Research on charging technology of the hottest bat

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Research on battery charging technology

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 and other departments. Batteries mainly include ordinary lead-acid batteries, alkaline cadmium nickel batteries and valve regulated sealed lead-acid batteries. Due to the problems of short service life, low efficiency, complex maintenance and environmental pollution caused by acid mist, the application range of ordinary lead-acid batteries is very limited. At present, they have been gradually eliminated by valve regulated sealed lead-acid batteries. The valve regulated sealed lead-acid battery adopts a sealed structure as a whole, which does not exist the phenomena of gas inflation 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 detect the electrolyte, adjust the acid and add water, which is also called maintenance free battery. It has been widely used in many fields, such as post and telecommunications, ship traffic, emergency lighting and so on. Alkaline nickel cadmium battery is characterized by small volume, high discharge rate, simple operation and maintenance, and long service life. However, its use is limited due to its low monomer voltage, easy leakage, high cost, and easy to pollute the environment. At present, it is mainly used in electric tools and various portable electronic devices

ordinary lead-acid batteries are mainly composed of plate groups, electrolyte and battery slots. Both positive and negative plates are composed of grids and active substances. The active substance on the positive plate is brown lead dioxide (PbO2), and the active substance on the negative plate is dark gray sponge pure lead (PB). The electrolyte is prepared with distilled water (H2O) and pure sulfuric acid (H2SO4) in a certain proportion. During the charging process, the proportion of electrolyte to the outlet of extruder is still low, and the active substances on the positive and negative plates react chemically, so as to turn electric energy into chemical energy for storage; During the discharge process, the electrolyte also reacts with the active substances on the positive and negative plates to convert the chemical energy stored in the battery into electrical energy for the load. In order for the chemical reaction to proceed normally, the electrolyte must have a certain concentration. Battery cell is the container of plate group and electrolyte. It must have good acid resistance, insulation performance and high mechanical strength

connect the load between the positive and negative plates of the battery, and the discharge process of the battery begins. At this time, the potential of the positive plate decreases, the potential of the negative plate increases, the active substances (PbO2 and Pb) on the positive and negative plates continue to change into lead sulfate (PbSO4), the sulfuric acid in the electrolyte gradually changes into water, and the proportion of the electrolyte gradually decreases, thereby increasing the internal resistance of the battery and reducing the electromotive force. If a DC power supply with a higher output voltage than the battery terminal voltage is connected between the positive and negative plates of the battery, the charging process of the battery begins. At this time, the positive plate potential increases due to the accumulation of positive charges, and the negative plate potential decreases due to the accumulation of negative charges. PbSO4 on the positive plate gradually becomes PbO2, and PbSO4 on the negative plate gradually becomes spongy Pb. At the same time, the synthesis of H2SO4 in the electrolyte gradually increases, the water molecules gradually decrease, the proportion of electrolyte gradually increases, and the terminal voltage of the battery also continuously increases

2 battery fast charging technology

the conventional charging method adopts the low current slow charging method. The initial charging of the new lead-acid battery takes more than 70h, and the ordinary charging also takes more than 10h. If the charging time is too long, it will not only prolong the time of charging monitoring and cause a waste of electric energy, but also limit the number of battery recycling and increase the maintenance workload. In addition, it is very inconvenient to use for occasions requiring continuous battery power supply, such as electric vehicles. Using fast charging method can shorten the charging time of battery, improve charging efficiency, save energy, and better meet the needs of industrial applications, which has great practical significance

Figure 1 acceptable charging current curve of the battery

Figure 2 pulse fast charging curve

Figure 3 block diagram of high-frequency switching charging power supply

in the mid-1960s, American scientist Thomas made a lot of experimental research on the outgassing problem in the charging process of the battery, and proposed an acceptable charging current curve of the battery based on the lowest outgassing rate, as shown in Figure 1 [1]. It can be seen from the figure that during the charging process, as long as the charging current does not exceed the acceptable current of the battery, a large number of bubbles will not be generated in the battery. Conventional charging generally adopts the two-stage charging method of constant current first and constant voltage later. In the early stage of the charging process, the charging current is far less than the acceptable charging current of the battery, so the charging time is greatly extended; At the later stage of the charging process, the charging current is greater than the acceptable current of the battery, so a large number of bubbles are generated in the battery. However, if the actual charging current can always be equal to or close to the acceptable charging current of the battery during the whole charging process, the charging speed can be greatly accelerated, and the air outlet rate can also be controlled within a very low range. This is the basic theoretical basis of fast charging. However, in the charging process, the polarization voltage generated in the battery will hinder its own charging, and significantly increase the outgassing rate and temperature rise. Therefore, the polarization voltage is an important factor affecting the charging speed. Therefore, in order to realize rapid charging, we must try to eliminate the influence of polarization voltage on battery charging. It can be inferred from the formation mechanism of polarization voltage that the magnitude of polarization voltage changes with the change of charging current. When the charging is stopped, the resistance polarization disappears, and the concentration polarization and electrochemical polarization also gradually weaken; If a discharge channel is provided for the battery to discharge in reverse, the concentration polarization and electrochemical polarization will disappear rapidly, and the temperature in the battery will also be reduced due to discharge. Therefore, in the process of battery charging, timely suspension of charging and appropriate addition of discharge pulse can quickly and effectively eliminate all kinds of polarization voltage, so as to improve the charging speed. At present, pulse charging and pulse discharge depolarization are the most recognized fast charging methods. Figure 2 shows the waveform of pulse charging and pulse discharge depolarization rapid charging. The research shows that using the switching charging power supply as shown in Figure 3 can effectively realize the rapid pulse charging of battery

3 charging control method

charging control mainly includes the automatic conversion of three stages: main charging, equalizing charging and floating charging, the automatic conversion from discharge state to charging state, the judgment of charging procedure and the control of stopping charging. Mastering the correct control method is conducive to improving the charging efficiency and service life of the battery

3.1 automatic conversion of main charge, equalizing charge and floating charge

at present, most domestic charging power supplies still use the three-stage charging method of main charge, equalizing charge and floating charge to charge the battery. The automatic conversion methods of each stage of charging are:

(1) time control, that is, the charging time of each stage is preset, and the conversion time is controlled by the time relay or CPU

(2) set the charging current or battery terminal voltage value of the conversion point,

when the actual current or voltage value reaches the set value, it will automatically convert

(3) the integrating circuit is used to monitor the capacity of the battery. When the capacity

reaches a certain value, a signal is sent to change the size of the charging current

among the above methods, time control is relatively simple, but this method lacks real-time information from the battery, and the control is relatively rough; The control circuit of capacity monitoring method is complex, but the control accuracy is high

3.2 judgment of charging degree

when charging the battery, the charging degree of the battery must be judged at any time in order to control the charging current. The main methods to judge the charging degree are:

(1) observe the change of terminal voltage after battery depolarization. Generally speaking, in the initial stage of charging, the change rate of battery terminal voltage is very small; In the middle stage of charging, the change rate of battery terminal voltage is very large; At the end of charging, the change rate of terminal voltage is very small [2]. Therefore, by observing the change of terminal voltage in unit time, we can judge the charging stage of the battery

(2) detect the actual capacity value of the battery and compare it with its rated capacity

value to judge its charging degree

(3) detect the battery terminal voltage for judgment. When the battery terminal voltage differs greatly from its rated value, it indicates that it is in the initial stage of charging; When the difference between the two is very small, it indicates that it is close to full

3.3 charge stop control

when the battery is fully charged, the charging current must be cut off in time, otherwise the battery will have a large number of overcharge reactions such as air outlet, water loss and temperature rise, which will directly endanger the service life of the battery. Industries such as PVC, POM and polyvinyl alcohol have suffered serious losses. Therefore, the charging condition of the battery must be monitored at any time to ensure that the battery is fully charged but not recharged. The main stop charging control methods are:

(1) when the constant current charging method is used for timing control, the charging time required for the battery can be easily determined according to the battery capacity and charging current. Therefore, as long as the charging time is preset, once the time comes, the timer can send a signal to stop charging or reduce to trickle charging. Timer can be acted by time relay, or its function can be undertaken by single chip microcomputer. This method is simple, but the charging time cannot be automatically adjusted according to the state of the battery before charging, so sometimes under charging and sometimes overcharging may occur during actual charging

(2) battery temperature control for Cd Ni batteries, the temperature change of the battery is not obvious when the battery is normally charged

. However, when the battery is overcharged, the internal gas pressure will increase rapidly, and the oxidation reaction on the negative plate will make the internal heating, and the temperature will rise rapidly (several degrees Celsius per minute). Therefore, observing the change of battery temperature can judge whether the battery is full. Usually, two thermistors are used to detect the battery temperature and ambient temperature respectively. When the temperature difference between the two reaches a certain value, the charging stop signal is sent. Because the dynamic response speed of thermistor is slower than that of 1923 mu, the full charge state of battery cannot be detected timely and accurately

(3) negative increment control of battery terminal voltage generally speaking, when the battery

is fully charged, its terminal voltage will show a downward trend. Therefore, the time when the battery voltage shows a negative increase can be regarded as the time when the battery stops charging. Compared with the temperature control method, this method has faster response speed. In addition, the negative increment of voltage is independent of the absolute value of voltage. Therefore, this off charge control method can be adapted to the charging of battery packs with different number of single cell batteries. The disadvantage of this method is that the sensitivity and reliability of the general detector are not high. At the same time, when the ambient temperature is high, the voltage reduction after the battery is fully charged is not obvious, so it is difficult to control

(4) using polarization voltage control. Generally, the

polarization voltage of the battery appears after the battery is just full, which is generally in the order of 50mV ~ 100mV. The relevant patented technology [3] is used to measure the polarization voltage of each single cell battery, so that each battery can be charged to the degree required by itself. The research shows that because each battery has at least some slight differences in geometric structure, chemical properties and electrical characteristics, it is more appropriate to determine the required charging level according to the characteristics of each single cell battery than to control the battery as a whole.

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