Passive equalization method and device for power battery pack

Abstract

The invention relates to the technical field of battery energy storage and battery management, in particular to a power battery pack passive equalization method and device capable of remarkably prolonging the service life of a battery pack and prolonging the endurance mileage. Whether single batteries in the power battery pack meet passive equalization conditions or not is judged, the passive equalization conditions are that the current power battery pack is in a charging state, the highest voltage Vmax of the single batteries in the battery pack is larger than or equal to 3.35 V, Vmax-Vmin is larger than or equal to 10 mV, Vmin is the lowest voltage value of the single batteries in the current power battery pack, and if the passive equalization conditions are met, the single batteries in the power battery pack are not in the charging state. And discharging the single battery with the highest battery voltage.

Description

Technical fields:

[0001] This invention relates to the fields of battery energy storage and battery management technology, specifically a passive balancing method and device for power battery packs that can significantly extend battery pack lifespan and increase driving range. Background technology:

[0002] Currently, most mobile machines or devices, such as automobiles, construction machinery, aerial work platforms, and battery energy storage systems, are powered by batteries, primarily lithium-ion batteries, especially lithium iron phosphate (LFP) batteries or ternary lithium batteries. Lithium-ion battery packs require a management system, or Battery Management System (BMS). Among the many functions of a BMS, capacity balancing is essential. Passive balancing is the most common method. Given the continuous improvement in battery quality and the decreasing capacity differences, passive balancing is a reasonable choice.

[0003] Typically, the passive balancing current is relatively small, around 40-60 mA. This is acceptable for batteries with capacities of 50Ah to 100Ah during the initial and mid-term of use. However, when the capacity is above 200Ah, or in the later stages of use for slightly lower capacity batteries, the passive balancing effect becomes difficult to observe, and the battery pack's State of Charge (SOC) decreases significantly. This ultimately leads to reduced driving range or operating time, increased energy loss, and deterioration of economic indicators. Therefore, it is necessary to appropriately increase the value of the passive balancing current.

[0004] There are many methods for passively increasing the balancing current of power battery packs, but the following principles should be followed: a. Ensure the expected increase is achieved, such as extending the time it takes for the SOC to drop to 90% of its original value; b. The measures should be simple and occupy little BMS space; c. The selected circuit components should have large market supply, low price, and high reliability; d. Minimize the use of MCU resources; e. When there is no balancing requirement, the circuit power consumption should be minimized; f. Ensure necessary isolation. Summary of the Invention:

[0005] This invention addresses the shortcomings and deficiencies of existing technologies by proposing a passive balancing method and apparatus for power battery packs that can meet the passive balancing requirements of power battery packs and effectively extend the battery pack's lifespan and range.

[0006] This invention achieves its purpose through the following measures:

[0007] A passive balancing method for a power battery pack, wherein the power battery pack contains at least two individual cells, characterized in that it determines whether the individual cells in the power battery pack meet the passive balancing conditions, wherein the passive balancing conditions are:

[0008] The current power battery pack is in a charging state, and the highest voltage of a single cell in the battery pack, Vmax, is ≥3.35V, and Vmax-Vmin is ≥10mV, where Vmin is the lowest voltage value of a single cell in the current power battery pack. If the passive balancing condition is met, the single cell with the highest battery voltage will discharge.

[0009] This invention also proposes a device based on the passive equalization method for power battery packs as described above, comprising a single-cell voltage detection module, an equalization module, and an MCU controller. The single-cell voltage detection module includes a low-segment battery voltage measurement module MODE1L and a high-segment battery voltage measurement module MODE1H. The low-segment battery voltage measurement module MODE1L and the high-segment battery voltage measurement module MODE1H are respectively used to measure the low-segment and high-segment single-cell voltage values ​​of two or more series-connected single cells in the power battery pack. The low-segment battery voltage measurement module MODE1L and the high-segment battery voltage measurement module MODE1H are respectively connected to controller MCU0 and controller MCU1. Controller MCU0 and controller MCU1 receive data from the single-cell voltage detection module, perform AD conversion, and store the data. Two optocouplers are connected between controller MCU0 and controller MCU1 to enable communication between them.

[0010] The system also includes an equalization module matrix, which contains two or more equalization modules that correspond one-to-one with two or more individual cells within the power battery pack. Each of the two or more equalization modules is connected to an address decoder. Each equalization module contains an equalization circuit, which can be a low-current equalization circuit or a high-current equalization circuit. Both the low-current and high-current equalization circuits include optocouplers, power devices, and optocoupler current-limiting resistors R1. The optocouplers are used to provide electrical isolation between the equalization command and the passively equalized battery. The power devices in the low-current equalization circuit are selected from switching transistors, while the power devices in the high-current equalization circuit are selected from MOSFETs.

[0011] In this invention, the address decoder adopts a 4-16 decoding method and the output is active low. It uses an optocoupler with CTR>=1.0 to directly drive the high hfe switching transistor and power MOSFET, while realizing electrical isolation between the main control MCU and the address decoder and each battery cell.

[0012] The single-cell battery voltage detection module described in this invention employs a high-voltage multiplexer analog switch. Specifically, it comprises two analog multiplexers packaged together. Each analog multiplexer detects voltages connected to eight input points. Each analog multiplexer has eight inputs and one output. The negative terminal of each single cell in the power battery pack is connected to the corresponding number of the SA terminal of the high-voltage multiplexer analog switch U1, with the DA terminal being its output. The positive terminal of each single cell is connected to the corresponding number of the SB terminal of U1, with the DB terminal being its output. DA and DB are connected to the negative and positive input terminals of the instrumentation amplifier U2, respectively. The output terminal of the instrumentation amplifier U2 is connected to voltage dividers R1 and R2. The voltage value taken from voltage divider R2 is fed into the A / D terminal of the MCU via emitter follower U4 for analog-to-digital conversion. The MCU selects SA and SB through the PX port, and U3 enables U1. The two MCUs store the received single-cell battery voltage values ​​in their storage space for future reference.

[0013] In the low-current equalization circuit described in this invention, the switching transistor should also meet the following requirements: Vces < 0.5V, Ic.max > 1A; a current-limiting resistor for the optocoupler connected to the optocoupler is provided to control the current of the light-emitting diode in the optocoupler to 10mA; a resistor R2 is also provided to limit the current through the OPTi phototransistor. The magnitude of this current is related to the CTR of the OPTi, and 10mA is selected for reliability and safety. A resistor R3 is connected between the collector of the switching transistor and the positive terminal of the single cell. By adjusting the resistance value of R3, the value of the passive equalization current is adjusted. A resistor R4 is connected between the emitter and base of the switching transistor for... To minimize unnecessary losses in the passive equalization circuit when equalization is not required: The phototransistor of the optocoupler OPTi has leakage current when it is off, with a value of approximately 1–10 μA. Without resistor R4, this leakage current is completely injected into the base of the switching transistor Qi. Assuming Qi's hfe is 200, the emitter current of Qi is 0.2 mA–2 mA, and the power is 0.7–7 mW. This loss may seem small, but it exists once the BMS is powered on and therefore cannot be ignored. After adding R4, when passive equalization is not required, the leakage current completely passes through it, setting the voltage between the base and emitter of Qi to 10*10. -6 *10 3 =0.01V, thereby further reducing the leakage current of transistor Qi.

[0014] The high-current equalization circuit described in this invention is suitable for equalization circuits operating at 1A to 5A, and the power device selected is a MOSFET. The MOSFET should meet the following requirements: Vgs(th) < 1.5V, Ron < 20mohm; when using the MOSFET, Vgs(th) < 1.5V, Ron < 20mohm; DSAt low voltage levels, such as 20V-30V, Id is around 20A-30A. Ron is only 20mΩ when Vgs = 3V, and only 5mΩ is generated when Qi is on. 2 With a power consumption of 0.02 = 0.5W, no heat sink is needed. This improves the reliability of the MOSFET and reduces the cost and space required for heat dissipation. Another advantage of using OSFET is that the passive balancing current can be adjusted, requiring only the replacement of MOSFET Qi and R3. Optocouplers are also compatible. Furthermore, using MOSFET for Qi reduces losses without passive balancing. Generally, the leakage current of MOSFET is around 1μA, which is significantly lower than that of switching transistors, only a fraction of the value of the same transistor. The role of resistor R4 is to effectively release the emitter current of OSFET, ensuring that excessive charge does not accumulate at the gate of Qi and cause it to be accidentally turned on.

[0015] Compared with the prior art, the present invention has the advantages of reasonable structure and low cost, and can effectively improve the service life and range of the power battery pack. Attached image description:

[0016] Appendix Figure 1 This is a flowchart of the present invention.

[0017] Appendix Figure 2 This is a schematic diagram of the principle of the present invention.

[0018] Appendix Figure 3 This is a schematic diagram of a voltage detection circuit in an embodiment of the present invention.

[0019] Appendix Figure 4 This is a schematic diagram of a structure of the equalization module matrix in an embodiment of the present invention.

[0020] Appendix Figure 5 This is a schematic diagram of the small current balancing circuit in an embodiment of the present invention.

[0021] Appendix Figure 6 This is a schematic diagram of the high-current equalization circuit in an embodiment of the present invention. Detailed implementation method:

[0022] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0023] Example:

[0024] This example is attached Figure 1 As shown, a passive balancing method and balancing device for power battery packs are proposed. In this example, the passive balancing method for the power battery pack is as follows:

[0025] First, determine whether the current power battery pack meets the passive balance conditions:

[0026] The Vmax of the battery pack is ≥3.35V.

[0027] In the battery pack, Vmax-Vmin ≥ 10mV,

[0028] During charging

[0029] These conditions are in an AND relationship, meaning they must all be fully satisfied for passive equilibrium to occur.

[0030] Secondly, passive balancing is only performed on the battery with the highest voltage; therefore, the exact serial number or designation of this battery must be known. Generally, this designation is common and universally accepted, and remains the same throughout the BMS program. For a 48V system, N is used here. O 0 to N O 15:

[0031] The enable signal for the address decoder is sent to H, which disables it. This is to ensure that the output of the address decoder, or Wj, will not be in a brief contention state, thus ensuring the unique correctness of the passive equalization.

[0032] Output the battery serial number value before decoding to PX.0~PX.3.

[0033] Typically, during the charging process, many operations, such as voltage measurement, current measurement, temperature measurement, SOC measurement and calculation, are performed cyclically. The duration of the cycle varies depending on the design of different companies, but is generally between a few tenths of a second and two seconds. The duration of passive equalization is approximately one cycle. Assuming this, passive equalization will be stopped at a suitable time point before the end of each cycle. The stopping operation is simple; just send the disable signal, PX.4, to H level.

[0034] To achieve the passive equilibration method described above, this example also provides a method as shown in the appendix. Figure 2 The passive balancing device for the power battery pack shown is specifically as follows:

[0035] The series-connected cells in the battery pack are divided into two segments: a low-segment battery pack L and a high-segment battery pack H. Figure 2In this module, MODE1_L is the voltage measurement module for the lower-end battery pack, and MODE1_H is the voltage measurement module for the higher-end battery pack. The measured battery voltage values ​​from MODE1_L and MODE1_H are sent to the respective A / D converters of MCU0 and MCU1, respectively. After A / D conversion, the values ​​are then sent to the memory areas of MCU0 and MCU1. Two optocouplers between MCU0 and MCU1 are used for communication between them: transmitting commands and data. MODE2 is the matrix for the equalization module. The equalization command is issued by MCU0, which selects one battery from the 16 cells for passive equalization. Of course, if no command is issued, equalization is not required.

[0036] One point needs clarification. In most applications, the nominal voltage of the battery pack is set at 48V. For lithium iron phosphate batteries, 15 cells are required in series, and occasionally 16 cells are used. With 15 cells, the peak charging voltage of the battery pack is 55.5V. This patent uses a high-voltage multiplexer analog switch with a withstand voltage limit of DC44V, which is lower than 55.5V. Therefore, using a two-stage voltage testing module satisfies the 48V requirement on the one hand, and provides the convenience of splitting and discarding one stage for the 24V requirement on the other.

[0037] This example presents the following: Figure 3 The voltage detection module MODE1 shown contains two 8-bit to 1-bit analog multiplexers packaged together, each detecting the voltage connected to only 8 input points. The negative terminal of each battery is connected to the corresponding number of SA on U1, and DA is its output; the positive terminal of each battery is connected to the corresponding number of SB on U1, and DB is its output. DA and DB are directly connected to the negative and positive input terminals of the instrumentation amplifier U2, respectively. Since the output voltage of U2 is equal to the voltage of one battery, which is higher than the maximum input value of the MCU's A / D converter, it is necessary to add voltage dividers R1 and R2. The voltage value taken from R2 is fed into the MCU's A / D converter via emitter follower U4 for analog-to-digital conversion. The MCU selects SA and SB through port PX, directing them to DA and DB, respectively. U3 is used to enable U1. The two MCUs, MCU0 and MCU1, store the voltage value of each battery detected by each MCU in a designated space for later retrieval.

[0038] This example presents the following: Figure 4 The equalization module matrix MODE2 shown is used, where 'a' is an equalization module, one for each battery cell. See the detailed circuit diagram below. Figure 5 and Figure 6b is the address decoder, using 4-to-16 decoding, with a low-level active output; R1 is the current-limiting resistor for the optocoupler, controlling the current of the LED to 10mA. During charging, if the difference between the highest and lowest battery voltages exceeds the design value, and the highest battery voltage is greater than 3.35V, an output command will be sent to PY to discharge the highest-voltage battery for a period of time, thus achieving a balancing effect.

[0039] In this example, MODE2, there are two types of equalization circuits: one is a low-current equalization circuit, see... Figure 5 Another type is the high-current balancing circuit, see... Figure 6 The main difference between the two lies in the selection of power components; the former uses low-power transistors, while the latter uses power MOSFETs.

[0040] Regarding the attached document Figure 5 The small current equalization circuit shown:

[0041] This circuit uses two key electronic components: a switching transistor Qi and an optocoupler OPTi. When Qi is saturated and conducting, current flows through resistor R3. Adjusting the resistance of R3 is, to a certain extent, equivalent to adjusting the passive balancing current. At this time, the voltage of the battery BATi is fully applied to the branch between R3 and Qi, and the magnitude of the current flowing through this branch is essentially the value of the passive balancing current. Since the base current of Qi is provided by OPTi, and the current provided by OPTi is less than or equal to 10mA, this ensures that the collector current of Qi will not be too high, meaning the passive balancing current will not be too high. OPTi provides electrical isolation between the balancing command and the passively balanced battery. When the input signal Wj is high, the OPTi LED is off, the OPTi phototransistor is off, and its emitter current, which is the base of the main transistor Qi, has no input current. This results in no current in the collector and emitter of Qi, and therefore no passive balancing current. When the input signal Wj is low, the OPTi LED lights up, the OPTi phototransistor is saturated and turned on, and its emitter current, which is the base of the transistor Qi, is injected with current. This causes the collector and emitter of Qi to generate a corresponding current, which is the passive balancing current.

[0042] exist Figure 5In the circuit, R1 ensures the normal LED emitting current of the OPTi, and is usually constant at 10mA. R2 limits the current through the OPTi phototransistor; this current is related to the CTR of the OPTi, and for reliability and safety, 10mA is chosen. R3 has already been introduced, but here we add an explanation: it should have sufficient heat dissipation capacity. The use of R4 is of great significance, namely, minimizing the unnecessary losses of the passive equalization circuit when equalization is not required. The OPTi phototransistor has leakage current when it is off, which is approximately 1-10μA. Without resistor R4, this leakage current is completely injected into the base of the switching transistor Qi. Assuming that the hfe of Qi is equal to 200, the emitter current of Qi is 0.2mA-2mA, and the power is 0.7-7mW. This loss seems small, but it exists once the BMS is powered on, so it cannot be ignored. After adding R4, when passive equalization is not required, the leakage current completely passing through it will set the voltage between the base and emitter of Qi to 10*10. -6 *10 3 =0.01V, thereby further reducing the leakage current of transistor Qi.

[0043] The cost of this circuit unit is very low, about 1.5 yuan.

[0044] Regarding the attached document Figure 6 The high-current equalization circuit shown:

[0045] This circuit is suitable for applications with relatively large passive balancing currents, such as 1A to 5A. In some applications, such as energy storage stations and current conversion stations, the battery capacity used may reach 500Ah, 1000Ah, or even 2000Ah. According to the battery design documents, the optimized balancing capability should be above the ampere level.

[0046] Figure 6 and Figure 5 The biggest difference is to put Figure 5 The Qi was changed from a switching transistor to a MOSFET. The purpose of the change is: first, to allow for the equalization of ampere-level current; second, to reduce the power consumption of the device, eliminate the need for a heat sink, and also reduce costs.

[0047] Since Qi uses a MOSFET, the V of the selected MOSFET... DS At low voltage levels, such as 20V-30V, Id is around 20A-30A, and Ron is only 20mΩ when Vgs = 3V. When Qi is turned on, only 5A is generated. 2 With a power consumption of *0.02 = 0.5W, a heat sink is not required in principle. This improves the reliability of the MOSFET and reduces the cost and space required for heat dissipation.

[0048] Another advantage of using MOSFETs for Qi is that the passive balancing current can be large or small. Only the MOSFET Qi and R3 need to be replaced. OPTi can be used interchangeably.

[0049] Furthermore, Qi's use of MOSFETs reduces losses when passive balancing is not available. Generally, the leakage current of a MOSFET is around 1μA, which is significantly lower than that of a switching transistor, only a fraction of the value of the same transistor.

[0050] The function of R4 is to effectively release the emitter current of OPTi, ensuring that Qi's gate does not accumulate excessive charge and be accidentally turned on.

[0051] In addition, attention must be paid to a MOSFET parameter: its turn-on threshold, Vgs(th). In passive equalization circuits, the supply voltage of Qi is typically between 3.2V and 3.7V. Many MOSFETs have a Vgs(th) value comparable to the Qi's supply voltage. Therefore, their on-resistance is high, leading to significant heat generation. To address this, MOSFETs with low Vgs(th) should be selected, such as 2V or even 1.3V devices. Applying a voltage of 2.5V or higher between the gate and source (GS) of the MOSFET will allow it to saturate and conduct, reducing its on-resistance to an ideal low value and improving the reliability of the MOSFET, i.e., the Qi.

[0052] Compared with existing technologies, this example has the following significant advantages: a. The circuit structure is simple, thus economical and highly reliable; b. With the continuous improvement of battery quality, the current required for passive balancing is getting smaller and smaller, thus reducing balancing losses and making the balancing effect more obvious, which promotes the wider application of passive balancing; c. The software and hardware overhead of BMS is reduced.

Claims

1. A passive equalization method for a power battery pack, the power battery pack having at least two or more single batteries, characterized in that, Judge whether the single battery in the power battery pack meets passive equalization condition, the passive equalization condition is: The current power battery pack is in the charging state, and the highest voltage Vmax of the single battery in the battery pack is greater than or equal to 3.35V, and Vmax-Vmin is greater than or equal to 10mV, wherein Vmin is the minimum voltage value of the single battery in the current power battery pack, if the passive equalization condition is met, the single battery with the highest battery voltage is discharged.

2. An apparatus based on the passive balancing method of the power battery pack of claim 1, comprising a single battery voltage detection module, a balancing module and an MCU controller, characterized in that, The single battery voltage detection module includes a low section battery voltage measurement module MODE1L and a high section battery voltage measurement module MODE1H, the low section battery voltage measurement module MODE1L and the high section battery voltage measurement module MODE1H are used for measuring the low section single battery voltage value and the high section single battery voltage value of two or more single batteries in series in the power battery pack respectively, the low section battery voltage measurement module MODE1L and the high section battery voltage measurement module MODE1H are connected with the controller MCU0 and the controller MCU1 respectively, the controller MCU0 and the controller MCU1 receive the data sent by the single battery voltage detection module, store after AD conversion, two optocoupler components are connected between the controller MCU0 and the controller MCU1 to realize the communication between the controller MCU0 and the controller MCU1; An equalization module matrix is further provided, two or more equalization modules corresponding to two or more single batteries in the power battery pack are arranged in the equalization module matrix, the two or more equalization modules are connected with an address decoder respectively, an equalization circuit is arranged in the equalization module, the equalization circuit includes a small current equalization circuit and a large current equalization circuit, an optocoupler device, a power device and an optocoupler current limiting resistor R1 are arranged in the small current equalization circuit and the large current equalization circuit, the optocoupler device is used for providing electrical isolation between the equalization instruction and the passive equalization battery, the power device in the small current equalization circuit is a switching transistor, and the power device in the large current equalization circuit is a MOSFET tube.

3. A passive balancing device for a battery pack according to claim 2, characterized in that The address decoder adopts a 4-16 decoding mode, and outputs a low level; the optocoupler with CTR >=1.0 is used to directly drive the high-hfe switching transistor and the power MOSFET, and to realize the electrical isolation between the main control MCU and the address decoder and each battery.

4. The passive balancing device for a battery pack of claim 2, wherein, The monomer battery voltage detection module adopts high-voltage multi-channel analog switch, specifically, two analog multi-channel switches are packaged together, each analog multi-channel switch only detects the voltage of 8 input points, each analog multi-channel switch has 8 inputs and 1 output, the negative pole of each monomer battery in the power battery pack is connected to the SA end of the high-voltage multi-channel analog switch U1 corresponding to the serial number, the DA end is its output, the positive pole of each monomer battery is connected to the SB end of U1 corresponding to the serial number, and the DB is its output, the DA and DB are connected with the negative input end and the positive input end of the instrument amplifier U2 respectively, the output end of the instrument amplifier U2 is connected with the voltage divider R1 and the voltage divider R2, the voltage value taken out from the voltage divider R2 enters the A / D end of the MCU through the emitter follower U4 to carry out analog-digital conversion, the MCU selects the SA and SB through the PX port, U3 is used for enabling U1, and the two MCUs store the received monomer battery voltage value in the storage space for checking.

5. The passive balancing device for a battery pack of claim 2, wherein, In the small current equalization circuit, the switch transistor also meets the following requirements: Vces < 0.5V, Ic.max > 1A; the small current equalization circuit is provided with a current limiting resistor connected with the optocoupler device, which is used for controlling the current of the light emitting diode in the optocoupler device to be 10ma, and is also provided with a resistor R2, which is used for limiting the current passing through the OPTi photosensitive triode, and the size of the current is related to the CTR of the OPTi, for reliability and safety, 10ma is selected, the resistor R3 is connected between the collector of the switch transistor and the positive pole of the monomer battery, the value of the passive equalization current is adjusted by adjusting the resistance value of R3, the resistor R4 is connected between the emitter and the base of the switch transistor, and is used for reducing the unnecessary loss of the passive equalization circuit to the maximum when the equalization is not needed.

6. The passive balancing device for a battery pack of claim 2, wherein, The large current equalization circuit is suitable for equalization circuit in the case of 1A-5A, and the power device is selected as MOSFET, and the MOSFET should meet the following requirements: Vgs(th)<1.5V, Ron<20mohm; when the selected MOSFET V DS is low, such as 20V-30V, Id is about 20A-30A, Ron is only 20mΩ when Vgs=3V, only 5 2 0.02=0.5W power consumption when Qi is turned on, and no radiator is needed. The passive equalization current can be large or small, and only the MOSFET Qi and R3 need to be exchanged. The optocoupler device is universal, and the resistance R4 is used for effective release of the emitter current of the OPTi, to ensure that the G pole of Qi will not accumulate excessive charge and be mis-triggered.

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