502030 lipo battery.Charging method based on high-voltage lithium-ion battery pack

General series charging: Currently, lithium battery packs are generally charged in series, mainly because the series charging method has a simple structure, low cost, and is easier to implement. However, due to the differences between single lithium batteries in terms of capacity, internal resistance, attenuation characteristics, and

General series charging


At present, lithium-ion battery packs are generally charged in series, 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 performance 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 It starts to be filled with electricity, and at this time, other batteries are not filled with electricity. If they continue to be charged in series, the single lithium-ion battery that is already filled with electricity may be overcharged.


Overcharging of lithium-ion batteries will seriously damage the performance of the battery, and may even cause explosions and personal injuries. Therefore, in order to guard against overcharging of single lithium-ion batteries, lithium-ion battery packs are generally equipped with a battery management system (batteryManagementSystem, bMS for short), which protects each single lithium-ion battery from overcharging through the battery management system. During series charging, 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 this single battery from being overcharged, which will cause other problems. 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, good cycle performance and other advantages. The technology also basically has large-scale Processing conditions. However, the performance of lithium iron phosphate batteries is somewhat different from other lithium-ion batteries, especially its voltage characteristics are different from lithium manganate batteries, lithium cobalt oxide batteries, etc. The following is a comparison of the charging curves of lithium iron phosphate and lithium manganate batteries and the corresponding relationship between lithium ion deintercalation:

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

It is easy to see from the curve in the figure above that when the lithium iron phosphate battery is quickly overcharged, 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. This will cause the battery to It is easy to reach the overcharge protection voltage. Therefore, the phenomenon of undercharging of some batteries in lithium iron phosphate battery packs is more obvious than that of lithium manganate battery packs.
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In addition, although some battery management systems have a balancing function, due to considerations such as cost, heat dissipation, reliability, etc., the balancing current of the battery management system is generally much smaller than the current of series charging, so the balancing effect is not very clear, and there may also be Some single batteries are not fully charged, which is more obvious for lithium-ion battery packs that require large current charging, such as lithium-ion battery packs for electric vehicles.


For example, 100 lithium-ion batteries with a discharge capacity of 100Ah are connected in series to form a battery pack. However, if 99 individual lithium-ion batteries are charged with 80Ah before the group is formed, and the other single lithium-ion battery is charged with 100Ah, the battery pack will When this battery pack is charged in series, the single lithium-ion battery with a charge of 100Ah will be overcharged first, thus reaching 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 battery manufacturers test the capacity before leaving the factory, they first charge the single battery with constant current, then charge with constant voltage, and then discharge it with constant current to measure the discharge capacity. Generally, the discharge capacity is approximately equal to the constant current charging capacity plus the constant voltage charging capacity. In the actual series charging process of battery packs, there is generally no constant voltage charging process for single batteries, so the constant voltage charging capacity will not be available, and the battery pack capacity will be smaller than the single battery capacity. Generally, the smaller the charging current, the smaller the proportion of constant voltage charging capacity, and the smaller the battery pack's lost capacity. Therefore, a battery management system and charger have been developed to coordinate and cooperate with series charging.


The battery management system and 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 allows the charger to understand the battery information in real time, thereby handling the battery charging time more effectively. Some problems occurred, the schematic diagram is as follows.

Figure 3 Integration method of power lithium-ion battery system Figure 4 Basic system of lithium-ion battery system Figure 5b Simplified diagram of coordinated series charging between MS and charger

The principle of the battery management system and charger coordinating the charging mode is: the battery management system monitors the current status of the battery (such as temperature, single cell voltage, battery operating current, consistency, temperature rise, etc.) and uses these parameters Estimate the maximum allowable charging current of the current battery; during the charging process, the battery management system and the charger are connected through communication lines to achieve data sharing. The battery management system transmits parameters such as total voltage, maximum single cell voltage, maximum temperature, temperature rise, maximum allowable charging voltage, maximum allowable single cell voltage, and maximum allowable charging current to the charger in real time, and the charger can operate according to the battery requirements. The management system supplies information to change its own charging strategy and output current.
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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 charges according to the designed maximum output current; when the battery voltage and temperature exceed the limits, the battery management system can detect it in real time and notify the charger in time. The machine changes the current output; when the charging current is greater than the maximum allowed charging current, the charger starts to follow the maximum allowed charging current, thus effectively preventing battery overcharging and extending battery life. Once a fault occurs during the charging process, the battery management system can set the maximum allowed charging current to 0, forcing the charger to shut down to prevent accidents and ensure charging safety.


In this charging mode, it not only improves the management and control functions of the battery management system, but also enables the charger to change the output current in real time according to the status of the battery, achieving the purpose of warning against overcharging of all batteries in the battery pack and optimizing 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 of some batteries in the battery pack being undercharged, especially when the battery pack has a large number of strings, poor battery consistency, and a large charging current. 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 requires multiple low-voltage, high-current charging power sources to charge each single battery. It has the disadvantages of high charging power cost, low reliability, low charging efficiency, and thick connection wire diameter. Therefore, there is currently no large-scale charging method. The range uses this charging method.


Large current charging in series and parallel charging with small current


Since the above three charging methods all have certain problems, I have developed a charging method that is most suitable for high-voltage battery packs, especially electric vehicle battery packs, which uses a battery management system and a charger to coordinate and cooperate with series high-current charging and constant charging. Voltage current limiting parallel small current charging mode, the schematic diagram is shown below.

Figure 7 Schematic diagram of battery management system and charger coordinating series charging and parallel charging

This charging method has the following characteristics:


(1) Since the bMS of this system has the function of guarding against overcharging, it ensures that the battery will not have overcharging problems. 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 overflowing, there will be no overcharging problem. .


(2) Since parallel charging is possible, a balancing circuit with low reliability and relatively high cost is not required. The charging effect is better than the series charging method with only a balancing 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 is 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 during charging can be flexibly controlled, and charging can be carried out at the same time; parallel charging can be carried out after series charging is completed; a parallel charging power supply can also be used according to the voltage in the battery pack. In this case, the battery with the lowest voltage is charged in turn.


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


(6) When there are a large number of single lithium-ion batteries in the lithium-ion battery pack, the lithium-ion battery pack can be divided into several lithium-ion battery pack modules. For each lithium-ion battery pack module, bMS and charger are used to coordinate and connect in series. Charging is carried out by combining current charging with constant voltage and current limiting parallel small current charging.


Its main purpose is to reduce the shortcomings of poor consistency between single cells when there are a large number of batteries in series in the battery pack, resulting in poor charging effect of the coordinated charging method of bMS and charger, so as to give full play to bMS and charger. Coordinate with the charging mode for maximum effect.


This method is particularly suitable for high-voltage battery packs, which are battery systems composed of quickly replaceable low-voltage (for example, 48V) battery module systems, so that they can be charged or repaired in parallel at battery replacement stations or charging stations (general users usually charge There is no need to charge in parallel), and a dedicated person will sort and regroup according to the actual situation.


The charging method of using the battery management system and the charger to coordinate with series high-current charging and constant voltage current limiting parallel low-current charging can effectively solve the overcharging, undercharging and other problems that are prone to occur when charging lithium-ion battery packs in series, and can prevent them. Parallel charging has problems such as high cost, low reliability, low charging efficiency, and thick connection wire diameter. It is currently the most suitable charging method for high-voltage battery packs, especially electric vehicle battery packs.


Conclusion


Lithium-ion batteries are an ideal power source due to their high operating voltage, small size, light weight, no memory effect, no pollution, small self-discharge, and long cycle life. In actual use, in order to obtain a higher discharge voltage, at least two single lithium-ion batteries are generally connected in series to form a lithium-ion battery pack. At present, lithium-ion battery packs have been widely used in notebook computers, electric bicycles and backup power supplies.