Application of the equalization function of multi-string lithium-ion batteries

　　For the design of multiple strings of batteries, what needs to be considered is the consistency of each battery cell in series. The consistency of the cell is generally divided into several parts: capacity, voltage, internal resistance and self-discharge rate. The most basic requirement for multiple strings of batteries is that the batteries connected in series must come from the same type of batteries, preferably from the same batch, to ensure the consistency of capacity, voltage, internal resistance and self-discharge.

　　With the popularization of smart terminals such as mobile phones and notebooks, the application of lithium-ion batteries has become more and more widespread. From the perspective of the number of batteries used, devices such as mobile phones and tablets usually use a single-cell battery and a string of two parallel batteries. However, notebook batteries and power batteries usually use multiple cells in series. Let's do some discussion on the use of multiple batteries in series.

　　For the design of multiple strings of batteries, what needs to be considered is the consistency of each battery cell in series. The consistency of the cell is generally divided into several parts: capacity, voltage, internal resistance and self-discharge rate. The most basic requirement for multiple strings of batteries is that the batteries in series must come from the same type of batteries, preferably from the same batch, to ensure the consistency of capacity, voltage, internal resistance and self-discharge.

　　But even for the same battery cell, due to individual differences in production lines and batch differences, their parameters are generally slightly different. In order to achieve better performance, batteries are usually grouped before shipment.

　　Regarding the capacity, usually when the battery cell factory produces the battery cell, there is a process to charge and discharge the battery cell completely, so that the capacity of each battery cell can be measured. Both voltage and internal resistance can be measured by the device when grouping. Therefore, in theory, the battery cell factory can group the battery cells according to the requirements when they ship. The packaging factory then assembles according to the principle of the same group configuration.

　　Regarding the parameter of self-discharge rate, generally manufacturers will not group, mainly relying on design considerations. The self-discharge rate is strongly related to the thickness of the diaphragm used in the cell design. Therefore, multiple strings of batteries will use a thicker diaphragm to ensure the consistency of self-discharge when designing the cell.

　　Can the battery cell grouping and design optimization we mentioned above completely avoid the unbalanced situation of the battery? Of course the answer is no. What we have been discussing so far has always been brand new batteries. If the battery is used by the user for a period of time, will there be an imbalance? of course.

　　The reasons for the inconsistency of batteries after use are mainly divided into the following two aspects:

　　1. After the user has used it for many times, the capacity of one of the battery cells decays faster, causing the battery to appear unbalanced in capacity.

　　2. After the battery is used and stored for a long time, the battery cells will be inconsistent after a long time accumulation due to the difference in the self-discharge rate.

　　What to do if the above two situations occur, usually there is a battery management system in the PACK of our battery, which is our BMS, and there is a function in this battery management system called the equalization function. When the battery is inconsistent, the battery management system can find out by monitoring the voltage of each battery cell in real time. Then start the equalization function of the battery to discharge the high-capacity cells until the cell voltage is consistent. According to the current design, the equalization function is usually activated in the battery charging and resting state.

　　If there is an extreme situation, such as a serious decrease in the capacity of a string of cells, the equalization function cannot make the cells consistent. At this time, there is a risk of overcharging the battery, and the battery can no longer be used. The battery management system judges that if this condition is reached based on the voltage, it will burn out the fuse on the battery's main circuit and permanently prohibit the use of this battery. If the user encounters this situation, he must go to the manufacturer's professional repair station to replace the battery.