Solutions for active balancing and passive balancing of lithium battery pack charging. The most important link we need to pay attention to when charging and discharging lithium battery packs is the balancing link. Currently, there are two solutions to solve the unbalanced situation of small lithium batteries in the battery pack: active balancing and passive balancing. The balancing step is to ensure that the input and output of the lithium battery pack are reasonable to each lithium battery, so that it can be used reasonably and effectively without damage.

　　Lithium battery pack charge balancing method

　　1. Traditional passive method: In a general lithium battery management system, each battery cell is connected to a load resistor through a switch. This passive circuit can discharge individual selected cells. But this method is only suitable for suppressing the voltage rise of the strongest battery cells in charging mode. In order to limit power consumption, such circuits generally only allow discharge with a small current of about 100mA, causing the charge balance to take up to several hours.

　　2. Active balancing method: There are many active balancing methods in relevant information, all of which require a storage element for transferring energy. If capacitors are used as storage elements, a huge switch array is required to connect them to all battery cells. A more efficient method is to store energy in a magnetic field. The key component in this circuit is a transformer.

　　Lithium battery passive balancing

　　Passive balancing of lithium batteries is based on the charging state of lithium batteries. Its premise is that before charging the lithium battery pack, each small battery in the battery pack has a different residual charge. This difference is determined by the different performance parameters of the battery. Due to the performance differences of each battery Differently, some batteries will overcharge due to their "weakness" under current impulse, causing overvoltage. Overvoltage is the greatest damage to lithium batteries. It will cause serious damage to the lithium battery pack and even make it impossible to use it again.

　　The method of passive balancing is: during charging, the current that causes the voltage of the small battery to rise rapidly can be shunted to the bypass resistor. By shunting unnecessary charging current to a resistor so that the resistor can dissipate the current, overcharging of the battery can be avoided. This power dissipation function of passive balancing limits the current drawn from the battery within a certain range, allowing all small batteries to be fully charged.

　　The disadvantage of the passive balancing method is that because power dissipation is used as a means, part of the balancing current appears as heat, which greatly reduces the charging efficiency of the lithium battery pack and increases the cost of manufacturing lithium batteries. In addition, it cannot solve the problem of power imbalance in the lithium battery pack when it is discharged and put on hold. Therefore, if you want to achieve the ultimate goal of passive balancing - a comprehensive balance of charge and discharge of the lithium battery pack, further measures need to be taken, but this will all Increase the manufacturing cost of lithium battery packs.

　　Solution for active balancing method of lithium battery pack charging

　　Active balancing abandons the passive balancing method of consuming current and changes it to a method of transmitting current. The device responsible for charge transfer is a power converter, which allows the small batteries in the lithium battery pack to transfer charge whether they are charging, discharging or idle, so that dynamic balance can be maintained between the small batteries.

　　Because the charge transfer efficiency of the active balancing method is extremely high, it can provide higher balancing current, which means that this method has better balancing ability when the lithium battery pack is charging, discharging and idle.

　　1. Strong fast charging capability: The active balancing function can make each small battery in the lithium battery pack reach balance faster. Therefore, fast charging is safer and suitable for high-rate charging with larger current.

　　2. When idle: Even if each small battery has reached the equilibrium state during charging, due to different temperature gradients, the internal temperature of some small batteries is higher and the internal temperature of some small batteries is lower, which will also cause the internal temperature of each small battery to Leakage speeds vary. Test data shows that the leakage rate doubles for every 10°C increase in battery temperature. The active balancing function ensures that small batteries in idle lithium battery packs "constantly" regain balance, which is beneficial to storage in the battery pack. The power can be fully utilized, so that when the battery pack ends, the remaining power of a single small lithium battery is minimal.

　　3. When discharging: There is no lithium battery pack with a discharge capacity of 100%. This is because the end of the working capacity of a group of lithium batteries is determined by a certain small lithium battery that is discharged first, and there is no guarantee that all small lithium batteries will be discharged. Lithium batteries can be fully discharged at the same time. Instead, there are individual small lithium batteries that retain unused residual energy. Through the active balancing method, when the lithium battery pack is discharging, the internal large-capacity lithium battery will distribute electric energy to the small-capacity lithium battery. Therefore, the small-capacity lithium battery can also be fully discharged, and there will be no residual electric energy in the battery pack. However, the battery pack with active balancing function has a larger actual storage capacity (that is, it can release electric energy closer to the nominal capacity).

　　As a final note, system performance with active balancing methods depends on the ratio between balancing current and battery charge/discharge efficiency. The higher the imbalance rate of a set of lithium batteries, or the greater the charge/discharge rate of a lithium battery pack, the higher the balancing current required. Of course, this current consumption used for balancing is quite cost-effective compared to the extra current obtained due to internal balancing. Moreover, this active balancing is also conducive to extending the life of the lithium battery pack.

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