Method for improving battery cell consistency of energy storage lithium battery

Abstract

The invention provides a method for improving the consistency of battery cells of an energy storage lithium battery. The method comprises the following steps: charging a battery provided with an active equalization device; in the battery charging process, the state of a battery cell is continuously monitored through a BMS, when it is found that the battery is in a poor consistency state, active equalization is started, and constant-current charging is adopted at least during active equalization; acquiring the maximum voltage of a single cell in the battery, and continuously monitoring the value of the maximum voltage; and according to the change of the maximum voltage, the current value of the constant current charging is adjusted, and the current value of the constant current charging is reduced along with the increase of the maximum voltage. The method can be directly applied to the charging process of the battery, the problem of poor consistency of the battery cells in the battery is solved all the time during charging, normal use of equipment is not affected, the service life of the battery cells is prolonged, the overlarge voltage difference at the charging tail end is reduced, and the cost of independent after-sales maintenance can be avoided.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium battery management, and particularly relates to a method for improving the consistency of energy storage lithium battery cells. Background Art

[0002] With the requirements of energy conservation, emission reduction, green, low-carbon and lead-free, the market demand for energy storage lithium batteries is increasing. The cell is an important part of the battery. Even if the same raw materials are used in the production of lithium iron phosphate cells, there may be slight differences in the production process of the battery, resulting in different performances of each cell inside the final battery. As the usage time of the product increases, the difference will become larger and larger. The battery pack may frequently trigger protection during use due to the overcharging and underdischarging of a certain cell, resulting in insufficient capacity within the available cycle. The conventional method is to directly replace the battery after finding that the battery has poor consistency; or remove the battery from the equipment point for separate maintenance, and disassemble the battery for separate equalization treatment with an equalizer; the above methods are not conducive to controlling the product cost and are time-consuming and laborious, and will also affect the normal use of the equipment party during maintenance. Summary of the Invention

[0003] This application mainly solves the problems that the lithium battery frequently triggers overcharge protection inverter alarms and the available capacity of the battery is insufficient due to the increase in the use time and the poor consistency of the internal cells at the end of charging. Aiming at this technical problem, a method for improving the consistency of energy storage lithium battery cells is proposed. The specific technical solutions are as follows:

[0004] A method for improving the consistency of energy storage lithium battery cells, the method comprising the following steps:

[0005] Charge the battery equipped with an active equalization device;

[0006] During the battery charging process, continuously monitor the state of the cells through the BMS. When it is found that the battery has a poor consistency state, turn on the active equalization, and at least use constant current charging during the active equalization;

[0007] Obtain the highest voltage of the single cells in the battery and continuously monitor the value of the highest voltage;

[0008] According to the change of the highest voltage, adjust the current value of the constant current charging, and the current value of the constant current charging decreases as the highest voltage increases.

[0009] Further, the method for judging the state of poor consistency of the battery is: during the charging process, continuously monitor the voltage value of each cell through the BMS, obtain the maximum voltage and the minimum voltage, calculate the voltage difference ΔV between the maximum voltage and the minimum voltage, and by setting a reference threshold Vre, when ΔV > Vre, it is judged that there is a problem of poor consistency.

[0010] Further, the method for determining the state of poor consistency of the battery is as follows: During the charging process, the BMS continuously monitors the voltage change rate of each battery cell per unit time, obtains the maximum voltage change rate and the minimum voltage change rate, calculates the ratio N of the maximum voltage change rate to the minimum voltage change rate, and by setting a reference threshold Nre, when N > Nre, it is determined that there is a problem of poor consistency.

[0011] Further, several continuous voltage change ranges are set, and according to the correspondence between the highest voltage and the voltage change range, the charging rate is set, and the voltage change range and the charging rate show a stepped decreasing trend.

[0012] Further, after the active equalization is turned on, when the highest voltage value of the battery cell decreases beyond the original voltage change range, the charging rate before the decrease of the highest voltage value is maintained.

[0013] Further, the maximum charging rate is 0.5C.

[0014] Further, five voltage change ranges are set: 0 - 3.4V, 3.4 - 3.5V, 3.5 - 3.55V, 3.55 - 3.6V, above 3.6V, and five corresponding charging rates are set as 0.5C, 0.3C, 0.2C, 0.1C, 0.05C.

[0015] The beneficial effects of the present invention are as follows: The method of the present application can be directly applied to the charging process of the battery, and during charging, by dynamically adjusting the charging rate and active equalization, the problem of poor consistency of the battery cells inside can be continuously improved, without affecting the normal use of the device, thereby extending the service life of the battery cells, reducing the excessive voltage difference at the end of charging, and also avoiding the cost of separate after-sales maintenance. Description of the Drawings

[0016] Figure 1 Shows the relationship between the voltage and the charging rate control during the battery charging process. Detailed Embodiments

[0017] In the following description, certain specific details are set forth in order to provide a thorough understanding of various embodiments. However, those skilled in the art should understand that the present invention can be practiced without these details. In other instances, well-known structures have not been shown or described in detail to avoid unnecessarily obscuring the description of the embodiments. Unless the context otherwise requires, throughout the specification and the appended claims, the word "comprising" shall be interpreted in an open, inclusive sense, i.e., as "including but not limited to".

[0018] As used throughout this specification, the phrase "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrase "in one embodiment" or "in an embodiment" throughout this specification are not necessarily all referring to the same embodiment. In addition, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. Further, as used in this specification and the appended claims, the singular forms "a", "an", and "the" include plural referents unless the context clearly dictates otherwise. It should also be noted that the term "or" is generally employed in its inclusive sense, i.e., "and / or", unless the context clearly dictates otherwise.

[0019] As a method for improving the consistency of energy storage lithium battery cells in this application, it should be noted that it needs to be implemented in cooperation with a supercapacitor equalization device. The supercapacitor equalization device is an active equalization device and has the following characteristics:

[0020] (1) Hybrid energy storage: A hybrid energy storage system composed of an energy storage battery (such as a lithium-ion battery) and a supercapacitor (or ultracapacitor). The energy storage battery has a high energy density and is suitable for long-term energy storage; while the supercapacitor has a high power density, fast charge and discharge capabilities, and a long cycle life, and is suitable for handling instantaneous high-power demands;

[0021] (2) Energy distribution and management: The supercapacitor is used to balance the voltage differences between energy storage batteries and handle instantaneous high-power demands or absorb regenerative energy; when it is detected that the voltage of a certain energy storage battery unit is too high or too low, this excess electrical energy can be temporarily transferred to the supercapacitor for storage, or energy can be released from the supercapacitor to supplement the battery unit with a lower voltage to achieve re-distribution of energy;

[0022] (3) Bidirectional DC-DC converter: In order to achieve energy transfer, a bidirectional DC-DC converter is usually used to connect the supercapacitor and the energy storage battery. The bidirectional DC-DC converter can switch between boost mode and buck mode, adjust the output voltage as needed, and ensure that energy can be effectively transferred between the supercapacitor and the energy storage battery;

[0023] (4) Intelligent control system: The whole process is monitored and regulated by an intelligent control unit (such as a battery management system BMS). It will real-time monitor parameters such as the voltage and temperature of each energy storage battery unit, and decide when and how to perform energy transfer according to a preset algorithm.

[0024] When the supercapacitor is used in an active equalization device, when the battery is charging and discharging at a large current and the consistency of the battery at the end of charging is very poor, the active equalization has little effect due to the too small equalization current, and the battery will still trigger protection. The method of this application aims to overcome this problem.

[0025] The method of this application is as follows:

[0026] Charge the battery in a constant current or constant voltage state;

[0027] During the battery charging process, continuously monitor the state of the battery cells through the BMS. When it is found that the battery shows a state of poor consistency, activate the active equalization;

[0028] Obtain the highest voltage of the individual battery cells in the battery and continuously monitor the value of the highest voltage;

[0029] According to the change range of the voltage value, adjust the current value of the constant current charging. And the larger the boundary extreme value of the change range of the voltage value, the smaller the current of the constant current charging.

[0030] In the above charging method, there is no restriction on the charging mode of the battery at the initial stage of charging. As the charging continues, the problem of the difference in the consistency of the battery cells will become prominent. Therefore, it is necessary to continuously monitor the state of the battery cells through the BMS to activate the active equalization according to the state of the battery cells.

[0031] The condition for activating the active equalization is that the BMS monitors that the battery cells show a state of poor consistency. The state of the battery cells showing a state of poor consistency specifically refers to voltage difference, capacity difference, internal resistance difference, self-discharge rate difference, temperature sensitivity difference, aging rate difference. And in this application, it specifically refers to the voltage difference between the battery cells. The voltage values reached by different individual batteries under the same charging conditions are different, resulting in that at the end of charging, some individual batteries may have reached their maximum charging voltage while other batteries are not fully charged.

[0032] Therefore, it is necessary to set a monitoring condition to judge whether and when the battery shows a problem of poor consistency. This application provides two methods. One is the voltage difference method, and the other is the boost rate method.

[0033] The voltage difference method means that during the charging process, continuously monitor the voltage value of each battery cell through the BMS, obtain the maximum voltage and the minimum voltage, calculate the voltage difference ΔV between the maximum voltage and the minimum voltage. By setting a reference threshold Vre, when ΔV > Vre, it is judged that there is a problem of poor consistency.

[0034] The boost rate method means that during the charging process, continuously monitor the voltage change rate per unit time of each battery cell through the BMS, obtain the maximum voltage change rate and the minimum voltage change rate, calculate the ratio N of the maximum voltage change rate and the minimum voltage change rate. By setting a reference threshold Nre, when N > Nre, it is judged that there is a problem of poor consistency.

[0035] Compared with the voltage difference method, the boost rate method can prospectively detect the problem of poor battery consistency, enabling the active equalization to intervene in the battery charging process earlier. Due to the poor battery consistency caused by the differences in the batteries themselves, during the charging process, some battery cells have a fast charging boost rate while others have a slow charging boost rate. However, the boost rate during the charging process is almost only related to the battery cell itself, while the voltage value during the charging process is also related to the charging process. Therefore, the voltage difference method requires a certain charging process to make a judgment, but the boost rate method can detect the consistency difference in the battery cell charging at an early stage of charging, and then enable the active equalization earlier. Through the active equalization, energy transfer is carried out to balance the voltage difference between the battery cells. This is beneficial to extending the constant current charging time when using constant current charging in the initial stage, and thus is also beneficial to reducing the charging time and improving the charging efficiency.

[0036] After the active equalization is enabled, the acquisition of the highest voltage value of the individual battery cells in the battery is continuous, aiming to perform subsequent charging control. During the charging control, different and continuous voltage change ranges are set according to the nominal voltage value of the battery cells, and a constant charging current is set for each voltage change range.

[0037] Taking a lithium iron phosphate battery with a nominal voltage value of 3.2V and a capacity of 1000Ah as an example, the conventional charging voltage of the lithium iron phosphate battery cell is 3.6V multiplied by the number of series, and the current is 0.5C. The ideal charging is from the constant current stage to the constant voltage stage. Five voltage change ranges are set, namely 0 - 3.4V, 3.4 - 3.5V, 3.5 - 3.55V, 3.55 - 3.6V, and above 3.6V. For each voltage change range, a constant charging rate is set respectively as 0.5C, 0.3C, 0.2C, 0.1C, and 0.05C. The charging process is detailed in Figure 1 。

[0038] During the above charging process, when the voltage reaches above 3.4V, the battery is basically above 90% of the battery capacity, and the voltage value is no longer within the voltage plateau range, and the voltage will rise rapidly. Therefore, implementing a current reduction strategy at this time can largely suppress the problem of battery cell inconsistency. As the charging progresses, the voltage rise change will accelerate again. Therefore, after 3.5V, the downshift is changed to every 50mv, which can better suppress the problem of battery cell inconsistency.

[0039] After performing charge control, the charging rate decreases with the charging time, and during this process, the maximum voltage of the battery also decreases. During this process, if the maximum voltage of the battery cell exceeds the original voltage change range due to the decrease, the charging rate still remains unchanged. Exemplarily, the maximum voltage of the battery cell is 3.52V, within the range of 3.5 - 3.55V, and the corresponding charging rate is executed at 0.2C. After a period of time, the maximum voltage of the battery cell decreases to 3.49V, within the range of 3.4 - 3.5V, but the charging rate does not need to be adjusted to 0.3C and still executes the 0.2C charging rate, that is, a step-down current charging method is executed throughout the charging process.

[0040] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than limiting it.

Claims

1. A method for improving the consistency of energy storage lithium battery cells, characterized in that: The method comprises the following steps: Charging a battery equipped with an active balancing device; During the battery charging process, the BMS continuously monitors the status of the battery cells. When the battery is found to be in a state of poor consistency, active balancing is turned on, at least constant current charging is used during active balancing; Obtain the highest voltage of the single cell in the battery and continuously monitor the value of the highest voltage; According to the change of the maximum voltage, the current value of the constant current charging is adjusted, and the current value of the constant current charging decreases as the maximum voltage increases.

2. The method for improving the consistency of energy storage lithium battery cells according to claim 1, characterized in that: The method for determining whether a battery has poor consistency is as follows: during the charging process, the BMS continuously monitors the voltage value of each battery cell, obtains the maximum voltage and the minimum voltage, calculates the voltage difference ΔV between the maximum voltage and the minimum voltage, and sets a reference threshold Vre. When ΔV>Vre, it is determined that a consistency problem has occurred.

3. The method for improving the consistency of energy storage lithium battery cells according to claim 1, characterized in that: The method for judging whether the battery has poor consistency is as follows: during the charging process, the BMS continuously monitors the voltage change rate per unit time of each battery cell, obtains the maximum voltage change rate and the minimum voltage change rate, calculates the ratio N of the maximum voltage change rate to the minimum voltage change rate, and sets the reference threshold Nre. When N>Nre, it is judged that a problem of poor consistency has occurred.

4. The method for improving the consistency of energy storage lithium battery cells according to claim 1, characterized in that: Several continuous voltage variation ranges are set, and the charging rate is set according to the correspondence between the highest voltage and the voltage variation range. The voltage variation range and the charging rate are in a step-down manner.

5. The method for improving the consistency of energy storage lithium battery cells according to claim 4, characterized in that: After active balancing is turned on, when the maximum voltage of the battery cell drops beyond the original voltage variation range, the charging rate before the maximum voltage drops is maintained.

6. The method for improving the consistency of energy storage lithium battery cells according to claim 1, characterized in that: The maximum charge rate is 0.5C.

7. The method for improving the consistency of energy storage lithium battery cells according to claim 1, characterized in that: Set five voltage change ranges: 0~3.4V, 3.4~3.5V, 3.5~3.55V, 3.55~3.6V, and above 3.6V, and set five charging rates accordingly: 0.5C, 0.3C, 0.2C, 0.1C, and 0.05C.

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