Charging method of high voltage lithium-ion battery pack

　Lithium-ion batteries are an ideal power source due to their high operating voltage, small size, light weight, no memory effect, no pollution, low 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 many fields such as laptops, electric bicycles, and backup power supplies.

　　Therefore, how to use lithium-ion battery packs well when charging is particularly critical. Here are several commonly used charging methods for lithium-ion battery packs and the most suitable charging method I think is described as follows:

　　1 Ordinary series charging

　　At present, lithium-ion battery packs are generally charged in series, mainly because the series charging method is simple in structure, low in cost, and easier to implement. However, due to the differences in performance such as capacity, internal resistance, attenuation characteristics, and self-discharge between single lithium-ion batteries, when charging a lithium-ion battery pack in series, the single lithium-ion battery with the smallest capacity in the battery pack will be fully charged first, and at this time, the other batteries have not been fully charged. If the series charging continues, the fully charged single lithium-ion battery may be overcharged.

　　Overcharging of lithium-ion batteries will seriously damage the performance of the battery and may even cause explosions and personal injury. Therefore, in order to prevent the overcharging of single lithium-ion batteries, lithium-ion battery packs are generally equipped with a battery management system (BMS) when in use. The battery management system protects each single lithium-ion battery from overcharging. 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 the single battery from being overcharged, which will cause other lithium-ion batteries to be unable to be fully charged.

　　After years of development, lithium iron phosphate power batteries have basically met the requirements of electric vehicles, especially pure electric cars, due to their advantages such as high safety and good cycle performance, and the process has basically met the conditions for large-scale production. However, there are certain differences in the performance of lithium iron phosphate batteries and other lithium-ion batteries, especially their voltage characteristics are different from lithium manganese oxide batteries and lithium cobalt oxide batteries. The following is a comparison of the charging curves of lithium iron phosphate and lithium manganese oxide lithium-ion batteries and the corresponding relationship between lithium ion insertion and extraction:

Figure 1 Corresponding relationship between lithium ion insertion and removal and charging curves of lithium manganese oxide 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 figure above that when the lithium iron phosphate battery is almost fully charged, the lithium ions are almost completely deintercalated from the positive electrode to the negative electrode, and the battery terminal voltage will rise rapidly, resulting in an upward charging curve, which will cause the battery to easily reach the overcharge protection voltage. Therefore, the phenomenon that some batteries in the lithium iron phosphate battery pack are not fully charged is more obvious than that of the lithium manganese oxide battery pack.

　　In addition, although some battery management systems have a balancing function, due to considerations such as cost, heat dissipation, and reliability, the balancing current of the battery management system is generally much smaller than the current of series charging. Therefore, the balancing effect is not very obvious, and some single cells may not be fully charged. This is more obvious for lithium-ion battery packs that require high current charging, such as lithium-ion battery packs used in 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 of the single lithium-ion batteries are charged with 80Ah before grouping, and the other single lithium-ion battery is charged with 100Ah, when the battery pack is charged in series, the single lithium-ion battery with a charge of 100Ah will be fully charged first, thus reaching the overcharge protection voltage. In order to prevent this single lithium-ion battery from being overcharged, the battery management system will cut off the entire series charging circuit, which means that the other 99 batteries cannot be fully charged, so that 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 it 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 battery pack series charging process, there is generally no constant voltage charging process for the single battery, so there will be no constant voltage charging capacity, and the battery pack capacity will be smaller than the single battery capacity. Generally, the smaller the charging current, the smaller the constant voltage charging capacity ratio, and the smaller the battery pack loss capacity. Therefore, a mode of coordinated series charging between the battery management system and the charger has been developed.

　　2 Coordinated series charging between the battery management system and the charger

　　The battery management system is the most comprehensive device for understanding the performance and status of the battery. Therefore, by establishing a connection between the battery management system and the charger, the charger can understand the battery information in real time, thereby more effectively solving some problems caused by battery charging. The schematic diagram is as follows.

Figure 3 Power lithium battery system integration solution

Figure 4 Basic system of lithium-ion battery system

Figure 5 Schematic diagram of BMS and charger coordinating series charging

       
        The principle of the battery management system and the charger coordinating the charging mode is as follows: the battery management system monitors the current state of the battery (such as temperature, single cell 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, the battery management system and the charger are connected through a communication line 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 change its charging strategy and output current according to the information provided by the battery management system.

　　When the maximum allowable charging current provided by the battery management system is higher than the current capacity designed by the charger, the charger charges according to the designed maximum output current; when the battery voltage and temperature exceed the limit, the battery management system can detect in real time and promptly notify the charger to change the current output; when the charging current is greater than the maximum allowable charging current, the charger begins to follow the maximum allowable charging current, which effectively prevents the battery from being overcharged and achieves the purpose of extending 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 shut down, avoiding accidents and ensuring charging safety.

　　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 status of the battery, so as to prevent all batteries in the battery pack from overcharging and optimize charging. The actual discharge capacity of the battery pack is also greater than the ordinary series charging method, but this method still cannot solve the problem of some batteries in the battery pack not being fully charged, especially when the battery pack has a large number of strings, poor battery consistency, and a large charging current.

        3 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, and its principle diagram is as follows.

Figure 6 Schematic diagram of parallel charging

　   However, the parallel charging method requires the use of multiple low-voltage, high-current charging power supplies to charge each single battery, which has the disadvantages of high charging power supply cost, low reliability, low charging efficiency, and thick connection wire diameter. Therefore, this charging method is not widely used at present.

　　4 Series high-current charging plus small-current parallel charging

　　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, that is, using the battery management system and the charger to coordinate the series high-current charging plus the parallel low-current charging mode with constant voltage and current limiting. 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 preventing overcharging, it ensures that the battery will not be overcharged. Of course, if the BMS cannot communicate and control with 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) Since parallel charging can be performed, there is no need for a low-reliability and relatively high-cost equalization circuit, and the charging effect is better than the series charging method with only an equalization circuit, and its maintenance and management are also simple and easy.

　　(3) Since the maximum current of series charging is much larger 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) During charging, the order of series charging and parallel charging and the number of parallel charging power supplies can be flexibly controlled. Charging can be carried out simultaneously; parallel charging can be carried out after the series charging is completed; or a parallel charging power supply can be used to charge the battery with the lowest voltage in turn according to the voltage situation in the battery pack.

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

　　(6) When there are a large number of single lithium-ion batteries in a lithium-ion battery pack, the lithium-ion battery pack can be divided into several lithium-ion battery pack modules, and each lithium-ion battery pack module is charged by a combination of series high-current charging with BMS and charger coordination and parallel low-current charging with constant voltage and current limiting.

　　Its main purpose is to reduce the disadvantage that when there are a large number of series batteries in the battery pack, the consistency between single batteries is relatively poor, which leads to poor charging effect of the charging method coordinated by BMS and charger, so as to maximize the effect of the charging mode coordinated by BMS and charger.

　　This method is particularly suitable for battery systems where high-voltage battery packs are composed of low-voltage (such as 48V) battery module systems that can be quickly replaced, so that they can be charged or repaired in parallel at battery replacement stations or charging stations (general users do not need to charge in parallel during normal charging), and they can be sorted and re-assembled by special personnel according to actual conditions.

　　In short, this charging method, which uses a battery management system and a charger to coordinate series high-current charging and parallel low-current charging with constant voltage and current limiting, can effectively solve the problems of overcharging and undercharging that are prone to occur in series charging of lithium-ion battery packs. It can also avoid the problems of high charging power supply cost, low reliability, low charging efficiency, and thick connecting wire diameter in parallel charging. It is currently the most suitable charging method for high-voltage battery packs, especially electric vehicle battery packs.