and using a nonaqueous electrolyte solution. Lithium metal batteries were first proposed and studied by Gilbert N. Lewis in 1912. In the 1970s, M.S. Whittingham proposed and began researching lithium-ion batteries. Due to the very active chemical properties of lithium metal, the processing, storage and use of lithium metal are very demanding on the environment. Therefore, lithium batteries have not been used for a long time. With the development of science and technology, lithium batteries have become the mainstream.
Lithium batteries can be roughly divided into two categories: lithium metal batteries and lithium ion batteries. Lithium-ion batteries do not contain metallic lithium and are rechargeable. The fifth-generation lithium-metal battery of rechargeable batteries was born in 1996, and its safety, specific capacity, self-discharge rate and performance price ratio are better than lithium-ion batteries. Due to its own high technical requirements, only a few companies in the country are producing such lithium metal batteries.
Lithium, atomic number 3, atomic weight of 6.941, it is the lightest of alkali metal elements. In order to improve safety and voltage, the scientists invented the graphite and cobalt acid lithium materials such as to store lithium atoms. The molecular structure of these materials, formed the nanometer level of small storage grid, can be used to store lithium atoms. In this way, even if the battery shell is broken, and oxygen to, also because of too much oxygen, into these small store, making lithium atoms will not avoid contact with the oxygen blast.
Lithium battery core overcharge to voltage is higher than 4.2 V, will start to produce side effects. Of charging the higher the pressure, the higher the risk also follow. Lithium batteries voltage higher than 4.2 V, the anode materials within less than half the number of lithium atoms left, store usually break down at this time, let the battery capacity to produce permanent decline. If continue to charge, due to the storage of the cathode has been full of lithium atoms, subsequent anode materials of lithium metal accumulation in the surface. These lithium atoms from lithium ion to the cathode surface to grow in the direction of the dendritic crystal. The lithium metal crystal through the diaphragm paper makes is negative short circuit. Sometimes in short circuit batteries before the first explosion, this is because in the process of overcharge, electrolyte materials can produce gas cracking, such as making the battery shell or blow up pressure valve is broken, and let oxygen in and accumulation in the lithium atoms on the surface of the cathode reaction, then exploded.
Therefore, when charging a lithium battery, you must set the upper voltage limit to take into account the battery life, capacity, and safety. The optimal charging voltage is limited to 4.2V. The lithium battery has a lower voltage limit when it is discharged. When the cell voltage is lower than 2.4V, some materials will start to be destroyed. Since the battery will self-discharge, the voltage will be lower for a longer time. Therefore, it is best not to put it at 2.4V to stop. During the period from the discharge of 3.0V to 2.4V, the energy released by the lithium battery is only about 3% of the battery capacity. Therefore, 3.0V is an ideal discharge cutoff voltage. In charge and discharge, in addition to the voltage limit, current limitation is also necessary. When the current is too large, lithium ions cannot enter the cell and will accumulate on the surface of the material.
When these lithium ions acquire electrons, crystals of lithium atoms are generated on the surface of the material, which is dangerous as overcharge. In the event of a broken battery case, it will explode. Therefore, the protection of the lithium ion battery must include at least three factors: the upper limit of the charging voltage, the lower limit of the discharging voltage, and the upper limit of the current. In the general lithium battery pack, in addition to the lithium battery core, there will be a protective board, which is mainly to provide these three protections. However, these three protections of the protection board are obviously not enough, and the global lithium battery explosion is still frequent. To ensure the safety of the battery system, a more careful analysis of the cause of the battery explosion must be performed.
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