A summary of several typical active equalization circuits

The single cell voltage is a direct measurement and can be measured online in real time, which makes it a favorable measure of system cell consistency. Not only that, in the common BMS management strategy, the condition of the single cell voltage is used as a trigger condition, there are discharge termination conditions, charge termination conditions, and the like.

We will hear such remarks for a long time. Japanese batteries are good, and domestic batteries are worse. The important point here is the consistency between the battery cells. For the battery life, the capacity is the most direct and most important parameter, so consistency is mainly directed to the capacity. Capacity is a parameter that can't be directly measured for a short time. According to experience, people have found that the capacity of a single cell has a one-to-one relationship with its open circuit voltage. Therefore, the vision of battery consistency in a system that has been loaded into a car eventually falls on the cell voltage.

A parameter in such a position, the difference in cell voltage uniformity is too large, it directly limits the battery pack charge and discharge power. Based on this, people use the battery equalization method to solve the problem of excessive battery cell voltage differences that are already in operation to increase the battery capacity. As a result, balance can be made to extend the cruising range and extend battery life. A picture in the literature illustrates the principle of active equilibrium. It can be seen from here that our balance is not very satisfactory, but only for the time being there is no better way.

We usually refer to the energy-consuming equilibrium as passive equilibrium, and other equilibrium as the active equilibrium. Although human intervention in the system is often not discussed theoretically, it is indispensable in practical applications. The single-cell charge equalization is a way to manually solve the inconsistency problem by charging the low-voltage cell.

There are many concrete implementation schemes for active equilibrium, which can be further divided into two categories: high-cut low-type and parallel-balanced type. It is generally questioned that active equalization affects battery life, and specifically refers to active balancing such as reducing fill-in. Several typical active equalization circuits are summarized below.

Cutting high fill-in is to transfer a part of the energy of the high-voltage cell to the low-voltage cell, thereby delaying the lowest cell voltage to reach the discharge. The cut-off threshold and the maximum cell voltage reach the charge termination threshold, and the effect of the system increasing charge and discharging power is obtained.

However, in this process, both the high-voltage and low-voltage cells are additionally charged. We all know that the battery life is called "cycle life." For this cell only, the extra charge and discharge burden will bring about a certain consumption of life, but for the battery pack system, the overall The last is to extend the system life or reduce the system life. At present, there is no clear proof of experimental data to prove.

The balancing of high and low fill-in includes capacitive equalization, inductive equalization, and transformer-type equalization. These three types of equalization methods include equalization in the charging process and equalization in the stationary process. There is also an active equilibrium, called parallel equilibrium, which only plays a role in the charging process.

Some people think that the equilibrium should be added during vehicle operation and at the end of the discharge process. However, it is generally believed that the fluctuation of the system current value is relatively large. If the balance is still based on the unit voltage, misjudgment is likely to occur and affect the equilibrium effect. Of course, with the development of technology, the SOC can be accurately estimated by other means, and the balance based on the SOC will no longer suffer from this problem.

Capacitive equalization

Let B1, B3 battery cells be the highest voltage and lowest cell in the group, respectively. In the figure, all the switches are normally open. When the equalizer issues an equalization command, the power switches S1 and Q2 are closed. At this time, the single battery B1 charges the capacitor, and the duty cycle of the power switch is controlled to control the charging power and time. After the end, the switches S3 and Q4 are closed, and the capacitor charges the single cell B3. At this time, the unbalance in the battery pack is reduced, and the equalization ends.

Inductive balance

During the charging process, the switch S is closed and the charger charges the battery pack. At this point, the switch on the right side of the battery pack is all disconnected, and the equalization system does not turn on Assuming that the voltage of the single cell battery B1 is significantly higher than that of other cells and reaches the equilibrium threshold, the equalization system is turned on at this time, the S1 and Q2 switch tubes are closed, and the inductance is connected in parallel with the single cell battery B1 to function as a shunt, and the inductor is stored from the charger. With the energy of the battery B1; When the S1, Q2 switch is set to 0, Q3, S4 switch is set to 1, the inductor to the charging process of the single battery B3 release a certain amount of energy.

In the process of standing, the switch S is disconnected. When the voltage of the cell B1 is higher than other cells and reaches the equilibrium threshold, the equalization system is turned on, the switches of S1 and Q2 are closed, the inductance is connected in parallel with the cell B1, and the inductor absorbs energy of B1. When the S1 and Q2 switches are off and the Q3 and S4 switches are closed, the inductor releases power to the cell B3.

Transformer balanced

The design of the parameters based on the flyback balanced transformer means that the transformer acts both as an energy source for absorption and a source for energy release. The conversion of absorbed and released energy lies in the conversion of energy between magnetic energy and electrical energy.

Similarly, suppose that the voltage of the single-cell B1 is the highest, S1 and Q2 are set to 1, and other switches are set to 0. At this time, the transformer is used as the energy source for absorption. The energy is converted from the energy given by the B1 battery to magnetic energy; S1 and Q2 are set to 0, Q1. S2 is set to 1, energy is transferred from the primary winding to the secondary winding, energy is released to the cell B3, and energy is converted back to energy by magnetic energy.

Parallel equalization

The ideal equalization method is that all battery energy and terminal voltage are the same, and the voltages of the individual cells in the parallel battery pack are always equal, because the principle of the communicator is the same, the water columns on both sides are always horizontal, and the parallel battery is also inherently high in the spontaneous monomer voltage. Low body voltage battery charging. However, if you want to apply this principle in a series battery pack, you need to slightly change the original battery pack topology.

As shown in the parallel topology shown below, each cell has a single-pole, double-throw switch relay, so n+1 relays are required in a n-series battery pack.

The control principle is as follows: Let B4 voltage in the battery pack be the highest, B2 voltage be the lowest, and the control relays S5, S3, Q4, and Q2 should be closed. At this time, two single battery cells are connected in parallel, and the two battery cells are automatically balanced and the voltage tends to be consistent. The disadvantage of this topology is that it cannot be balanced during the charging process. It can only be paralleled and balanced when it is depolarized.

Parallel equalization also has multiple forms of implementation. In addition to the above, we also see the scheme shown in the following figure:

In parallel, the balance is that, in the charging process, the charging current is shunted, and the battery with low voltage is charged more, while the voltage with higher charging is less. As a result, there is no need for a â€œhijacking wealthâ€ program, avoiding the extra charge and discharge burdens of the highest and lowest voltage batteries, and there is no doubt that the impact of the equalization process on the life of individual batteries will drag down the system life.

Balance between modules

This kind of form is rare in practical application, but the module blueprint that the chip supplier offers has already appeared the function that the adjacent module can mutually balance. A schematic is as follows.

Comparison of several balanced methods

Active equilibrium choice

There are industry insights that summarize a set of methods for choosing a balanced approach based on your own engineering experience:

â¶ For battery packs up to 10AH, the use of energy-consumption type may be a better choice and the control is simple.

For a few tens of AH battery packs, it is advisable to use a one-to-many flyback transformer in combination with the battery sampling section to do battery equalization.

â¸ For hundreds of AH battery packs, it may be better to use a separate charging module because hundreds of AH batteries have more than 10 A of equalizing current. If the number of series sections is more, the equalizing power is large. It may be safer to use external dc-dc or ac-dc equalization outside the lead to the battery.

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