Lithium-ion battery pack charging strategy analysis

Lithium-ion battery is an ideal power source because of its high working 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-cell lithium-ion batteries are generally used in series to form a lithium-ion battery pack. At present, lithium-ion battery packs have been widely used in various fields such as notebook computers, electric bicycles and backup power supplies.

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Therefore, how to use the lithium-ion battery pack during charging is particularly important. Several charging methods commonly used in lithium-ion battery packs and the most suitable charging methods that I consider are 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 easy to implement. However, due to the difference in capacity, internal resistance, attenuation characteristics, self-discharge, etc. between the single-cell lithium-ion batteries, when the lithium-ion battery pack is charged in series, the single-cell lithium-ion battery with the smallest capacity in the battery pack will The first battery is fully charged, and at this time, the other batteries are not fully charged. If the series charging is continued, the fully charged single-cell lithium-ion battery may be overcharged.

Lithium-ion battery overcharge can seriously damage the performance of the battery, and may even cause personal injury caused by the explosion. Therefore, in order to prevent over-charging of the single-cell lithium-ion battery, the lithium-ion battery pack is generally equipped with a battery management system (Battery Management). System, referred to as BMS), protects each single-cell lithium-ion battery by battery management system. 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 cannot be fully charged.

After years of development, lithium iron phosphate power battery has basically met the requirements of electric vehicles, especially pure electric cars, because of its high safety and good cycle performance. condition. However, the performance of lithium iron phosphate batteries is different from other lithium ion batteries, especially the voltage characteristics are different from those of lithium manganate batteries and lithium cobalt oxide batteries. The following is a comparison of the charging curves of lithium iron phosphate and lithium manganese oxide batteries with lithium ion deintercalation:

Fig.1 Corresponding relationship between lithium ion deintercalation and charging curve of lithium manganate battery

Fig. 2 Corresponding relationship between lithium ion deintercalation and charging curve of lithium iron phosphate battery

It is not difficult to see from the curve above that when the lithium iron phosphate battery is fully charged, the lithium ion is almost completely decoupled from the positive electrode to the negative electrode, and the voltage at the battery terminal rises rapidly, and the charging curve is upturned, which causes the battery to be very It is easy to reach the overcharge protection voltage. Therefore, the phenomenon that some of the batteries in the lithium iron phosphate battery pack are not fully charged is more obvious than the lithium manganate battery pack.

In addition, although some battery management systems have equalization functions, due to cost, heat dissipation, reliability and other aspects, the equalization current of the battery management system is generally much smaller than the current of series charging, so the equalization effect is not obvious, and it will appear. Some single cells are not fully charged, which is more obvious for lithium-ion battery packs that require high current charging, such as lithium-ion battery packs for electric vehicles.

For example, 100 lithium-ion batteries each having a discharge capacity of 100 Ah are connected in series to form a battery pack, but if 99 of the single-cell lithium-ion batteries are charged at 80 Ah in the group before, and another single-cell lithium-ion battery is charged at 100 Ah, When the battery pack is charged in series, the single-cell lithium-ion battery with a charge of 100 Ah will be fully charged to achieve the overcharge protection voltage. In order to prevent the single lithium-ion battery from being overcharged, the battery management system will The entire series charging circuit is cut off, which makes the other 99 batteries unable to fully charge, so that the entire battery pack discharge capacity is only 80Ah.

Generally, when the battery manufacturer tests the capacity at the factory, the single battery is firstly charged with constant current and then charged at a constant voltage, and then discharged by a 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 series charging process of the actual battery pack, there is generally no constant voltage charging process for the single battery, so the constant voltage charging capacity will be absent, and the battery pack capacity will be smaller than the single battery capacity. The smaller the general charging current, the smaller the ratio of the constant voltage charging capacity, and the smaller the loss capacity of the battery pack. Therefore, the battery management system and the charger are coordinated to cooperate with the series charging mode.

2 Battery management system and charger coordinately cooperate with series charging

The battery management system is the most comprehensive device for understanding the performance and status of the battery. Therefore, establishing a connection between the battery management system and the charger enables the charger to know the battery information in real time, thereby more effectively solving the battery charging. Some problems arise, and the schematic is as follows.

Figure 3 Power lithium battery system integration solution

Figure 4 Lithium-ion battery system basic system

Figure 5 Schematic diagram of BMS and charger coordination and series charging

The principle of the battery management system and the charger coordinating with the charging mode is: the battery management system monitors the current state of the battery (such as temperature, 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 a communication line to realize data sharing. The battery management system transmits parameters such as total voltage, maximum cell voltage, maximum temperature, temperature rise, maximum allowable charging voltage, maximum allowable cell voltage, and maximum allowable charging current to the charger in real time, and the charger can be based on the battery. The information provided by the management system changes its own charging strategy and output current.

When the maximum allowable charging current provided by the battery management system is higher than the current capacity of the charger design, the charger is charged according to the designed maximum output current; when the voltage and temperature of the battery exceeds the limit, the battery management system can detect and notify the charging in real time. The machine changes 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, thus effectively preventing the battery from being overcharged and achieving the purpose of prolonging the battery life. In the event of a fault in the charging process, the battery management system can set the maximum allowable charging current to 0, forcing the charger to stop, avoiding accidents and ensuring the safety of charging.

In the charging mode, the management and control functions of the battery management system are perfected, and the charging machine can change the output current in real time according to the state of the battery, thereby preventing the overcharge of all the batteries in the battery pack and optimizing the charging. The actual discharge capacity of the battery pack is also greater than the ordinary series charging method, but this method can not solve the problem that some batteries in the battery pack are not fully charged, especially when the number of battery packs is large, the battery consistency is poor, and the charging current is relatively high. Big time.

3 parallel charging

In order to solve the problem of overcharging and charging of some single cells in the battery pack, a parallel charging method has been developed, and the schematic diagram is as follows.

Figure 6 Schematic diagram of parallel charging

However, the parallel charging method requires a plurality of low-voltage, high-current charging power sources to charge each of the single cells, and there are defects such as high cost of the charging power source, low reliability, low charging efficiency, and large connecting wire diameter, so there is no large current The range uses this charging method.

4 series high current charging plus small current parallel charging

Since the above three charging methods 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, the battery management system and the charger are coordinated to cooperate with the series high-current charging and constant. The mode of parallel small current charging of the voltage limiting current, the schematic diagram is as follows.

Figure 7 Schematic diagram of battery management system and charger coordinated with series charging plus parallel charging

This charging method has the following characteristics:

(1) Since the BMS of this system has a function of preventing overcharging, it is ensured that the battery does not have an overcharge problem. 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 source is generally the same as the voltage value when the single lithium ion battery in the lithium ion battery pack is fully charged, the problem of overcharging does not occur. .

(2) Since parallel charging is possible, an equalization circuit with low reliability and relatively high cost is not required, and the charging effect is better than the series charging method with only the equalization circuit, and the maintenance management thereof is also simple and easy.

(3) Since the maximum current of series charging is much larger than the current of parallel charging (generally 5 times or more), it can be ensured 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 during charging and the number of parallel charging power sources can be flexibly controlled, and charging can be performed at the same time; parallel charging can be performed after series charging is completed; or a parallel charging power source can be used according to the voltage in the battery pack. The situation is to charge the battery with the lowest voltage in turn.

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

(6) When the number of single-cell lithium-ion batteries in the lithium-ion battery pack is large, the lithium-ion battery pack can be divided into several lithium-ion battery pack modules, and each lithium-ion battery pack module is coordinated with the BMS and the charger. Charging is performed in combination with current charging and parallel low current charging of constant voltage current limiting.

The main purpose is to reduce the consistency of the cells in the battery pack when the number of cells in the battery pack is relatively large, which leads to the poor charging effect of the charging method coordinated by the BMS and the charger, so as to exert the BMS and the charger. Coordinate with the maximum effect of the charging mode.

This method is especially suitable for high-voltage battery packs, which are battery systems consisting of a quick-changeable low-voltage (eg 48V) battery module system, so that they can be charged or repaired in parallel at the battery exchange station or charging station (normal user charging) It is not necessary to charge in parallel), and the person can sort and re-group according to the actual situation.

In short, this parallel charging method using battery management system and charger to cooperate with series high current charging and constant voltage limiting current can effectively solve the problem of overcharging and charging of Lithium-ion battery packs. The utility model can avoid the problems of high cost, low reliability, low charging efficiency and thick connecting wire diameter of the charging power supply for parallel charging, and is currently the most suitable charging method for the high voltage battery pack, especially the electric vehicle battery pack.