Lithium Battery Balancing Management

The inconsistency of battery refers to the battery cell difference of capacity, internal resistance, and the terminal voltage of the battery cell in the same group of battery, which is determined by the nature of the battery.

Battery dispersion refers to the difference between the average level and the capacity, internal resistance, terminal voltage, and other parameters of a battery cell in the same group. It is a state of the cell in the battery pack. The dispersion of the battery is the performance of the inconsistency of the battery. There are many sources of battery inconsistency. First of all, the material of the battery is not completely consistent, and the details of each battery can not be controlled in the process. After manufacturing, due to the influence of the external environments, such as temperature, potential balance, etc., there will be different degrees of variation. In addition, the difference of self-discharge rate and different degrees of overcharge and over-discharge will further brush the inconsistency between single cells.

Generally, the standard deviation of voltage can be used to measure the dispersion degree of batteries in the battery pack. The use of the battery pack will not be affected if the voltage is less than 2%. The performance of 2% ~ 8% battery pack is obviously affected when it is used at a high rate and high power. There are few working conditions that can provide a normal function of more than 8%.

BMS

In the case of no single cell damage, the dispersion of the battery pack can be considered to be mainly caused by imbalance, which can be reduced by equalization, so that the battery pack can play its design function in a wider range. Equalization technology includes an equalization circuit and equalization control strategy. The equalization circuit is a kind of power and energy transfer circuit, which has developed many varieties. It can realize the discharge of battery cell, energy transfer between the battery cell and battery pack, and power transfer between the battery cell and battery cell.

The balance strategy is the difficulty of equilibrium management. First of all, it is necessary to calculate the dispersion of the battery pack to determine whether equalization is needed. However, due to the difference of capacity, the dispersion of the same battery at different times in the same cycle is different. For example, when all the monomers are fully charged, the dispersion can be considered as zero. But after reaching the end of discharge, the dispersion reaches the maximum. Therefore, to determine the target point of charge and discharge, we can judge whether the current dispersion needs to be balanced. In the use of the interval to calculate the possible maximum dispersion, if the dispersion is too large to be used normally, it needs to be balanced. The wider the goal of using the interval, the higher the requirement of equilibrium. Then select the balanced monomer, how to determine which monomer to participate in this equalization, the purpose of equalization is to maintain the normal function of the battery pack in the whole target SOC range. If we want to achieve the goal simply and effectively, the choice of the monomer participating in the equilibrium will have a great influence. How to set the starting point and endpoint of equilibrium depends on the performance of each cell in the selected interval. The higher the coincidence degree of the two intervals is, the better.

However, as mentioned above, the SOC estimation of the whole battery has errors, and the SOC estimation of each cell also has errors, which is within the interval width of double error, The system is unable to determine whether the equilibrium is at the beginning or at the end. Because the system does not know which monomer is a positive error and which cell is a negative error.

There is a way to do differential amplification between the voltage of a battery cell and the minimum voltage in the set interval so that the slope of the battery discharge curve can be made very large, and the curve looks very steep. Then, it is assumed that the cell capacity consistency is normal in the strategy so that the voltage of each cell at the beginning and the end of the target range can be estimated, and the function of each cell can be judged according to the voltage. After selecting a target interval, the cell with the fastest voltage change has the lowest voltage at the low end of the interval, and the highest voltage at the high end of the interval; the cell with the slowest voltage change has the highest voltage at the low end of the interval, and the lowest voltage at the high end of the interval. This strategy only involves the accuracy of voltage measurement, which generally does not cause overcharge and over-discharge.

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