Lithium-ion power battery cells generally adopt a constant current-constant voltage charging method, that is, first use a fixed rate of current (0.3C or 1C, etc.) to charge, reach the set charging cut-off voltage (3.6V, 4.0V, etc.) For constant voltage charging, the charging is completed after the charging current is lower than a certain value (such as 0.03C). Relevant studies have shown that the charging current and charging cut-off voltage not only have a significant impact on the charging time and charging energy of lithium-ion power batteries, but also have an important impact on their service life. Lithium-ion power battery systems are generally used in groups of multiple cells in series. If the constant current-constant voltage charging method is still used, it may cause overcharging of some battery cells. On the basis of studying the influence of charging current, charging voltage, overcharge and other charging factors on the service life, with the purpose of prolonging the service life of the battery, an optimized charging method for lithium-ion power battery cells and battery packs is proposed.
1. Influence of charging factors on the life of lithium-ion power batteries
1) Influence of overcharging on the life of lithium-ion power batteries
When the lithium-ion power battery is overcharged, a lot of heat is generated inside the battery, and at the same time, a lot of bubbles are generated in the electrolyte, which causes the active material on the positive and negative plates of the lithium-ion power battery to peel off, which seriously affects the activity of the battery and increases the internal resistance. Capacity has also dropped. At the same time, overcharging may also cause the battery to expand and deform, and even cause serious consequences such as fire and explosion. Research by Wang Hongwei et al. shows that at an ambient temperature of 20°C to 40°C, overcharging will cause the lithium-ion power battery to expand and deform, and the higher the temperature, the faster the temperature rises when the lithium-ion power battery is overcharged, and the higher the maximum temperature. more likely to be dangerous. Therefore, during the use of the lithium-ion power battery, it is necessary to ensure the normal operation of the charger and the protection circuit to avoid overcharging.
2) Influence of charging current and charging voltage on battery life
The charging voltage and charging current directly affect the charging energy and charging speed of the lithium-ion battery. Taking a certain lithium-ion power battery as an example, as shown in Figure 1, as the charging current increases, the charging capacity in the constant current stage becomes smaller, and the constant current charging capacity at 100A charging current is reduced by 8.36% compared with 20A charging current. The total charging time of the battery decreases with the increase of charging current, and the total charging time of 100A constant current charging is reduced by 76.1% compared with the total charging time of 20A charging current. This shows that increasing the charging current has little effect on the total energy charged, but can significantly improve the charging speed. However, high-rate charge-discharge current will cause the battery system to deviate from the equilibrium state, and accelerate the aging of positive and negative materials, thereby shortening the battery life. Therefore, power battery manufacturers need to comprehensively consider charging time and battery life when designing charging strategies. Two charging modes can be set: under normal circumstances, low-current charging should be selected as far as possible when charging lithium-ion power batteries, so as to prolong the battery life; in urgent cases, high-current charging can be used to shorten the charging time, although this will damage battery life.
In general, when the charging current is the same, the higher the charging cut-off voltage, the greater the total energy charged by the lithium-ion power battery. However, the higher charge cut-off voltage will cause partial decomposition of the battery cathode material, the performance of the electrolyte will also decline, and the separator will also be oxidized due to contact with the high-potential cathode material. Taking a lithium-ion power battery as an example, as shown in Figure 2, when the charging voltage is reduced from 4.2V to 4.1V, the capacity retention of the lithium-ion battery is better as the number of charging and discharging increases, that is, the battery Longer cycle life. Relevant studies have shown that reducing the charge cut-off voltage by 0.1~0.3V can prolong the battery cycle life by 2~5 times.
2. Charging strategy based on lifetime optimization
By studying the influence of charging current, charging voltage and overcharge on the life of lithium-ion power battery, in order to prolong the service life of lithium-ion power battery cells and battery packs, this paper proposes the optimization of lithium-ion power battery cells and battery packs. charging strategy.
1) Charging strategy of lithium-ion power battery cells
In order to prolong the service life of the battery, the charger and the charging protection circuit should be safe and reliable. The thermistor can be used to detect the temperature of the battery, and stop charging when the battery temperature exceeds the high temperature threshold to prevent overcharging. After the battery is fully charged, disconnect the voltage in time, otherwise, metal lithium will be generated inside the battery, resulting in permanent capacity loss, and may cause a short circuit inside the battery.
According to different positive and negative materials and battery structure, the charging parameters of lithium-ion power batteries will be different. When determining the charging method of a single battery, factors such as the composition material and structure of the battery, charging time, charging capacity and battery life should be comprehensively considered, and parameters such as charging current, charging cut-off voltage and charging termination current should be optimized and designed. The charging methods of lithium-ion batteries can be divided into ordinary charging and fast charging: ordinary charging is suitable for general household charging or long-term parking charging, using small rate charging current and charging cut-off voltage, charging voltage can be 3.8~4.0V, And use a small charging termination current (such as C/10 or less) to strengthen the protection of the battery; fast charging is suitable for charging in emergency situations, which will greatly damage the battery life, use a large rate current for a short time (1h) Charge more than 90% of the battery inside. The number of fast charging times should be minimized during use.
Appropriately reducing the charge cut-off voltage can significantly improve the service life of the battery. In order to take into account the charging time, the cut-off voltage can be set according to the depth of discharge (DOD): when the depth of discharge is 100%, in order to shorten the charging time, the cut-off voltage can be increased, such as 3.8V; when the depth of discharge is 0, set the cut-off voltage The voltage is 3.5V. In this way, when the depth of discharge is between 0 and 100%, the charge cut-off voltage can be set between 3.5 and 3.8V according to a linear relationship. This approach helps extend battery life and can reduce the overall charging time for Li-ion power batteries.
2) Charging strategy of lithium-ion power battery pack
When charging a lithium-ion power battery pack, if a single constant current-constant voltage charging is used, it is likely to cause some cells to be overcharged. Therefore, the charging of the battery pack needs to be controlled according to the state of the lithium-ion power battery cells to prevent overcharging of the cells, protect the cycle life of the single cells, and help prolong the cycle life of the battery pack. Taking the charging process of a lithium-ion power battery system as an example, the standard charging steps are: charging with a constant current of 1C, when the voltage of a single battery reaches 3.5V or more, the charging current is reduced to C/2. When the voltage of the single battery reaches 3.55V or more, reduce the charging current to C/4, when the voltage of a single battery reaches 3.6V, reduce the charging current to C/8, when the highest voltage of the single battery reaches the cut-off voltage (3.65V) or the battery pack The charging process is completed after the total voltage reaches a certain cut-off voltage.
When the vehicle is parked for a long time, the power supply should be cut off, and the vehicle should be parked in a ventilated, rain-proof, moisture-proof, sun-proof, and fire-fighting place, and should be kept away from flammable and corrosive items. When the vehicle is parked for more than a month, the battery pack must be kept at a state of charge of about 50%, the connecting wire of the battery pack must be unplugged, and the power battery system must be charged and discharged with a small current every three months for maintenance.