Review of Lithium Iron Phosphate Battery Balancing Technology

　　0 Preface

　　Large-scale energy storage power stations generally have large design capacities and require multiple battery cells to be connected in series and parallel to meet design requirements. Taking lithium iron phosphate batteries as an example, the operating voltage range of a single cell is usually about 2.8~4V. If each battery cell is 200Ah and the rated voltage is 3.2V, it needs to reach a capacity of 2.4MWh. 252 batteries can be The cells are connected in series to form a battery pack, and then 15 battery packs are connected in parallel, then: 3.2V × 252 cells x 200Ah

　　During the mass production of battery cells, due to fluctuations in raw materials and production processes, the capacity, internal resistance, voltage and self-discharge rate of the battery cells will have certain deviations. At the same time, as the number of charge and discharge cycles increases during the use of the battery cells, Due to the influence of storage time, temperature, etc., the capacity attenuation of battery cells will also be inconsistent, resulting in inconsistent battery cells in the same battery pack. In large-scale energy storage, the imbalance of the battery pack is one of the main reasons that affects the performance of the battery pack and reduces the life of the battery pack.

　　1Common concepts of large-scale energy storage

　　Battery capacity refers to the amount of electricity released by the battery under certain conditions (discharge rate, temperature, discharge cut-off voltage, etc.), represented by the letter c, and the unit is ampere-hour (Ah). According to QB/T2502-2000 "General Specification for Li-ion Batteries", the rated capacity of the battery is the capacity when it is discharged to the termination voltage at a rate of 0.2C when the ambient temperature is (20±5)°C.

　　The internal resistance of the battery is divided into ohmic internal resistance and polarization internal resistance. The ohmic internal resistance is composed of electrode materials, electrolyte, diaphragm resistance and contact resistance of various parts. The ohmic resistance does not change with the frequency of the excitation signal. In the same charge and discharge cycle, Ohmic resistance is almost unchanged except for the influence of temperature rise. Polarization internal resistance refers to the resistance caused by polarization during electrochemical reactions, including resistance caused by electrochemical polarization and concentration polarization. Internal resistance is one of the most important characteristic parameters of a battery. It is an important parameter that characterizes battery life and battery operating status. It is also a major indicator of how easily electrons and ions are transported within the electrode.

　　The working voltage of the battery refers to the voltage measured between the positive and negative electrodes after the battery is connected to the load. When discharging, the battery's operating voltage is lower than the open circuit voltage due to polarization and internal resistance. During charging, the operating voltage is higher than the open circuit voltage and rises as charging proceeds until it is fully charged.

　　State of charge SOC (state of charge): the ratio of the remaining battery capacity to the rated capacity, commonly expressed as a percentage.

　　The inconsistency of battery packs refers to the certain differences in parameters such as voltage, internal resistance, and charge amount when batteries of the same specification and model are connected in series or parallel. According to the way inconsistency affects battery pack performance and the reasons for the expansion of inconsistency during use, battery inconsistency can be divided into capacity inconsistency, resistance inconsistency and voltage inconsistency.

　　2 Factors affecting battery consistency and mitigation measures

　　2.1 Factors affecting battery consistency

　　The reasons for the inconsistency of lithium iron phosphate batteries in group applications are multi-faceted, mainly due to inconsistencies in processes and materials during production, and secondly due to inconsistencies in the operating environment.

　　The first is the inconsistency caused during production, which mainly refers to the initial capacity, DC internal resistance, self-discharge phenomenon and charge-discharge efficiency between batteries due to process problems and uneven materials during the production process. has a difference. For example, the quality, thickness, area, and smear thickness of lithium battery electrode plates are not exactly the same; the welding quality of the electrode group directly affects the ohmic resistance consistency of the lithium-ion battery; the amount of impurities in the battery determines the self-discharge rate of the battery. ;The quality, density and injection volume of the electrolyte have a great influence on the battery capacity.

　　2.2 Analysis of reasons for the expansion of battery pack inconsistency during operation

　　During use, the differences in the initial performance parameters of the battery accumulate and amplify during use, mainly in capacity and internal resistance.

　　(1) The capacity decay speed is different, resulting in different available capacities.

　　Due to the different ability of each single cell in the battery pack to absorb current, the Coulombic efficiency of each single cell is not exactly the same during the charge and discharge cycle, resulting in a gradual difference in the available capacity of the battery. When there is a certain difference in capacity between individual batteries, the battery with a small capacity is filled first, but the charging process of the battery pack is not over at this time. The capacity of the small-capacity battery will continue to decrease due to overcharging. During the discharge process, the small-capacity battery will be discharged first. Since the battery pack is still continuing to discharge, the small-capacity battery will be over-discharged, further reducing its capacity. This inconsistency will become more serious after multiple charge and discharge cycles, and may even have a serious impact on the cycle life of the battery.

　　(2) Inconsistency in battery internal resistance results in different operating temperatures and different discharge depths.

　　For battery strings connected in series, the currents during the discharge process are equal and the internal resistance of the battery is large. The energy loss is large, the heat is generated, and the temperature rises quickly, which accelerates the chemical reaction rate. The continued rise in temperature will cause serious battery deformation or explosion. as a result of. During the charging process, a battery cell with a larger internal resistance is assigned a higher charging voltage. Compared with other batteries, it will reach the preset charging cut-off voltage earlier. At this time, in order to prevent the cell from overcharging, the energy management system will Stop charging the entire group, and after multiple cycles, the inconsistency expands.

　　For parallel-connected battery strings, during the discharge process, the discharge current is inversely proportional to the internal resistance. Therefore, the discharge current is different, and the energy released by the battery is different, making the battery discharge depth under the same working conditions different.

　　2.3 Relevant standard requirements

　　At present, there are no corresponding standards for the consistency testing methods and specifications of large-scale energy storage batteries. Only the inconsistency testing and analysis methods are clarified in QC/T43-2006 "Lithium-ion Batteries for Electric Vehicles", that is, based on simple simulation conditions Test data analysis of battery module consistency.

　　This standard stipulates that each batch of products should be randomly sampled for factory inspection before leaving the factory. For the 20°C discharge performance inspection item of the factory inspection, the 3h rate discharge capacity difference of all battery samples should not be greater than ±5%. BYD batteries used in wind and solar energy storage demonstration power stations have capacity inconsistency controlled within ±2% before leaving the factory.

　　2.4 Measures to alleviate battery inconsistency

　　Battery manufacturers mainly take the following measures to ensure the consistency of battery packs:

　　(1) Before the batteries leave the factory, on the one hand, the process consistency level is improved, and on the other hand, the battery cells to be grouped are screened based on voltage and internal resistance to enhance matching.

　　(2) Strengthen maintenance during use, measure battery cell voltage regularly, adjust and replace cells with abnormal voltage in a timely manner, and charge batteries with low voltage in voltage measurement separately to restore their performance.

　　(3) Avoid overcharging and deep discharge of the battery. When the SOC of lithium iron phosphate battery is less than 10% or greater than 90%, the voltage change rate is large and it is easy to lose control.

　　(4) Install an energy balancing system on the battery pack to intelligently manage the charge and discharge of the battery pack.

　　In summary, there are many objective factors that affect the inconsistency of lithium batteries, which are unavoidable both in production and use. The impact of inconsistency on the lifetime of the entire group is an important factor affecting the economics of scale energy storage. This paper studies the balanced system of measure (4).

　　3 Balanced circuit topology

　　The battery pack balancing circuit refers to equipping the battery pack with an additional circuit and control management system to ensure that the state of charge of each single cell in the battery pack is the same, and to prevent overcharge and over-discharge of the battery pack during use. Performance is not compromised.

　　Currently, there are two types of commonly used balancing circuits for lithium iron phosphate batteries: energy dissipative circuits and non-energy dissipative circuits. Energy dissipation circuits are relatively simple. Non-energy dissipation circuits are divided into two types: one is composed of energy storage components (inductors or capacitors) and control switches, and the other mainly applies DC-DC conversion technology to control the inductor. Energy storage components such as capacitors and capacitors realize energy transition and achieve the purpose of recharging or discharging battery cells.

　　3.1 Energy dissipation type

　　The basic structure of the energy dissipation balancing circuit is shown in Figure 1. Batteries B1, B2...Bn are connected in parallel with shunt resistors R1, R2...Rn respectively. When the voltage of battery B1 is too high, the control circuit will bypass the control switch S1. When closed, the corresponding shunt resistor R1 generates heat, preventing the voltage of B1 from being higher than the voltage of other cells. Through repeated detection by the control circuit, after multiple rounds of cycles, the entire group is consistent. The advantages of this method are simple structure, high reliability and low cost. The disadvantages are large energy consumption, slow equalization speed, low efficiency, and resistor heat dissipation will affect the normal operation of the system, so it is only suitable for battery packs with smaller capacities.

　　3.2 Switched capacitor method non-energy dissipation type

　　The topology of the switched capacitor method is shown in Figure 2. The capacitor c stores the energy of the battery cell with a higher voltage through the switching of switches at all levels, and then releases it to the battery cell with a lower voltage.

　　The energy storage component in this topology can be a capacitor or an inductor, and the principles are similar. This balancing method has a simple structure, is easy to control, and has relatively small energy loss. However, when the voltage difference between adjacent batteries is small, the balancing time will be longer and the balancing speed is slow; the balancing efficiency is low, and it is not suitable for high-current fast charging situations. Be applicable.

　　3.3DC. DC converter method non-energy dissipation type

　　The circuit topology using DC-DC converter balancing is mainly divided into two types: centralized and distributed. Theoretically, there is no loss and the balancing speed is fast. It is the mainstream solution for lithium battery balancing now.

　　3.3.1 Centralized transformer balancing method

　　The forward and flyback structures are shown in Figure 3(a) and (b) respectively. Each battery cell is connected in parallel with a transformer secondary winding, and the number of turns of each secondary winding is equal, so that the unit with a lower voltage The more energy the body can obtain, thereby achieving balance for the entire group.

　　The advantages of this balancing structure are fast balancing speed, high efficiency and low loss. The disadvantage is that when the voltage is relatively high and the battery pack has a large number of cells connected in series, it will be more difficult to accurately match the secondary winding of the transformer. It is also difficult to compensate for the voltage difference caused by the leakage inductance of the transformer. It has many components and is large in size. It is not easy to modularize, and the switching tube has high voltage resistance.

　　3.3.2 Distributed equilibrium method

　　The structure of the distributed balancing method is to configure a parallel balancing circuit for each unit, which is divided into an isolated circuit with a transformer and a non-isolated circuit.

　　(1) Non-isolated topology

　　The non-isolated topology is a two-way balancing based on the balancing of adjacent cells. Since the structure without a transformer is relatively simple, it is more suitable for situations where the number of battery packs in series is small. Buck-Boost circuit and Chuuk circuit are two common topologies, as shown in Figure 4. The control strategy is that the balancing circuit stops working when the voltage difference between adjacent units reaches the allowable range.

　　(2)Isolated topology

　　Figure 5 shows the basic isolation topology. Each balancing circuit is a buck-boost circuit with an isolation transformer. The advantage is that the balancing efficiency is high, and the voltage endured by the switching device has nothing to do with the number of series series. This balancing structure is more suitable for situations where the number of series battery packs is large; its main disadvantage is that there are many magnetic components in the circuit and the volume is Large, mutual inductance is easy, the transformer has leakage inductance, and it is difficult to keep the coils completely consistent.

　　4 Balanced control strategy

　　Balance control strategies are generally divided into three types in terms of balance targets: external voltage, maximum available capacity, and real-time SOC.

　　The control strategy with external voltage as the balancing target is to measure the external voltage of the battery cells in real time during the charging and discharging process, discharge the batteries with high voltage in the group, and charge the batteries with low voltage, thereby adjusting the voltage of the battery group to be consistent. This is the most widely used equalization method at present. Its control method is easy to implement and does not have high requirements on the algorithm. The disadvantage is that it is difficult to guarantee the accuracy and efficiency of equalization with a single voltage equalization. Especially for parallel battery cells, this strategy cannot be applied for equalization. .

　　The balancing strategy with capacity and real-time SOC as the balancing target refers to controlling the remaining capacity or SOC of each battery to be equal during the charging and discharging process. Since capacity and SOC are battery parameters that cannot be directly measured, they are secondary quantities that need to be calculated from measurable primary quantities (voltage, current, temperature, etc.). The accuracy of the calculation is restricted by the calculation method and battery model. Battery aging, self-discharge, and temperature are also influencing factors. It is difficult to accurately grasp the specific capacity and SOC of each single battery. Therefore, this control strategy is currently rarely used.

　　5 Introduction to two equilibrium schemes and analysis of the development trend of equilibrium technology

　　Actual-scale energy storage systems often use a combination of the above technologies. This article lists two solutions. The first solution is one of the balancing solutions used in wind and solar energy storage power stations, and the second solution is a new balancing circuit developed by KAIST University in South Korea.

　　5.1 Resistive discharge DC-DC power supply circuit

　　The battery energy management unit BMU (Battery Management Unit) detects the cell voltage in real time. According to the balancing strategy and the charge and discharge status of the battery pack, when it is determined that the SOC of a certain battery cell is low and needs to be replenished, the DC-DC output is enabled. After the power is replenished to the target value, the power replenishment equalization stops automatically.

　　When it is judged that the voltage of a certain battery cell is too high and needs to be discharged, the corresponding discharge circuit is opened, and the discharge resistors (R1~R12) shown in Figure 6 discharge the cell. When the discharge reaches the target value (or the discharge temperature is too high), the discharge equalization will automatically stop. The internal balancing principle block diagram of the battery module is shown in Figure 6.

　　5.2 New equalization circuit based on buck-boost

　　Figure 7 shows the balancing circuit developed by KAIST University in South Korea. Each battery is equipped with diodes and switching tubes at both ends, forming a one-way path for balancing current. One end of the battery is connected to point A on one side of the energy storage inductor, and the other end is connected to point A on one side of the energy storage inductor. Point B on the other side of the inductor is connected. In order to reduce switching devices, the balancing branches at the two ends only have one switching tube and one diode.

　　The balancing strategy is to transfer the battery power with high voltage to the battery with low voltage by detecting the working voltage of the battery cell. For example, if B1 has the highest voltage and B2 has the lowest voltage, switches S1d and S1d2c are turned on, the inductor stores energy, and B1 voltage reaches the predetermined level. After setting the equilibrium voltage, S1d is closed, S2c3d is opened, and the inductor releases energy to B2.

　　The advantage of this circuit is that each switch can be a discharge circuit for one battery and a charging circuit for another battery at the same time. For example, switch S1d2e is not only the discharge circuit switch of battery B1, but also the charging circuit switch of battery B2. The principle is similar to the buck-boost circuit, but only one energy storage inductor is used, so it is small in size and low in cost.

　　5.3 Balanced technology developmentDevelopment trend analysis

　　Analyzing the above two options, we can draw the following conclusions:

　　(1) The first option is to use the battery operating voltage as the balancing target and use centralized DC. The DC converter topology is used as a power supply circuit with a high frequency of use, and uses a resistive dissipative balancing circuit as a discharge circuit. In this way, the current in the power supply circuit only needs to flow in one direction, reducing the number and cost of switching devices. In the control strategy Mainly replenishing power, supplemented by discharging, it can meet the dual requirements of balanced efficiency and cost.

　　(2) The second option is the switched capacitor method and distributed DC. The combined application of the DC converter method avoids the shortcomings of the switched capacitor method of multiple switching devices and low balancing efficiency, and at the same time reduces the need for distributed DC. The use of magnetic components in the DC converter method reduces the size.

　　(3) The distributed DC-DC converter balancing circuit can basically be lossless. Each balancing circuit has the same structure and works independently of each other. It has high modulation flexibility and is easy to modularize. When the number of batteries is increased in the battery pack, the balancing module Has the advantage of being easy to expand. Among them, buck-boost transformation equalization has greater flexibility than one-way equalization and is suitable for various working conditions, such as electric vehicles, so its application prospects are broader.

　　(4) The switched capacitor method is a commonly used topology at present. Energy is quickly transferred through the capacitor group to achieve precise dynamic and static balance without any additional matching device or high-precision error requirements. It does not require a closed-loop control strategy and balanced charging. The process ends on its own. However, there are many switching devices, and when the voltage difference between adjacent batteries is small, it takes a long time to reach equilibrium.

　　6 Conclusion

　　Scaled energy storage technology can cooperate with new energy power generation to achieve functions such as smooth output, peak shaving and valley filling, and has good application prospects. Reliable and economical balancing technology is an important technical guarantee for realizing scaled energy storage applications. Due to the long service life of large-scale energy storage power stations, the number of battery cells is huge, there are generally no complex working conditions, and the requirements for balancing speed are not high. Therefore, the main requirements for balancing circuits are fewer switching elements, simple structure, low loss, and low cost. , high reliability.

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