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 properly when charging is particularly important. Here are several commonly used charging methods for lithium-ion battery packs and the most suitable charging method I think:


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 easy 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 be fully charged first, while at this time, other batteries are not 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 explosion 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. 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 fail 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, the performance of lithium iron phosphate batteries is somewhat different from other lithium-ion batteries, especially its voltage characteristics are different from lithium manganese oxide batteries and lithium cobalt oxide batteries. The following is a comparison of the charging curves and the corresponding relationship between lithium ion insertion and extraction of two lithium-ion batteries, lithium iron phosphate and lithium manganese oxide:

 


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 and the other single lithium-ion battery is charged with 100Ah before forming the battery pack, when the battery pack is charged in series, the single lithium-ion battery with a charge of 100Ah will be fully charged first, thereby 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 cell at a constant current, then charge at a constant voltage, and then discharge at a 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. However, in the actual battery pack series charging process, there is generally no constant voltage charging process for the single cell, so there will be no constant voltage charging capacity, and the battery pack capacity will be smaller than the single cell 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 of battery management system and charger has been developed.


2 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 battery's performance and status. 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 that arise during 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 coordinated series charging between BMS and charger The principle of coordinated charging mode between battery management system and charger 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 designed current capacity of the charger, the charger will charge 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 starts to follow the maximum allowable charging current, which effectively prevents the battery from overcharging 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 the charging. The actual discharge capacity of the battery pack is also greater than the ordinary series charging method. However, this method still cannot solve the problem that some batteries in the battery pack are not fully charged, especially when the battery pack has many strings, poor battery consistency, and 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 sources to charge each single battery, which has the disadvantages of high charging power source cost, low reliability, low charging efficiency, and thick connecting wire diameter. Therefore, this charging method is not widely used at present.


4. Series high current charging plus low 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, a battery management system and a charger are coordinated to cooperate in series high-current charging plus parallel low-current charging 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, 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 that of parallel charging (generally more than 5 times), it can ensure that a higher capacity can be charged in a shorter time, thus achieving 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; series charging can be completed before parallel charging; or a parallel charging power supply can be used to charge the battery with the lowest voltage in turn according to the voltage conditions in the battery pack.


(5) With the development of technology, the parallel charging power source can be a contactless charging power source (wireless charging power source) or a solar cell power source, 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. Each lithium-ion battery pack module can be charged by combining series high-current charging with constant voltage and current-limited parallel low-current charging in a coordinated manner between the BMS and the charger.


Its main purpose is to reduce the disadvantage of poor charging effect of the charging method coordinated by BMS and charger, which occurs when there are a large number of batteries in series in the battery pack, due to relatively poor consistency between single cells, 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 quickly replaceable low-voltage (e.g. 48V) battery module systems, 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 can be sorted and re-assembled by dedicated personnel based on 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.