lithuim ion battery 18650

　　Overview of lithuim ion battery 18650 Balancing Method

　　In systems where batteries are used as energy storage units, since battery cells often have relatively low capacity and cannot meet the requirements of large-capacity systems, battery cells need to be connected in series to form a battery pack to increase the supply voltage and storage capacity. For example, in Most fields such as electric vehicles and microgrid systems require batteries to be connected in series. Due to the manufacturing process of the battery cells themselves, there are differences between different cells such as electrolyte density, electrode equivalent resistance, etc. These differences lead to the same charge and discharge current of each cell in the series battery pack. The capacity of each cell is different, which affects the operation of the entire battery pack. In the worst case, in a battery pack, the remaining capacity of one cell is close to 100%, and the remaining capacity of another cell is 0. Then the battery pack can neither be charged nor discharged, and cannot be used at all. Therefore, balancing the lithuim ion battery 18650 is very important, especially when a large number of battery cells are connected in series.

　　The lithuim ion battery 18650 balancing methods mainly include resistance consumption balancing method, switched capacitor method, bidirectional DC-DC converter method, multi-winding transformer method, multi-module switching balancing method, switched inductance method, etc.

　　1. Resistance consumption balancing method

　　The resistance consumption balancing method is to use the resistance connected to the battery cell to release energy higher than that of other cells to achieve the balance of each cell, as shown in Figure 1. Each battery cell is connected to a resistor through a triode, and the battery cell discharges the resistor by controlling the on and off of the triode. This structure has simple control, fast discharge speed, and can discharge multiple cells at the same time. But the shortcomings are also obvious. It consumes a lot of energy. It can only discharge the cells but not charge them. Moreover, other battery cells must be based on the lowest cell as the standard to achieve balance, which results in low efficiency.

　　2. Switched capacitor method

　　The switched capacitor method is to connect a capacitor in parallel with a switching device between every two adjacent batteries, as shown in Figure 2. Energy transfer between two adjacent batteries is achieved by controlling the duty cycle of the switching device drive signal pWM. For example, if the battery cell capacity B1 is higher than B2, when G1 is turned on and G2 is turned off, capacitor C1 and battery cell B1 are connected in parallel, and B1 transfers energy to C1; when G1 is turned off and G2 is turned on, capacitor C1 and battery cell B2 are connected in parallel. C1 transfers energy to B2, completing the energy transfer within this cycle. By analogy, by controlling the opening and closing of switching devices, capacitors are used to transfer energy one by one.

　　This circuit can be equivalent to the circuit shown in Figure 3. An equivalent resistor is connected between each two battery cells, and the equivalent resistance value can be derived as given in equation 1. In this method, the energy is transferred one by one, so the equilibrium time is long. According to equation 1, the equivalent resistance can be adjusted by changing the switching frequency of the switching device and the capacitance value, and the charge and discharge current can be changed.

　　In the formula: f is the switching frequency; t=RC; D is the duty cycle.

　　The switched capacitor method has simple control and can achieve charging and discharging balance, but because it transfers energy step by step, the balancing speed is slow.

　　3. Bidirectional DC-DC converter method

　　In this method, each battery cell is connected to a bidirectional DCDC converter and then connected in series, as shown in Figure 4. Since the voltage level of the battery cell is relatively low, the battery cell is generally used as the low-voltage side. When charging the battery pack, according to the control strategy in Figure 5, constant voltage charging of each battery cell can be achieved. If the voltage outer loop of this control strategy is opened, constant current charge and discharge control can be performed according to the need for balancing. During discharge, if the connected load is heavy, the inductors of some bidirectional DC-DC converters may operate in an intermittent state.

　　This balancing method can charge and discharge all battery cells at the same time, and control the charge and discharge current according to the capacity of different battery cells. This method has flexible control and short charge and discharge equalization time. However, since each battery cell requires a bidirectional DC-DC converter, the cost is higher.

　　4. Multi-winding transformer equalization method

　　The multi-winding transformer method is to connect each battery cell to a secondary side of the transformer, as shown in Figure 6. When balancing the voltage of the battery pack, the secondary voltage of the control transformer is first higher than the lowest battery cell. At this time, the diode in this cell circuit is turned on, and the diodes connected to other cells are turned off due to the back pressure. Charge the battery cell with the lowest voltage. When this cell reaches the second to last highest voltage, then increase the secondary voltage and charge the two lowest cells. Continue in this way. The charging voltage is shown in Figure 7. .

　　The multi-winding transformer with this charging method is complex in design and expensive. The number of windings needs to be changed according to the number of battery cells. It is not easy to expand the battery pack and can only be balanced by charging the battery cells.

　　Multi-module switch selection equalization method#e#

　　5. Multi-module switch selection equalization method

　　The structure of this method is shown in Figure 8. Since there are a large number of series battery cells, these cells can be divided into M modules, each module having K cells. Each battery cell has a set of switches connected to the bidirectional DC-DC converter. The switch consists of two MOSFETs connected in reverse series. When the cell is not selected for charging and discharging, the control chip controls the corresponding MOSFET to turn off, and the cell When the controller selects a cell to charge when it is disconnected from the converter, it turns on the corresponding optocoupler through the control chip, turns on the MOSFET, and connects the battery cell to the DC-DC converter, as shown in the figure 9 shown.

　　This method can charge and discharge any cell individually, and the charge and discharge current can be controlled. However, it can only charge one cell at a time, so the charge and discharge equilibrium time of the entire battery pack is long, especially when the number of cells is large. in the case of.

　　6. Switched inductance method

　　The switched inductor method is to connect two adjacent battery cells to an inductor through a MOSFET, as shown in Figure 10. If the cell capacity B1 is greater than B2, first turn on the switch Q1 and turn off the switch Q2, and B1 will supply the inductor. L1 is charged, then Q1 is disconnected and Q2 is closed. At this time, the inductor releases the stored energy to B2. In order to ensure that Q1 and Q2 are not turned on at the same time, a dead zone is added. During the dead zone time, the inductor L1 continues to flow through B2 and D2. . At the same time, B2 can also transfer energy to B3, and can also realize the flow of energy in the opposite direction until all battery cells have the same capacity.

　　Figure 10 Circuit structure diagram of switched inductance method

　　The switched inductance method can realize the simultaneous transfer of energy between adjacent battery cells and reduce the balancing time. For N battery cells, 2N-2 MOSFETs and N-1 inductors are required.

　　7.Conclusion

　　The balance of the capacity of each cell in the battery pack plays a very important role in the working efficiency and safety of the series battery pack. Long-term imbalance will shorten the life of the entire battery pack and seriously affect the work of the entire system. This article introduces the working principles, advantages and disadvantages of various battery balancing methods. From this we can see that no method is perfect. It needs to be comprehensively considered according to the application situation, balancing time, number of series connections, cost and other factors for practical application. choose.

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