Active equalization of power lithium battery -Lithium - Ion Battery Equipment

Active equalization method for power lithium battery system -Lithium - Ion Battery Equipment

We will hear such remarks for a long time. The battery in Japan is good, but the battery in China is poor. The important point here is the consistency between battery cells. For vehicle endurance, capacity is the most direct and important parameter, so consistency mainly refers to capacity. Capacity is a parameter that cannot be directly measured in a short time. According to experience, it is found that the capacity of a single electric core has a one-to-one relationship with its open circuit voltage. Therefore, the focus on the battery consistency of the system that has been installed and operated is finally on the cell voltage.

Single voltage is a direct measurement value, which can be measured online in real time, which makes it a favorable condition to measure the consistency level of the system core. Not only that, in common BMS management strategies, there are discharge termination conditions, charging termination conditions, etc. when the monomer voltage value is used as the trigger condition. For a parameter in such a position, the consistency difference of individual voltage is too large, which directly limits the charging and discharging capacity of the battery pack. Based on this, people use the battery equalization method to solve the problem that the voltage difference of single battery pack in operation is too large, so as to improve the capacity of the battery pack. Thus, we can infer that the equalization method extends the range and battery life. A picture in the literature vividly illustrates the principle of active equalization. It can be seen from this that our balance is not very ideal, but there is no better way for the time being.(Lithium - Ion Battery Equipment)

We usually call energy consuming equilibrium passive equilibrium, and other equilibrium active equilibrium. While human intervention on the system is often not discussed in theory, it is indispensable in practical application. Single charging equalization is a way to solve the inconsistency problem by manually charging low voltage cells separately. There are many specific implementation schemes of active equalization, which can be further divided into two categories, namely, cut high to fill low type and parallel equalization type. It is usually questioned that active equalization affects the battery life, especially such active equalization as cutting high and filling low. Several typical active equalization circuits are summarized below.

Cutting high and filling low is to transfer part of the energy of the already high voltage cell to the low voltage cell, thus delaying the lowest single voltage touch discharge. The cut-off threshold and the time when the highest monomer voltage touches the charging termination threshold can obtain the effect that the system can increase the charging and discharging power. However, in this process, both the high voltage unit and the low voltage unit are additionally charged and discharged. As we all know, the battery life is called "cycle life". Just for this cell, it is certain that the extra charge and discharge burden will lead to the consumption of life. However, for the battery pack system, in general, whether the system life is extended or reduced has not been proved by clear experimental data.

The equalization of cutting high and filling low includes capacitive equalization, inductive equalization and transformer equalization. These three equalization methods include equalization during charging and equalization during quiescence. There is also an active equalization, called parallel equalization, which only plays a role in the charging process. Some people also think that equalization should be added at the end of the vehicle operation and discharge process. However, it is generally believed that the fluctuation of the system current value is relatively large. If the equalization is still carried out based on the unit voltage, it is likely to lead to miscalculation and affect the equalization effect. Of course, with the development of technology, SOC can be accurately calculated directly by other means, and the balance based on SOC will not be troubled by this problem.

Capacitive equalization

Set B1 and B3 cell as the highest and lowest cell in the group. All switch tubes in the figure are normally open. When the equalizer sends an equalization command, power switch tubes S1 and Q2 are closed. At this time, the single battery B1 charges the capacitor, and controls the duty cycle of the power switch tube to control the charging power and time. After charging, switch tubes S3 and Q4 are closed, and the capacitor charges the single battery B3. At this time, the imbalance in the battery pack decreases, and the equalization is over.

Inductive equalization

During charging, switch tube S is closed and the charger charges the battery pack. At this time, all switch tubes on the right side of the battery pack are disconnected, and the equalization system is not turned on. When the voltage of single battery B1 starts to be significantly higher than that of other batteries and reaches the equalization threshold, the equalization system is turned on, S1 and Q2 switch tubes are closed, and the inductance is connected in parallel with single battery B1, which plays the role of shunt. The inductance stores the energy from the charger and battery B1; When S1 and Q2 switch tubes are set to 0 and Q3 and S4 switch tubes are set to 1, the inductance will release certain energy to the single battery B3 during the charging process.

During the quiescent process, the switch tube S is disconnected. When the voltage of single battery B1 is higher than that of other batteries and reaches the equalization threshold, the equalization system is opened, S1 and Q2 switch tubes are closed, and the inductance is connected in parallel with single battery B1, and the inductance absorbs B1 energy; When S1 and Q2 switch tubes are disconnected and Q3 and S4 switch tubes are closed, the inductance will release electricity to the single battery B3.

Transformer type equalization

The parameter design is based on the flyback equalizing transformer, that is, the transformer is used as both the absorption energy source and the release energy source. The conversion of absorption and release energy lies in the conversion of energy between magnetic energy and electric energy.

Similarly, if the voltage of single battery B1 is the highest, set S1 and Q2 to 1, and set other switch tubes to 0. At this time, the transformer is used as the energy absorption source, and the energy is converted from electrical energy to magnetic energy from B1 battery; S1 and Q2 are set to 0, and Q1 and S2 are set to 1. The energy is transferred from the primary winding to the secondary winding, and released to the single battery B3. The energy is converted from magnetic energy to electrical energy again.

Parallel equalization

The ideal equalization mode is that all batteries have the same energy and terminal voltage, and the single battery voltage in the parallel battery pack is always the same. Because, like the principle of the connector, the water column on both sides is always horizontal, and the parallel battery also naturally charges the battery with low single voltage with high single voltage. However, if you want to apply this principle in the series battery pack, you need to slightly change the topology of the primary battery pack.

As shown in the figure below, each single battery has a single pole double throw switching relay, so n+1 relays are required in n series battery packs.

The control principle is as follows: set B4 voltage in the battery pack to be the highest and B2 voltage to be the lowest, and control relays S5, S3, Q4 and Q2 are closed. At this time, two single batteries are connected in parallel, and the two single batteries are automatically balanced, and the voltage tends to be the same. The disadvantage of this topology is that it cannot be equalized during charging, and it can only be equalized in parallel when depolarizing.

Parallel equalization can also be realized in many forms. In addition to the above, we also see the scheme shown in the following figure:

In general, parallel equalization means that during the charging process, the charging current is shunted to charge more cells with low voltage and less cells with high voltage. Therefore, the process of "robbing the rich to help the poor" does not need to occur, which avoids the extra charge and discharge burden of the highest and lowest voltage cores, and it is unnecessary to doubt that the life of individual cores is affected by the equalization process, which will drag down the system life.

Balance between modules

This form is rare in practical applications, but the adjacent modules can balance each other in the scheme blueprint provided by chip suppliers. A schematic diagram is shown below.

Comparison of several equalization methods

Selection of active equalization

According to the understanding of the industry, a set of methods for selecting the balanced mode is summarized based on their own engineering experience:

1) For the battery pack within 10AH, the energy consumption type may be a better choice with simple control.

2) For dozens of AH battery packs, it should be feasible to use one driving many flyback transformers and combine the battery sampling part to do battery equalization.

3) For hundreds of AH battery packs, it may be better to use an independent charging module, because for hundreds of AH batteries, the equalizing current is more than 10 A. If the number of series connections is more, the equalizing power is large, and the lead is connected to the battery, it may be safer to use external DC-DC or AC-DC for equalizing.

This paper is organized by "Power Battery Technology" from Liu Xiugang's article "Research on Active Equalization Strategy in the Charging Process of Power Battery Pack", Qiu Shi's article "Research on Active Equalization System of Lithium Iron Phosphate Power Battery Pack", Lv Wenqiang's article "Research on Coordinated Control Strategy of Balanced Charging of Power Battery Pack", electric vehicle resource network, new electronic TomanMicro Electronics, and other pictures are from Internet public materials. It is only used for learning and communication. Please indicate the source for forwarding.



Contact Us

24 hours online service