A battery balancing method

Abstract

The present invention discloses a battery balancing method, comprising the following steps: step 1, obtaining voltage and capacity information of each battery string, and recording the lowest voltage Vl; obtaining the precision value interval of the AFE used as the balancing switch threshold Vt; step 2, dividing the cell voltage into different voltage zones according to the open circuit voltage-remaining capacity characteristics of each battery and the voltage sampling precision characteristics of the AFE; step 3, obtaining the voltage of each battery string at a fixed frequency, recording the battery string where the lowest voltage is located as Bn, and recording the highest voltage Vh; according to the precision value interval of the AFE, presetting the balancing start voltage Vs of each voltage zone as the voltage difference between the highest voltage Vh and the lowest low voltage Vl; presetting the balancing shut-off voltage Vc of each voltage zone according to the precision value interval of the AFE and the open circuit voltage-remaining capacity characteristics of the battery; step 4, starting balancing when the start voltage Vs is greater than the balancing switch threshold Vt.

Description

Technical Field

[0001] The invention relates to a battery equalization method. Background Art

[0002] Lithium batteries are increasingly used in the field of electric mobility because they are more environmentally friendly, lightweight, and have a longer lifespan than lead-acid batteries. The voltage of lithium battery cells is relatively low, between 2V and 4.5V, while the batteries used in electric vehicles in the field of mobility are mostly above 24V, so multiple cells need to be connected in series to increase the total output voltage to meet the vehicle's voltage requirements.

[0003] Lithium batteries have very strict requirements on the operating voltage range. Taking a certain type of ternary lithium battery as an example, its operating voltage range is not allowed to be lower than 2.5V or higher than 4.2V. Therefore, when used in series, if the voltages of the cells are not equal (i.e. unbalanced), then during charging or discharging, the voltage of a certain string of cells will be too high or too low, resulting in incomplete charging or discharging, causing the overall capacity of the battery pack to be low, affecting user use.

[0004] However, due to differences in manufacturing, working environment, or the capacity consumed by each string of lithium batteries in the battery management system (BMS), imbalance may occur. To solve this problem, the BMS usually needs to have a function that can balance each string of lithium batteries (i.e., balancing function).

[0005] In order to achieve the purpose of balancing, there are two main methods currently used: active balancing and passive balancing. Active balancing uses energy transfer, that is, transferring the capacity of high-capacity batteries to low-capacity batteries; passive balancing uses energy consumption, that is, consuming the capacity of high-capacity batteries through resistance. Active balancing has high efficiency, but the system is complex and costly, and is suitable for areas with large battery capacity and not very cost-sensitive; passive balancing has low balancing efficiency, but the system is simple and cost-effective, and is more suitable for areas with small capacity but cost-sensitive.

[0006] The current mainstream passive balancing methods are:

[0007] 1. Overvoltage balancing, that is, when the voltage of a certain string of lithium batteries is detected to be higher than the set value, the balancing circuit is controlled to work, consuming the capacity of the string of lithium batteries. The advantage of this method is that the circuit is simple, and there is no need to pay attention to the voltage of other lithium batteries. A simple voltage comparison can be achieved. Most of the current analog protection chips use this method. The disadvantages of this method are also obvious, mainly in the following three points:

[0008] (1) The so-called balancing function should only work when there is an imbalance between the battery strings, rather than being forced to work when the voltage is higher than a certain value, which causes unnecessary energy waste;

[0009] (2) Because it can only work when it is set above a certain voltage, the setting value cannot be too low. If it is too low, too much electricity will be discharged, resulting in a lower output capacity of the battery pack. The problem caused by this is that the balancing circuit can work for too short a time and the balancing effect is not obvious. Although the method proposed in the patent (CN201110092543.7) solves the problem of short balancing time, it requires close cooperation with the charger, which limits the applicability of the method;

[0010] (3) Device errors may cause an originally balanced battery pack to become unbalanced. For example, if the voltage sampling error of a device is ±20mV (this indicator is a good parameter for this type of device), and the balancing start threshold is set to 4200mV (taking ternary lithium batteries as an example, the charging cut-off voltage is 4200mV), it is possible that the originally balanced batteries (the actual voltage of each battery cell is 4200mV) will mistakenly start balancing because the device thinks that the current voltage has been reached or not reached, ultimately resulting in a voltage difference of 40mV.

[0011] 2. Voltage difference balancing, this method is currently adopted by most digital protection chips (AFE), which effectively overcomes the shortcomings of the "overvoltage balancing" method. This method requires that the voltage is higher than the set value and the difference between the highest string battery voltage and the lowest string battery voltage is higher than the set value. The balancing will only be started when both conditions are met. Therefore, a lower start-up voltage can be set to make the balancing time longer. At the same time, by setting a suitable start-up threshold, false start-up caused by device errors can be avoided. As in the example described above, the voltage difference balancing can set the balancing start-up voltage to 4100mV and the start-up voltage difference to 30mV. However, the shortcomings of this method are still obvious, as follows:

[0012] (1) This method usually sets the equalization on and off voltage difference thresholds to the same value. However, some solutions can set the on value and the off value to different values. However, due to the sampling accuracy and drift of the AFE, the off value is not allowed to be set to 0mV. Therefore, the best result of equalization is to control each string of batteries within the voltage difference threshold (such as 30mV). For lithium batteries (especially lithium iron phosphate batteries), the capacity difference corresponding to this difference may be large (more than 50% of the full charge capacity). If you want to reduce this threshold, you can only use an AFE with higher accuracy, and such AFE is often expensive.

[0013] (2) Even for ordinary AFE, after calibration at room temperature (25℃±10℃), the voltage difference of each string collected can be made very small (e.g. within 5mV). However, because ordinary voltage difference equalization methods do not introduce other conditions, they can only set the threshold value according to the maximum error value marked in the chip specification (for the same chip, this value is ±15mV, that is, the threshold value is 30mV), which greatly reduces the equalization effect.

[0014] (3) It is necessary to set the equalization start voltage to prevent the battery pack from being exhausted due to the ADC sampling noise of the AFE after the equalization is turned on. In this way, the equalization process only occurs when the battery pack has a high power level, while in actual use, the power level is often not fully charged (such as between 20% and 80%), which results in a shorter time for equalization. What's worse, an option of equalization only during charging is added to this equalization method to prevent the battery pack from being exhausted due to the constant equalization.

[0015] (4) Lithium batteries are in a very good equilibrium state after discharge, because the voltage difference at the end of discharge will be very obvious, which means that as long as all the batteries are fully discharged, the battery pack can be in a balanced state (the remaining capacity of all batteries is 0%). However, the voltage difference method directly abandons this equilibrium state due to the above factors;

[0016] 3. Capacity balancing: This method calculates the capacity difference of each battery string under appropriate conditions based on the battery voltage and remaining capacity characteristics, and then integrates the balancing current and time of each battery string in the subsequent use process to consume or transfer the capacity difference of each string. This method has a good balancing effect, but requires the processor to perform a large number of calculations on the balancing process. The algorithm is very complex, and because the calculation amount is recalculated, it places very high reliability requirements on the algorithm itself, sampling state judgment, and processor memory, which is difficult to implement in a system where processor resources are already tight. Summary of the invention

[0017] The present application proposes a battery balancing method, which aims to provide a passive balancing method to solve the problems of power waste, battery exhaustion due to AFE sampling accuracy and drift, and complex calculation in the prior art.

[0018] In order to achieve the above technical solution, this application adopts the following technical solution:

[0019] A battery balancing method comprises the following steps: step 1, obtaining the voltage and capacity information of each battery string, and recording the lowest voltage Vl of the battery string; obtaining the information of AFE, and obtaining the precision value interval of the AFE used as the balancing switch threshold Vt; step 2, dividing the cell voltage into different voltage zones according to the open circuit voltage-remaining capacity characteristics of each battery and the voltage sampling precision characteristics of AFE; step 3, obtaining the voltage of each battery string at a fixed frequency, recording the battery with the lowest voltage as Bn, and recording the highest voltage Vh; according to the precision value interval of AFE, presetting each cell The balanced start-up voltage Vs of the voltage zone is the voltage difference between the highest voltage Vh and the lowest low voltage Vl; the balanced shutdown voltage Vc of each voltage zone is preset according to the precision value interval of AFE and the open circuit voltage-remaining capacity characteristics of the battery; step 4, when the start-up voltage Vs is greater than the balanced switch threshold Vt, start balancing; compare the voltage of each battery string with the voltage of the battery Bn where the lowest voltage is recorded, and balance the voltage of each battery string to the voltage of the battery Bn where the lowest voltage is located; step 5, when the voltage of the remaining battery strings reaches the balanced shutdown voltage Vc, stop balancing.

[0020] Preferably, in step 2, the method for dividing the battery cell voltage into different voltage zones is: measuring the current voltage of each battery string; judging whether the difference between the residual voltage SOC value corresponding to the battery Bn where the lowest voltage is located and the residual voltage SOC value corresponding to the sum of the voltage of the battery string Bn and the balancing switch threshold value Vt is less than 0.1%; if it is less than 0.1%, it is one voltage zone; otherwise, it is another voltage zone.

[0021] Preferably, if the SOC value difference is less than 0.1%, the balancing shutdown voltage Vc is set to 0mV, that is, the balancing termination condition is that the voltages of all other battery strings are not higher than the voltage of the battery Bn where the lowest voltage is located; otherwise, the balancing shutdown voltage is set to Vc1, that is, the termination condition is that the voltages of all other battery strings are not higher than the sum of the voltage of the battery Bn where the lowest voltage is located and the balancing switch threshold voltage Vt.

[0022] Preferably, when the voltage of each battery is obtained at a fixed frequency, the balancing is stopped; and when the voltage of each battery is stopped, the balancing is continued.

[0023] Due to the adoption of the above technical solution, the present invention uses the lowest voltage as the standard for balancing, and there is no need to worry about the balancing being carried out continuously due to AFE sampling drift, thereby consuming the battery; and different balancing conditions are set for different voltage intervals, so the entire battery voltage interval can be balanced, which greatly extends the balancing time; this method is simple to calculate and only relies on one recorded quantity, and all other parameters are obtained in real time, which greatly reduces the reliability requirements for the processor and memory. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a schematic diagram of the process of the patent of this invention;

[0025] Figure 2 This is the sampling accuracy value table of AFE;

[0026] Figure 3 FIG. 4 is a graph showing an open circuit voltage-remaining capacity characteristic of a battery according to an embodiment of the present invention. DETAILED DESCRIPTION

[0027] The technical solution of the present invention is further specifically described below through embodiments and in conjunction with the accompanying drawings.

[0028] The battery balancing method of the present invention comprises the following steps:

[0029] Step S1, obtain the voltage and capacity information of each battery string, record the lowest voltage Vl in all battery strings, obtain the AFE information, and obtain the precision value interval of the AFE used as the balancing switch threshold Vt.

[0030] Because the sampling accuracy of AFE can be obtained from the data sheet, the value will be different in different temperature and voltage ranges. For example, for the following chip, because the cell voltage range is 2V to 4.5V, the value only needs to be taken according to the temperature, see Figure 2 :

[0031] (1) 22℃~28℃, Vt is 10mV;

[0032] (2) 0℃~60℃, Vt is 20mV;

[0033] (3) -40℃~85℃, Vt is 30mV.

[0034] According to the data sheet, the balancing switch threshold Vt is determined.

[0035] Step S2, dividing the cell voltage into different voltage zones according to the open circuit voltage-remaining capacity characteristics of each battery and the voltage sampling accuracy characteristics of the AFE.

[0036] The method of dividing the cell voltage into different voltage zones is:

[0037] S21 measures the current voltage of each battery string;

[0038] S22 determines whether the difference between the residual voltage SOC1 value corresponding to the battery Bn where the lowest voltage is located and the residual voltage SOC2 value corresponding to the sum of the voltage of the battery string Bn and the balancing switch threshold value Vt is less than 0.1%; if it is less than 0.1%, it is one of the voltage zones; otherwise, it is another of the voltage zones.

[0039] See also Figure 3 As shown, it is an open circuit voltage-remaining capacity characteristic diagram of a battery of an embodiment. In the figure, the battery voltage and the remaining capacity are not in a linear relationship. When the battery cells are at different voltages, the battery capacity represented by the voltage difference is different. In the figure below, the X-axis represents the open circuit voltage and the Y-axis represents the remaining capacity. The entire open circuit voltage-remaining capacity characteristic diagram can be roughly divided into the first and third zones with relatively small differences, the fourth and fifth zones with large differences, and the second zone in the middle, which can belong to the zone with small differences or the zone with large differences, based on the relationship between the remaining voltage capacity and the corresponding voltage difference.

[0040] Step S3, obtain the voltage of each battery string at a fixed frequency, record the battery with the lowest voltage as Bn, and record the highest voltage Vh; preset the balanced start voltage Vs of each voltage zone according to the precision value range of AFE; preset the balanced shutdown voltage Vc of each voltage zone according to the precision value range of AFE and the open circuit voltage-remaining capacity characteristics of the battery.

[0041] The value of the balanced start voltage Vs is the current voltage difference, that is, the voltage difference between the current highest voltage Vh and the lowest low voltage Vl. The balanced start voltage Vs is preset according to the initial detection data.

[0042] If the SOC value difference is less than 0.1%, the balancing shutdown voltage Vc is set to 0mV, that is, the balancing end condition is that the voltages of all other battery strings are not higher than the voltage of the battery Bn where the lowest voltage is located; otherwise, the balancing shutdown voltage is set to Vc1, that is, the end condition is that the voltages of all other battery strings are not higher than the sum of the voltage of the battery Bn where the lowest voltage is located and the balancing switch threshold voltage Vt.

[0043] Even if the AFE sampling accuracy is poor, resulting in over-balancing (i.e., it was already balanced, but due to sampling errors, it was over-balanced), its impact on the actual capacity of the battery cell does not exceed 0.1%.

[0044] Step S4, when the equalization start voltage Vs is greater than the equalization switch threshold Vt, equalization starts. Compare the voltage of each battery string with the voltage of the battery Bn where the lowest voltage is recorded, and equalize the voltage of each battery string to the voltage of the battery Bn where the lowest voltage is located. The equalization method can be active or passive, and is not limited. Change the purpose of equalization to: equalize the voltage of all other battery strings to a voltage close to the voltage of the battery string number Bn with the lowest voltage recorded. The advantage of this is that there is no need to worry about whether the equalization will continue due to AFE sampling drift and thus kill the battery. When the voltage of the remaining battery strings in the same voltage zone reaches the voltage of the battery string Bn where the lowest voltage is located, stop balancing.

[0045] When the voltage of each battery string is obtained at a fixed frequency, it does not necessarily have to be a very high frequency. The voltage can be obtained at a lower frequency, such as once every 60 seconds, and balancing is stopped when the voltage is obtained to ensure that the obtained voltage is not affected by the balancing effect.

[0046] Step S5: When the voltage of the remaining battery strings reaches the balancing shutdown voltage Vc, the balancing is stopped.

[0047] This application sets different voltage zones, and uses the voltage of the battery string with the lowest voltage in each zone to be compared in real time with the voltage in the initial stage, and uses the battery string with the lowest voltage recorded as the comparison object, so there is no need to worry about whether the equalization will continue to be carried out due to AFE sampling drift, thereby killing the battery. Because there is no need to worry about killing the battery, and different equalization conditions are set for different voltage intervals, each voltage interval of the entire battery voltage can be equalized, and the equalization can work for a longer time and the equalization effect is better; this method is simple to calculate, and only relies on one record, and all other parameters are obtained in real time, which greatly reduces the reliability requirements for the processor and memory.

[0048] The above-described embodiments are only used to illustrate the present invention and are not used to limit the scope of the present invention. Equivalent changes and modifications made to the present invention by those skilled in the art should all fall within the scope of the claims attached to the present invention.

Claims

1. A battery balancing method, characterized in that: The following steps are involved: Step 1, obtain the voltage and capacity information of each battery string, and record the lowest voltage Vl of the battery string; obtain the information of AFE, and obtain the precision value interval of the AFE used as the balancing switch threshold Vt; Step 2: Divide the cell voltage into different voltage zones according to the open circuit voltage-remaining capacity characteristics of each battery and the voltage sampling accuracy characteristics of the AFE; Step 3, obtain the voltage of each battery string at a fixed frequency, record the battery with the lowest voltage as Bn, and record the highest voltage Vh; according to the precision value range of AFE, preset the balanced start voltage Vs of each voltage zone as the voltage difference between the current highest voltage Vh and the lowest low voltage Vl; preset the balanced shutdown voltage Vc of each voltage zone according to the precision value range of AFE and the open circuit voltage-remaining capacity characteristics of the battery; Step 4, when the equalization start voltage Vs is greater than the equalization switch threshold Vt, equalization starts; the voltage of each battery string is compared with the voltage of the battery Bn where the lowest voltage is recorded, and the voltage of each battery string is equalized to the voltage of the battery Bn where the lowest voltage is located; Step 5: When the voltage of each battery string reaches the balancing shutdown voltage Vc, stop balancing.

2. The battery equalization method according to claim 1, characterized in that: In step 2, the method of dividing the cell voltage into different voltage zones is: Measure the current voltage of each battery string; Determine whether the difference between the residual voltage SOC value corresponding to the battery Bn where the lowest voltage is located and the residual voltage SOC value corresponding to the sum of the voltage of the battery string Bn and the balancing switch threshold value Vt is less than 0.1%; if less than 0.1%, it is a voltage zone; Otherwise, it is another voltage range.

3. The battery equalization method according to claim 2, characterized in that: If the SOC value difference is less than 0.1%, the balancing shutdown voltage Vc is set to 0 mV, that is, the balancing end condition is that the voltages of all other battery strings are not higher than the voltage of the battery Bn where the lowest voltage is located; Otherwise, the balancing shutdown voltage is set to Vc1, that is, the termination condition is that the voltages of all other battery strings are not higher than the sum of the voltage of the battery Bn with the lowest voltage and the balancing switch threshold Vt voltage.

4. The battery equalization method according to claim 1, characterized in that: When the voltage of each battery is obtained at a fixed frequency, balancing is stopped; when the voltage of each battery is stopped, balancing is continued.

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