Lithium ion batteries are an ideal power source due to their high working voltage, small size, light weight, no memory effect, no pollution, small self discharge, and long cycle life. In practical use, in order to obtain a higher discharge voltage, at least two individual 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 various fields such as laptops, electric bicycles, and backup power supplies.
Therefore, how to use lithium-ion battery packs well during charging is particularly crucial. Now, several commonly used charging methods for lithium-ion battery packs and the most suitable charging method in my opinion are described as follows:
1. Ordinary series charging
At present, the charging of lithium-ion battery packs generally adopts series charging, which is important because the series charging method has a simple structure, low cost, and is relatively easy to implement. However, due to the differences in capacity, internal resistance, attenuation characteristics, and self discharge performance between individual lithium-ion batteries, when charging a lithium-ion battery pack in series, the smallest individual lithium-ion battery in the battery pack will be fully charged first, while other batteries have not yet been fully charged. If series charging continues, the fully charged individual lithium-ion battery may be overcharged.
However, overcharging lithium-ion batteries can seriously damage their performance and even lead to explosions causing personal injury. Therefore, in order to prevent overcharging of individual lithium-ion batteries, lithium-ion battery packs are generally equipped with a Battery Management System (BMS) to protect each individual lithium-ion battery 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 to be unable to fully charge.
After years of development, lithium iron phosphate power batteries have been able to meet the requirements of electric vehicles, especially pure electric cars, due to their advantages such as high safety and good cycling performance. The technology also has the basic conditions for large-scale production. However, there are certain differences in the performance of lithium iron phosphate batteries compared to other lithium-ion batteries, especially in their voltage characteristics, which are different from lithium manganese oxide batteries, lithium cobalt oxide batteries, and so on.
In addition, although some battery management systems have balancing functions, 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 individual batteries 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, if 100 lithium-ion batteries with a discharge capacity of 100Ah are connected in series to form a battery pack, but if 99 individual lithium-ion batteries are charged with 80Ah before the group is formed, and the other individual lithium-ion battery is charged with 100Ah, when the battery pack is connected in series for charging, the individual lithium-ion battery with 100Ah will be fully charged first to reach the overcharge protection voltage. In order to prevent this individual lithium-ion battery from being overcharged, the battery management system will cut off the entire series charging circuit, making it impossible for the other 99 batteries to fully charge, resulting in a discharge capacity of only 80Ah for the entire battery pack.
When battery manufacturers test the capacity at the factory, they first charge the individual battery with constant current, then charge it with constant voltage, and then discharge it with constant current to measure the discharge capacity. The general discharge capacity is approximately equal to the constant current charging capacity plus the constant voltage charging capacity. However, in the actual series charging process of battery packs, there is generally no constant voltage charging process for individual batteries, so the constant voltage charging capacity will not be available, and the battery pack capacity will be smaller than the individual battery capacity. The smaller the charging current, the smaller the proportion of constant voltage charging capacity, and the smaller the capacity loss of the battery pack. Therefore, a battery management system and a coordinated series charging mode have been developed.
2. Coordination and coordination between battery management system and charger for series charging
The battery management system is the most comprehensive device for understanding the performance and status of batteries. Therefore, establishing a connection between the battery management system and the charger can enable the charger to understand battery information in real time, thereby more effectively solving some problems that may arise during battery charging,
The principle of coordinated charging mode between battery management system and charger is that the battery management system monitors the current state of the battery (such as temperature, individual battery voltage, battery working current, consistency, and temperature rise), and uses these parameters to estimate the maximum allowable charging current of the current battery; During the charging process, the battery management system and charger are connected through communication lines to achieve data sharing. The battery management system transmits real-time parameters such as total voltage, maximum individual battery voltage, maximum temperature, temperature rise, maximum allowable charging voltage, maximum allowable individual battery voltage, and maximum allowable charging current to the charger. The charger can change its charging strategy and output current based on the information provided by the battery management system.
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 voltage and temperature of the battery exceed the limit, the battery management system can detect and promptly notify the charger to change the current output; When the charging current exceeds the maximum allowable charging current, the charger begins to follow the maximum allowable charging current, effectively preventing the battery from overcharging and achieving the goal of extending the battery life. Once a malfunction occurs during the charging process, the battery management system can set the maximum allowable charging current to 0, forcing the charger to stop, preventing accidents and ensuring the safety of charging.
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 state of the battery, achieving the goal of preventing all batteries in the battery pack from overcharging and optimizing charging. The actual discharge capacity of the battery pack is also greater than that of ordinary series charging methods. However, this method still cannot solve the problem of some batteries in the battery pack being under charged, especially when there are many battery packs in series, poor battery consistency, and high charging current.
3. Parallel charging
However, the parallel charging method requires multiple low voltage and high current charging power sources to charge each individual battery, which has drawbacks such as high cost, low reliability, low charging efficiency, and thick connection wire diameter. Therefore, this charging method is currently not widely used.
4. Series high current charging and parallel low current charging
Due to the limitations of the three charging methods mentioned above, I have developed a charging method that is most suitable for high-voltage battery packs, especially for electric vehicle battery packs. This method uses a battery management system and a charger to coordinate with series high current charging and a parallel low current charging mode with constant voltage and current limiting.
This charging method has the following characteristics:
(1) Due to the BMS of this system's function of preventing overcharging, it ensures that the battery will not experience overcharging issues. Of course, if the BMS cannot communicate and control with parallel charging power sources, since the constant voltage value of parallel charging power sources is generally the same as the voltage value of single lithium-ion batteries in lithium-ion battery packs when fully charged, there will be no overcharging problem.
(2) Due to the ability to charge in parallel, it is not recommended to use a low reliability and relatively high cost balancing circuit. Moreover, the charging effect is better than the series charging method with only balancing circuits, and its maintenance and management are also simple and easy to implement.
(3) Due to the fact that the maximum current of series charging is much greater than that of parallel charging (usually more than 5 times), it can ensure that a higher capacity can be charged in a shorter period of time, thereby achieving the maximum effect of series charging.
(4) The sequence of series and parallel charging during charging, as well as the number of parallel charging power sources, can be flexibly controlled, allowing for simultaneous charging; Parallel charging can be carried out after the series charging is completed; It is also possible to use a parallel charging power supply to charge the battery with the lowest voltage alternately based on the voltage in the battery pack.
(5) With the development of technology, parallel charging power sources can be non-contact charging power sources (wireless charging power sources) or solar cell power sources, making parallel charging simple.
(6) When there are a large number of individual lithium-ion batteries in a lithium-ion battery pack, the lithium-ion battery pack can be divided into several modules, and each module of the lithium-ion battery pack can be charged using a combination of BMS and charger coordination, series high current charging, and parallel low current charging with constant voltage and current limiting.
Its important purpose is to reduce the disadvantage of poor consistency between individual batteries when there are a large number of series connected batteries in the battery pack, which leads to poor charging effectiveness of the charging method coordinated by BMS and charger, in order to achieve the maximum effect of BMS and charger coordinated charging mode.
This method is particularly suitable for high voltage battery packs, which are battery systems composed of quickly replaceable low voltage (such as 48V) battery modules. This allows for parallel charging or repair at battery replacement or charging stations (general users may not need to charge in parallel during normal charging), and is sorted and reassembled by a dedicated person according to the actual situation.
In summary, this charging method that uses a battery management system and a charger to coordinate with series high current charging and constant voltage current limiting parallel small current charging can effectively solve the problems of overcharging and insufficient charging that are prone to occur in series charging of lithium-ion battery packs. It can also prevent problems such as high cost, low reliability, low charging efficiency, and thick connection wire diameter of parallel charging power sources. It is currently the most suitable charging method for high-voltage battery packs, especially for electric vehicle battery packs.