AA rechargeable battery

　　Charging method based on high-voltage lithium-ion battery pack

　　General series charging At present, lithium battery packs are generally charged in series, mainly because the series charging method is simple in structure, low in cost and easy to implement. However, due to the differences in capacity, internal resistance, attenuation characteristics, and self-

　　General series charging

　　At present, the charging of lithium-ion battery packs generally adopts series charging, which is mainly because the series charging method has a simple structure, low cost, and is easier to implement. However, due to the differences in capacity, internal resistance, attenuation characteristics, self-discharge and other performances between single lithium-ion batteries, when charging lithium-ion battery packs in series, the single lithium-ion battery with the smallest capacity in the battery pack will At first, it is full of electricity, and at this time, other batteries are not yet full of electricity. If you continue to charge in series, the single lithium-ion battery that has already been full of electricity may be overcharged.

　　However, overcharging of lithium-ion batteries will seriously damage the performance of the battery, and may even cause an explosion to cause personal injury. Therefore, in order to guard against overcharging of single lithium-ion batteries, lithium-ion battery packs are generally equipped with a battery management system (battery Management System, Abbreviated as bMS), through the battery management system, each single lithium-ion battery is protected from overcharging. When charging in series, if the voltage of a single lithium-ion battery reaches the overcharge protection voltage, the battery management system will cut off the entire series charging circuit and stop charging to prevent the single battery from being overcharged, which will cause other Lithium-ion batteries cannot be fully charged.

　　After years of development, lithium iron phosphate power lithium-ion batteries have basically met the requirements of electric vehicles, especially pure electric cars, due to their high safety and good cycle performance. processing conditions. However, the performance of lithium iron phosphate batteries is different from other lithium ion batteries, especially its voltage characteristics are different from those of lithium manganese oxide batteries and lithium cobalt oxide batteries. The following is a comparison of the charging curves of lithium iron phosphate and lithium manganate lithium-ion batteries and the corresponding relationship between lithium ion deintercalation:

　　Figure 1 Corresponding relationship between lithium ion deintercalation and charging curve of lithium manganate battery Figure 2 Corresponding relationship between lithium ion deintercalation and charging curve of lithium iron phosphate battery

　　It is not difficult to see from the curve in the above figure that when the lithium iron phosphate battery is rapidly overflowing, the lithium ions are almost completely deintercalated from the positive electrode to the negative electrode, the battery terminal voltage will rise rapidly, and the charging curve will rise, which will cause the battery It is easy to reach the overcharge protection voltage. Therefore, the phenomenon that some batteries in the lithium iron phosphate battery pack are not fully charged will be more obvious than that in the lithium manganate battery pack.

　　Low temperature lithium iron phosphate battery 3.2V 20A -20℃ charge, -40℃ 3C discharge capacity ≥70%

　　Charging temperature: -20~45℃ -Discharging temperature: -40~+55℃ -40℃ supports maximum discharge rate: 3C -40℃ 3C discharge capacity retention rate≥70%

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　　In addition, although some battery management systems have an equalization function, due to considerations such as cost, heat dissipation, and reliability, the equalization current of the battery management system is generally much smaller than the current of series charging, so the equalization effect is not very clear, and there will be The situation that some single cells are not fully charged is more obvious for lithium-ion battery packs that need to be charged with a large current, such as lithium-ion battery packs for electric vehicles.

　　For example, connect 100 lithium-ion batteries with a discharge capacity of 100Ah in series to form a battery pack. When this battery pack is charged in series, the single lithium-ion battery with a charge of 100Ah will be fully charged first, so as to reach the overcharge protection voltage. In order to prevent this single lithium-ion battery from being overcharged, the battery management system will The entire series charging circuit is cut off, which prevents the other 99 batteries from being fully charged, so the discharge capacity of the entire battery pack is only 80Ah.

　　Generally, when the battery manufacturer leaves the factory to detect the capacity, the single battery is first charged with a constant current, then charged with a constant voltage, and then discharged with a constant current to measure the discharge capacity. Generally, the discharge capacity is approximately equal to the constant current charge capacity plus the constant voltage charge capacity. In the actual battery pack charging process in series, there is generally no constant voltage charging process for the single battery, so the constant voltage charging capacity will be lost, and the capacity of the battery pack will be smaller than the capacity of the single battery. Generally, the smaller the charging current, the smaller the proportion of constant voltage charging capacity, and the smaller the loss capacity of the battery pack. Therefore, a mode of coordinating and coordinating series charging between the battery management system and the charger has been developed.

　　The battery management system and the charger coordinate and cooperate in series charging

　　The battery management system is the device that has the most comprehensive understanding of the performance and status of the battery. Therefore, establishing a connection between the battery management system and the charger can enable the charger to understand the information of the battery in real time, so as to handle the charging time of the battery more effectively. There are some problems, the schematic diagram is as follows.

　　Fig. 3 Integration method of power lithium-ion battery system Fig. 4 Basic system of lithium-ion battery system Fig. 5 b Schematic diagram of coordination and coordination of MS and charger for serial charging

　　The principle of coordinating the charging mode between the battery management system and the charger is: the battery management system monitors the current state of the battery (such as temperature, single battery voltage, battery operating current, consistency, and temperature rise, etc.), and uses these parameters to Estimate the maximum allowable charging current of the current battery; during the charging process, connect the battery management system and the charger through the communication line to realize data sharing. The battery management system transmits parameters such as total voltage, maximum cell voltage, maximum temperature, temperature rise, maximum allowable charging voltage, maximum allowable cell voltage, and maximum allowable charging current to the charger in real time, and the charger can The information supplied by the management system changes its own charging strategy and output current.

　　Low temperature high energy density 18650 3350mAh-40℃ 0.5C discharge capacity ≥60%

　　Charging temperature: 0~45℃ Discharging temperature: -40~+55℃ Specific energy: 240Wh/kg -40℃ discharge capacity retention rate: 0.5C discharge capacity≥60%

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　　When the maximum allowable charging current supplied by the battery management system is higher than the designed current capacity of the charger, the charger will charge according to the designed maximum output current; when the voltage and temperature of the battery exceed the limit, the battery management system can detect it in real time and notify the charger in time The charger changes the current output; when the charging current is greater than the maximum allowable charging current, the charger starts to follow the maximum allowable charging current, which effectively prevents the battery from overcharging and prolongs the battery life. Once a fault occurs during the charging process, the battery management system can set the maximum allowable charging current to 0, forcing the charger to stop, guarding against accidents, and ensuring the safety of charging.

　　In this charging mode, the management and control functions of the battery management system are improved, and the charger can change the output current in real time according to the state of the battery, so as to prevent overcharging of all batteries in the battery pack and optimize charging. The actual discharge capacity of the battery pack is also greater than the general series charging method, but this method still cannot solve the problem that some batteries in the battery pack are not fully charged, especially when there are many battery packs in series, the battery consistency is poor, and the charging current is low. big time.

　　Parallel charging

　　In order to solve the problem of overcharging and undercharging of some single cells in the battery pack, a parallel charging method has been developed. The schematic diagram is as follows.

　　Figure 6 Schematic Diagram of Parallel Charging

　　However, the parallel charging method needs to use multiple low-voltage, high-current charging power sources to charge each single battery, which has defects such as high cost of charging power sources, low reliability, low charging efficiency, and thick connection wires. Range uses this charging method.

　　Series high current charging plus small current parallel charging

　　Since there are certain problems in the above three charging methods, I have developed a charging method that is most suitable for high-voltage battery packs, especially electric vehicle battery packs, that is, using a battery management system and a charger in coordination with a series of high-current charging plus constant charging. The mode of parallel connection of small current charging with pressure limiting current, the schematic diagram is shown below.

　　Fig. 7 Schematic diagram of battery management system and charger coordinated with series charging plus parallel charging

　　This charging method has the following characteristics:

　　(1) Since the bMS of this system has the function of guarding against overcharging, it is guaranteed that the battery will not be overcharged. Of course, if the bMS cannot communicate and control the parallel charging power supply, since the constant voltage value of the parallel charging power supply is generally the same as the voltage value of the single lithium-ion battery in the lithium-ion battery pack when it is fully charged, there will be no overcharging problem. .

　　(2) Because it can be charged in parallel, there is no need for an equalizing circuit with low reliability and relatively high cost, and the charging effect is better than the series charging method with only an equalizing circuit, and its maintenance and management are also simple and easy.

　　(3) Since the maximum current of series charging is much greater than the current of parallel charging (generally more than 5 times), it can ensure that a higher capacity can be charged in a shorter time, thereby exerting the maximum effect of series charging.

　　(4) The sequence of series charging and parallel charging and the number of parallel charging power sources can be flexibly controlled during charging, and charging can be performed at the same time; parallel charging can be performed after series charging is completed; a parallel charging power supply can also be used according to the voltage in the battery pack The battery with the lowest voltage is charged in turn.

　　(5) With the development of technology, the parallel charging power supply can be non-contact charging power supply (wireless charging power supply) or solar battery power supply, thus making parallel charging easy.

　　(6) When the number of single lithium-ion batteries in the lithium-ion battery pack is large, the lithium-ion battery pack can be divided into several lithium-ion battery pack modules, and each lithium-ion battery pack module uses bMS and chargers to coordinate and cooperate with each other in series. Charging is carried out by combining current charging with parallel connection of small current charging with constant voltage and current limiting.

　　Its main purpose is to reduce the relatively poor consistency between single cells when there are a large number of batteries in series in the battery pack, which leads to the poor charging effect of the coordinated charging method of bMS and charger, so as to make full use of bMS and charger. Harmonize with charging mode for maximum effect.

　　This approach is especially suitable for high-voltage battery packs that are composed of rapidly replaceable low-voltage (eg 48V) battery module systems, so that parallel charging or repairing can be performed at battery replacement stations or charging stations (general users usually charge It is not necessary to charge in parallel), and special personnel will sort and regroup according to the actual situation.

　　The battery management system and the charger are used to coordinate the charging method of series high current charging and constant voltage current limiting parallel small current charging, which can effectively solve the problems of overcharging and undercharging that are easy to occur when charging lithium-ion battery packs in series, and can be alert Parallel charging is the most suitable charging method for high-voltage battery packs, especially electric vehicle battery packs.