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The disconnection detection function is described above. This time, the equalization function is also one of the main functions of BMS.

The concept of cell balance is believed to be familiar to everyone. The main reason is that the current cell consistency is not good enough. It needs to be improved through balance. Just as there are no two identical leaves in the world, you can't find two identical cells. So in the final analysis, equalization is to solve the shortcomings of the battery cell and is a means to make up for it. Basically, it is the development and breakthrough of battery related technologies (such as group technology); Instead of always thinking about breakthroughs in equalization technology, how to improve equalization current and efficiency. The battery cell in the future does not need to be balanced, or even BMS, so I will lose my job.

What are the inconsistencies of the cell?

It mainly includes four points: SOC, internal resistance, self-discharge current and capacity. However, equalization can not completely solve these four differences. Equalization can only make up for the difference in SOC, and by the way, it can solve the problem of inconsistent self-discharge. But for internal resistance and capacity, balance is powerless.

So how is the inconsistency of the cell caused?

There are mainly two aspects: one is the inconsistency caused by the cell production and processing, and the other is the inconsistency caused by the cell use environment. The reason for the inconsistency in production comes from the processing technology, materials and other factors. It is relatively simple to say this, but the actual situation is very complex; It is easy to understand the inconsistency of the environment. Because each cell is located in a different position in the PACK, the environment must be different. For example, the temperature will be slightly different. After long-term accumulation, the cell will be inconsistent.

As mentioned earlier, equalization is used to eliminate the SOC difference of the cells. In an ideal state, it keeps the SOC of each cell the same at all times, so that all cells can reach the upper and lower voltage limits of charging and discharging synchronously, so that the usable capacity of the battery pack can be increased. There are two scenarios for SOC differences. One is that the cell capacity is the same, but the SOC is different; The second is that the SOC varies with the capacity of the cell.

The following figure is Scenario 1. The capacity of the cell is the same, but the SOC is different; The cell with the smallest SOC reaches the lower limit of discharge first (assuming 25% SOC is the lower limit), and the cell with the largest SOC reaches the upper limit of charge first; Under the effect of equalization, all cells maintain the same SOC for charging and discharging.

Equalization is more troublesome for cells with different capacities (Scenario 2), as shown in the figure below, the SOC is different with different cell capacities; In this way, the cell with the least capacity is the first to be fully charged and discharged; Under the effect of equalization, all cells maintain the same SOC for charging and discharging.

So equalization is a very important function for the current cell. There are two schemes to realize the balancing function: active balancing and passive balancing; Passive equalization is to discharge with resistance, and active equalization is to let the charge flow between the cores. In fact, there are some disputes about the names of these two, so we will not expand them; Among them, passive equalization is applied more in reality, while active equalization is less.

For passive equalization, how to determine the equalization current of BMS? To be irresponsible, the bigger the better, but considering the cost, heat dissipation and space, we need to make a compromise.

Before selecting the equalizing current, it is necessary to clarify whether the difference in SOC belongs to Scenario 1 or Scenario 2. At present, many situations are closer to Scenario 1: the capacity and SOC of the cell are almost the same at the beginning, but with the use, especially the self-discharge difference of the cell, the SOC of each cell is gradually different, so the equalizing ability should at least be able to eliminate the impact of the self-discharge difference. According to this, the following is the calculation formula of the equalizing current, which is very simple.

Image from liionbms

If the self-discharge of all cells is consistent, then there is no need for equalization; However, if the self-discharge current is different, the SOC difference will be caused. Equalization is to make up for the self-discharge current difference. In addition, since the average equalization time of each day is limited, and the self-discharge is continuous every day, the time factor should also be taken into account. The figure above is a diagram of the difference between the equalizing current and the self-discharge current according to the different equalizing time every day. So if we know the self-discharge rate of the cell, we can calculate a required equalizing current, but this calculation result is a lower limit, and the actual equalizing capacity must be greater than this value.

Summary:

In this paper, we will briefly introduce the causes and consequences of balancing. The calculation of passive balancing current is mentioned later. In fact, this is still a relatively ideal and rough value. The actual situation is more complex, but it is at least justified; Generally, the passive equalization current on BMS is about 100mA, and its origin is based on historical experience, which is a little unclear. The next article is going to discuss the circuit part involved in equalization in combination with AFE. All the above are for reference only.

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