602030 lipo battery

Introduction to the BMS balancing function of power 602030 lipo battery

Due to the differences in manufacturing and use processes, power lithium battery monomers are naturally inconsistent. The inconsistency is mainly manifested in monomer capacity, internal resistance, self-discharge rate, charge and discharge efficiency, etc. The inconsistency of monomers is transmitted to the power lithium battery pack, which inevitably leads to the loss of power lithium battery capacity, and then causes a decrease in life. Studies have shown that a 20% capacity difference in a single cell will lead to a 40% capacity loss in the battery pack.

The inconsistency of battery monomers will further deteriorate over time under the influence of random factors such as temperature and vibration conditions, causing the parameters to move towards the discrete direction without hesitation. Just like the world is always moving towards the direction of entropy increase. The trend cannot be reversed, but it can be intervened to reduce its deterioration rate. One of the methods is to perform balancing on the battery cell through the battery management system.

1 Triggering balancing

The industry has long recognized the importance of balancing. The research on battery balancing has a long history, and the methods and conclusions obtained are also diverse.

1.1 Triggering parameters

The first problem faced by balancing is under what conditions to start the system balancing function. There are two common routes. One is to use the single-cell voltage as the monitoring target. When the single-cell voltage difference enters a certain range, the balancing begins to play a role. The other route is to use SOC as the target. It is believed that SOC is the parameter that truly reflects the needs of the battery cell. When the difference between the single-cell SOC and the average SOC reaches a certain value, the balancing process is triggered.

In fact, SOC is a more comprehensive parameter. If the calculation is reasonable and accurate, it can cover the influence of the single-cell voltage. However, if SOC is used as the target parameter, the system design must include the collection and calculation of the SOC-related data of each series battery cell.

1.2 What state can be balanced

Another question is in what process the balancing is performed. Does it start balancing as long as the parameter threshold is reached regardless of the process, or is it artificially stipulated that the balancing only occurs during the charging process, the discharging process, or the static process when the battery has no working tasks.

The views on this issue are not very consistent, and each management system has different settings. I think the balancing process should be designed in any process, but it is necessary to consider whether it is most beneficial to the battery pack.

Balance at the end of charging. After the highest cell voltage reaches the charging cut-off voltage, the system starts the balance function and discharges part of the power of the cell with the highest voltage, so that the system can further charge more power, or let the high-power cell charge the lowest-power cell. The ideal state is that all cells reach the cut-off voltage at the same time.

Balance at the end of the discharge process. When the lowest cell voltage has reached the discharge cut-off voltage, the system starts the balance. After the lowest voltage disappears, the system can still run for a distance.

There are two problems in this process. On the one hand, only if the system is equipped with an active balance function can the purpose of continuing to drive for a distance be achieved. If there is only passive balance, the high power is discharged, which can only play the purpose of removing the accumulated energy gap of the cell; then, another problem is that even if all cells return to the same starting line at the end of discharge, due to the capacity difference between the cells, charging balance may still need to be performed when the charging ends.

Balance during vehicle operation. One problem here is that due to the different current sizes and the influence of different system internal resistance sizes, it is often difficult to obtain accurate values for dynamic SOC and cell voltage, which may be very unfavorable for balance during operation.

2 Balancing strategy

2.1 Concept

What is passive balancing

Passive balancing uses resistors to consume the energy of high-voltage or high-charge cells to reduce the gap between different cells. It is a kind of energy consumption.

What is active balancing

Active balancing uses energy storage devices to transfer part of the energy of cells with more energy to cells with less energy. It is energy transfer.

Some experts believe that the above two statements should correspond to dissipative balancing and non-dissipative balancing. Whether it is active or passive should depend on the event that triggers the balancing process. When the system reaches that state, it is passive. If it is set manually, the balancing program is set when it can be unbalanced, which is called active balancing.

For example, at the end of discharge, the cell with the lowest voltage has reached the discharge cut-off voltage, while other cells still have electricity. At this time, in order to discharge as much electricity as possible, the system transfers part of the electricity of the high-energy cell to the low-energy cell, so that the discharge process continues until all the electricity is discharged. This is a passive balancing process. If the system predicts that there will be imbalance when the discharge is terminated when the power is 40%, then it starts the balancing process. This is active balancing.

I recently saw this part of the content and put it here for reference.

2.2 Balancing control strategy

Among the current balancing control strategies, some use single cell voltage as the control target parameter, and some people propose that SOC should be used as the balancing control target parameter. Let's put aside the discussion of which of the two control targets is better, and give an example to illustrate the general form of the balancing strategy.

Take single cell voltage as an example. Set the trigger threshold of balancing control, such as the difference between the extreme value and the average value reaches 50mV to start the balancing process, and 5mV to end the balancing. The management system collects the voltage of each single cell terminal according to a fixed collection cycle, first calculates the average value, and then calculates the difference between the voltage of each battery cell and the voltage mean, and the battery cell numbers are arranged according to the difference. The difference is compared with the set threshold. If the largest difference is within the threshold range, the balancing program is triggered. The subsequent strategy is related to the specific balancing implementation form.

3 Overview of balancing hardware

3.1 Based on transformer

The primary side of the transformer with more turns is connected in parallel to the total positive and negative of the entire battery pack, and the secondary side with fewer turns can be connected in parallel to any battery cell through the switching of the switch. The transformer uses mutual inductance to transfer energy between the primary and secondary sides.

The balancing process is generally like this. The secondary side is first connected in parallel to the high-energy battery cell, and the energy is transferred to the primary side to form the terminal voltage of the primary side, which is loaded on the entire battery pack to charge the entire battery pack; the secondary side is connected in parallel to the low-energy battery cell, and through the transformation ratio, a voltage higher than the terminal voltage of the low-energy battery cell is obtained to charge the battery cell.

3.2 Based on bidirectional DCDC

The approach proposed in some literature is to make a difference between the SOC of each battery and the average SOC, and line up the batteries according to the difference. According to the principle of one-to-one and one-to-red, the maximum positive difference and the maximum absolute negative difference are paired, and the high-voltage battery cell charges the low-voltage battery cell through low-voltage DCDC. In the same way, all cells with a difference exceeding a certain limit are traversed until a battery that does not need to be matched is encountered.

3.3 Based on inductance

The basic idea is to temporarily store the energy of the high-energy cell in the inductor, and when the circuit switch changes position, the inductor and the low-energy cell are connected to form a loop, and then the energy in the inductor is put into the low-energy cell.

A specific example. Compare the terminal voltages of two adjacent batteries A and B, A is high and B is low; the balancing circuit first connects the inductor to A for a short time, charges part of the energy into the inductor, and disconnects; then the inductor and B form a loop, and the inductor charges B. Energy can only be transferred between adjacent cells through inductance, but the first and last cells in a string of cells can also achieve energy transfer in this way, thus forming a closed loop of energy transfer. After multiple comparisons and transfers, the single cell voltage in the system can theoretically be balanced.

3.4 Based on capacitors

Similar to the basic idea of applying inductors, it is also to try to temporarily store part of the energy of high-energy cells in capacitors, and transfer the energy to low-energy cells by configuring switch circuits.

There are generally three ways to apply capacitors: multi-capacitor balancing, single-capacitor balancing, and double-layer capacitor balancing.

The principle of multi-capacitor balancing is similar to that of single-capacitor balancing. The difference is that in multi-capacitor circuits, the capacitor only switches between two nearby batteries, while single-capacitor balancing uses different on-off combinations of switches so that the capacitor can be connected in parallel at both ends of any cell.

Connect a capacitor in parallel to both ends of a high-energy cell, and part of the energy is transferred to the capacitor in the form of charging. When the cell and capacitor voltages are balanced, the switch is disconnected, and the capacitor is transferred to both ends of the low-energy cell. When the cell and capacitor voltages are balanced, the process is repeated. The cell itself has internal resistance, and the power supply potential for charging the cell must be slightly higher than the cell. After several transfers, when the capacitor is finally connected in parallel with the low-energy cell, it is found that it can no longer charge the cell because the voltage difference is not enough. At this time, the balancing process is declared over.

Double-layer capacitor balancing is to add a capacitor connected in parallel to the two ends of the entire series battery pack on the basis of multiple capacitors, so that the energy transfer between the first and last sections of a string of cells becomes possible, and the balancing efficiency is also improved.

3.5 Based on resistance

Connecting resistors in parallel to both ends of the cell allows the resistors to consume part of the battery energy, which is the passive balancing method mentioned earlier.

There are two forms of parallel resistors. One is a fixed connection. The resistor is connected in parallel to both ends of the battery for a long time. When the cell voltage is high, the current passing through the resistor is large and the power consumption is large. When the battery voltage is low, the resistor consumes less power. The voltage at the battery end is balanced through the voltage-sensitive characteristic of the resistor. This is a theoretically feasible method, but it is rarely used in practice.

Another parallel resistor method is to connect the resistor in parallel to both ends of the cell through a switch circuit. The switch is triggered by the management system signal. When the system determines which cell voltage or SOC is high, its parallel resistor is connected to consume its energy.

4 Limitations of balancing

Passive balancing, the current cannot be fully adjusted according to actual needs, because the energy consumed by the resistor is converted into heat, which will have adverse effects on the battery management system and the battery pack;

Active balancing requires the configuration of corresponding circuits and energy storage devices, which are large in size and costly. These two conditions together determine that active balancing is not easy to promote and apply.

Each charging and discharging process of the battery pack is accompanied by an additional charging and discharging process of a part of the battery, which invisibly increases the number of battery cycles. Regarding the battery cells that need to be charged and discharged to achieve balancing, whether the additional workload causes them to age beyond the general battery cells, and thus causes a greater performance gap with other battery cells, no research has made a clear judgment. The first reason is that the instability of the material precision and purity determined by the basic industrial level leads to inconsistent performance of the final product. Cell monomers produced using different batches of positive electrodes, negative electrodes and electrolytes are generally not mixed. Even if the parameters in the sorting process are very consistent, the sorting methods basically cannot reflect the state of the battery cells after a period of use in the future, so the current treatment method is to prevent mixed use.

Another reason is the process consistency problem in the production process of the battery cells. The production process of battery cells is relatively complicated, and the general process is as follows.

In the whole process, the consistency of each step is very important, but the most difficult to ensure consistency is the coating process. The coating thickness and uniformity and material activity are not easy to strictly control by mechanical means, and are important processes that cause monomer differences. Differences in the manufacturing process can only be compensated in the sorting process.

1.2 Use process

Cyclic use process

The position of the monomer in the entire battery pack is not the same. The monomer wrapped in the center of the module and the monomer in the outermost layer of the module have huge differences in heat dissipation conditions;

The relative position with the module collector copper busbar is also unlikely to cause inconsistency in the thermal environment of the monomer. The copper busbar is a good conductor of heat and has a higher heat dissipation capacity than the battery cell. The different positions of the battery cell relative to the collector copper busbar will cause different heat dissipation conditions between each other.

Studies have shown that during the working process, the inconsistency of temperature will have the most significant impact on the inconsistency of the battery cell, causing the battery cell to go from inconsistency to greater inconsistency.

The superposition of different thermal environments leads to differences in the working temperature conditions of the monomer. High-temperature operation causes the battery to deteriorate, and the internal resistance after degradation increases, which in turn increases the temperature rise of the battery cell. The difference in thermal environment is the beginning of this negative feedback.

Stationary process

During the static state during use, imagine that the single body is in an electric car. In the parking state, all power on the car is cut off, including the thermal management system of the battery pack (if the battery pack originally has a thermal management system), and the battery pack is in a natural temperature field. What affects it is the relative position of the battery cell, which causes different thermal environments.

Each battery cell is at a different distance from the battery pack shell, and the degree of influence by the external temperature change will be different. Before reaching thermal equilibrium, the temperature conditions of different batteries are different.

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