Charging method based on high voltage CR2032 button cell battery pack

General series charging

Currently, 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 the lithium-ion battery pack in series, the single lithium-ion battery with the smallest capacity in the battery pack will be fully charged first, while the other batteries are not fully charged at this time. 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 injuries. Therefore, in order to prevent the occurrence of overcharging of single lithium-ion batteries, lithium-ion battery packs are generally equipped with a battery management system (battery management system, referred to as bMS) when used, and each single lithium-ion battery is protected by the battery management system such as 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 lithium-ion batteries have basically met the requirements of electric vehicles, especially pure electric cars, due to their high safety and good cycle performance, and the process has basically met the conditions for large-scale processing. However, the performance of lithium iron phosphate batteries is somewhat different from that of other lithium-ion batteries, especially their voltage characteristics are different from those of lithium manganese oxide batteries, lithium cobalt oxide batteries, etc. The following is a comparison of the charging curves and the corresponding relationship between lithium ion deintercalation and lithium ion deintercalation of two lithium-ion batteries, lithium iron phosphate and lithium manganese oxide:

Figure 1 Correspondence between lithium ion deintercalation and charging curve of lithium manganese oxide batteryFigure 2 Correspondence between lithium ion deintercalation and charging curve of lithium iron phosphate batteryIt is not difficult to see from the curves in the above figure that when the lithium iron phosphate battery is quickly 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 phenomenon, 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 clear, 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 being grouped, 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 the discharge capacity of the entire battery pack is only 80Ah.

Generally, when battery manufacturers detect 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.

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, so as to more effectively deal with some problems that arise during battery charging. 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 5 Schematic diagram of coordinated series charging between the battery management system and the charger The principle of the coordinated 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 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. 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 voltage and temperature of the battery exceed the limit, the battery management system can test it in real time and notify the charger to change the current output in time; 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, prevent accidents, and ensure 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 battery status, 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 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 the battery pack has a large number of strings, poor battery consistency, and a large charging current.

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 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 supplies to charge each single cell, which has the defects of high charging power cost, low reliability, low charging efficiency, and thick connection wire diameter. Therefore, this charging method is not widely used at present.

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, 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 the battery management system and the charger coordinating the series charging plus the 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 that of parallel charging (generally more than 5 times), it is possible to ensure that a higher capacity can be charged in a shorter time, thereby 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 performed simultaneously; parallel charging can be performed after 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 in the battery pack.

(5) With the development of technology, the parallel charging power supply can be a contactless 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 combining series high-current charging with constant voltage and current-limited parallel low-current charging with the coordination of the bMS and the charger.

Its main purpose is to reduce the disadvantage that when there are more batteries in series in the battery pack, the consistency between the single cells is relatively worse, which leads to the poor charging effect of the charging method coordinated by the bMS and the charger, so as to give full play to the maximum effect of the charging mode coordinated by the bMS and the charger.

This method is particularly suitable for high-voltage battery packs 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-grouped by special personnel according to actual conditions.

The charging method of using the battery management system and the charger to coordinate the series high-current charging plus the parallel low-current charging with constant voltage and current limiting can effectively solve the problems of overcharging and undercharging that are easy to occur in the series charging of lithium-ion battery packs, and can prevent the high cost, low reliability, low charging efficiency, and thick connection wire diameter of the charging power supply of parallel charging. 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, 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 various fields such as laptops, electric bicycles, and backup power supplies.

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