An estimation method for the safety margin of a lithium battery pack

Abstract

The present invention discloses a method for estimating the safety margin of a lithium battery pack, which relates to the technical field of batteries. By collecting the dynamic changes of the external excitation of the battery pack, a sampling value array of the current of the battery pack and the voltage of each battery cell during the dynamic change of the external excitation of the battery pack is obtained. After sorting and comparing each group in the sampling value array of the battery cell voltage, the voltage imbalance degree is calculated; after performing a ratio operation on the increment of the battery cell voltage in two adjacent groups of data in the sampling value array of the battery cell voltage and the absolute value of the current increment in a sampling value array of a group of battery pack currents, the battery cell voltage coherence coefficient is calculated; after performing operations on three adjacent groups of data of the same battery cell voltage in the sampling value array of the battery cell voltage, the safety coefficient of the battery cell is calculated; according to the results calculated by synchronous operation, the safety margin value of the current lithium battery pack is obtained; the current safety of the lithium battery pack is quantitatively evaluated in real time through the safety margin parameter value of the battery pack.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and particularly relates to a method for estimating the safety margin of a lithium battery pack. Background Art

[0002] With the continuous popularization of the application of lithium batteries, the safety of lithium battery packs has attracted more and more attention from users. Especially in the field of electric transportation, once a safety accident such as thermal runaway occurs in a lithium battery pack, it often brings heavy casualties to people and property.

[0003] At present, the real-time safety judgment of batteries mainly judges the temperature of the internal battery cells of the battery pack. If the temperature reaches the upper limit value, safety warnings will be given and the output of the battery pack will be cut off. In some application scenarios with higher safety requirements, such as lithium battery packs for electric vehicles, a more complex battery pack heat dissipation control system will be adopted to keep the battery temperature within a constant range by adjusting the speed of the fan or the circulation pump, so as to avoid the occurrence of battery thermal runaway. However, this solution will significantly increase the complexity and cost of the system, and cannot accurately quantitatively evaluate the real-time safety of the battery pack, thus limiting its application scenarios.

[0004] Limited by the current technical bottleneck, the safety of lithium battery packs cannot be as stable as that of other types of chemical batteries such as lead-acid batteries. Therefore, how to solve the safety of lithium battery packs during use has become a major pain point in the application of the lithium battery industry. At present, the feasible related technologies are divided into active safety protection and passive safety protection. The so-called passive safety protection is to improve the shell of the lithium battery, the installation process and the production and manufacturing process of the lithium battery cells, so that it is not easy to generate safety problems such as thermal failure and explosion. Even if they occur, the losses caused can be minimized. Active safety protection is to predict the possibility of battery thermal failure, so as to issue warnings or take battery power-off measures to avoid battery safety accidents. However, this method requires accurate judgment of the real-time safety performance of the battery, and there is no mature and reliable judgment method at present. Therefore, a method for estimating the safety margin of a lithium battery pack is proposed to solve the above problems. Summary of the Invention

[0005] Aiming at the deficiencies of the prior art, the present invention provides a method for estimating the safety margin of a lithium battery pack to solve the problems existing in the prior art.

[0006] To achieve the above object, the present invention is realized through the following technical solutions:

[0007] A method for estimating the safety margin of a lithium battery pack includes the following steps:

[0008] By collecting and recording the changes in the battery pack current and the cell voltages during the dynamic change of the external excitation of the battery pack, multiple arrays of sampled values of the battery pack current and the cell voltages are obtained;

[0009] Sort and compare the arrays of sampled values of the cell voltages for each group, obtain the absolute value coefficient of the deviation between the maximum and minimum values of the cell voltages, and calculate the voltage imbalance degree of the battery pack;

[0010] Perform a ratio operation on the absolute value of the increment of the cell voltages in the arrays of sampled values of the cell voltages for two adjacent groups and the increment of the current in the array of sampled values of a battery pack current, obtain the dynamic impedance coefficient of each cell, and calculate the coherence coefficient of the cell voltages of the battery pack;

[0011] After performing operations on three adjacent sets of data in the sampling array of the cell voltages of a group, and then performing operations on the sampled values of the cell voltages of this group, two process parameters are obtained, and the safety coefficient of the battery pack cells is calculated;

[0012] According to the voltage imbalance degree of the battery pack, the coherence coefficient of the cell voltages of the battery pack, and the safety coefficient of the battery pack cells obtained after synchronous calculation, calculate the safety margin value of the current lithium battery pack.

[0013] Furthermore, the battery pack voltage and current acquisition process provides a time reference for the external excitation loading process, and the total execution time tp is obtained.

[0014] Furthermore, the absolute value coefficient of the deviation Kvd between the maximum and minimum values of the cell voltages is:

[0015]

[0016] Where k is the number of cells in the battery pack, Vco is the array of sampled values of the cell voltages of n groups with a length of k, Vavg is the arithmetic mean of all Vco values in this group, and Vdmax is the absolute value of the deviation between the maximum and minimum values of Vco within this group.

[0017] Furthermore, the voltage imbalance degree Kub of the battery pack is:

[0018]

[0019] Furthermore, the expression of the dynamic impedance coefficient Krd of each cell is:

[0020]

[0021] Where Ib is the battery pack current.

[0022] Furthermore, the expression of the coherence coefficient Kco of the cell voltages of the battery pack is:

[0023]

[0024] Among them, Krdmax and Krdmin are the maximum and minimum values in a set of Krd with a length of k.

[0025] Furthermore, the expression of the process parameter K1 is:

[0026]

[0027] The expression of the process parameter K2 is:

[0028]

[0029] The expression of the battery pack cell safety factor Ksc is:

[0030]

[0031] Furthermore, the expression of the battery pack safety margin Ms is:

[0032] M s = K c1 *(1 - K ub ) + K c2 *(1 - K co ) + (1 - K c1 - K c2 )*(1 - K sc )

[0033] Among them, Kc1 and Kc2 are weighting coefficients.

[0034] Furthermore, the calculation of the battery pack cell voltage imbalance degree, the calculation of the battery pack cell voltage coherence coefficient, and the calculation of the battery pack cell safety factor are executed in parallel, and the battery pack external excitation loading and the battery pack voltage and current acquisition are executed synchronously.

[0035] The present invention provides a method for estimating the safety margin of a lithium battery pack, having the following beneficial effects:

[0036] (1) By using the safety margin parameter value of the battery pack to quantitatively evaluate the current safety of the lithium battery pack in real time, and without relying on the temperature value of the battery pack cells, the battery management device can determine, based on the interval in which this value is located, the maximum current at which the current lithium battery pack can be charged and discharged without affecting the use safety of the battery; especially when the lithium battery pack may undergo thermal runaway, this parameter value will change significantly, and users can, based on this change, cut off the input and output of the lithium battery pack in advance, start the cooling measures, and issue a fault warning, thereby reducing the probability of lithium battery accidents;

[0037] (2) By adopting the technical solution proposed by the present invention, the problem of safety performance evaluation during the use of lithium battery packs can be effectively solved, enabling the safety evaluation results to be quantified and highly real-time;

[0038] (3) Accurately evaluate the current usage safety of lithium batteries, while fully considering the differences of each cell in the battery pack;

[0039] (4) Quantify the usage safety of the lithium battery pack into the safety margin index of the battery pack, enabling users to intuitively understand the real-time safety of the lithium battery pack;

[0040] (5) The calculation of the safety margin of the lithium battery pack does not rely on the temperature sensor of the battery pack, can predict the possible thermal runaway risk of the lithium battery pack, and belongs to the active safety protection technology of the lithium battery pack; at the same time, the calculation process of the safety margin of the lithium battery pack is faster and can be completed in seconds. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 It is a schematic diagram of the calculation process of the safety margin of the lithium battery pack of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0042] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments.

[0043] To accurately evaluate the real-time safety performance of the lithium battery pack, the present invention provides a method for calculating the safety margin of the lithium battery pack.

[0044] As Figure 1 shown, the calculation process of the safety margin of the lithium battery pack consists of the battery pack external excitation loading process, the battery pack voltage and current acquisition process, the battery pack cell voltage imbalance calculation process, the battery pack cell voltage coherence coefficient calculation process, the battery pack cell safety factor calculation process, and the battery pack safety margin calculation process; the working sequence of each process is: first execute the battery pack initial voltage and current acquisition process, and then sequentially execute the battery pack external excitation loading process and the battery pack voltage and current acquisition process. Among them, the battery pack voltage and current acquisition process provides time feedback for the battery pack external excitation loading process to ensure that the latter completes the change of the battery pack external excitation within the specified time according to the strict time sequence requirements, and accurately records the voltage sum of each cell in the battery pack and the battery pack current corresponding to each excitation change. After the data recording meets the requirements, the battery pack cell imbalance calculation process, the battery pack cell voltage coherence coefficient calculation process, and the battery pack cell safety factor calculation process are synchronously started, and the operation results generated by the three processes are synchronously output to the battery pack safety margin calculation process, so as to calculate the safety margin value of the current lithium battery pack.

[0045] The external excitation loading process of the battery pack is to dynamically change the external loading load of the battery pack, causing regular changes in the voltage and current of the battery pack. The time of this loading process is controlled by the battery pack voltage and current acquisition process, and the change law of the loading load presents a one-to-one corresponding functional law with time.

[0046] The described battery pack voltage and current acquisition process real-time collects and records the change situation of the battery pack current Ib and the change situation of each cell voltage Vco during the dynamic change of the external excitation of the battery pack, where the k value corresponds to the number of cells in the battery pack; at the same time, this acquisition process also provides a time reference for the external excitation loading process. At the end of this process, it has completed n groups of voltage and current samplings, obtaining an array of Ib sampling values with a length of n, n arrays of Vco sampling values with a length of k, and the total execution time tp of this process. This process will synchronously transmit the above data to the battery pack cell imbalance degree calculation process, the battery pack cell voltage coherence coefficient calculation process, and the battery pack cell safety factor calculation process.

[0047] The battery pack cell voltage imbalance degree calculation process sorts and compares each group in the n groups of Vco sampling value arrays with a length of k, finds the absolute value of the deviation Vdmax between the maximum and minimum values of Vco in this group. If this value is 0, then the deviation absolute value weight coefficient Kvd = 1; otherwise, take the arithmetic mean of all Vco values in this group to obtain Vavg; subsequently, the expression of the deviation absolute value weight coefficient Kvd of each group of Vco data can be obtained as follows:

[0048]

[0049] For the n groups of Vco data, n Kvd values can be obtained, and then the expression of the voltage imbalance degree Kub of the battery pack can be obtained as follows:

[0050]

[0051] The calculated Kub parameter value will be output to the battery pack safety margin calculation process.

[0052] The battery pack cell voltage coherence coefficient calculation process is to perform a ratio operation on the absolute value of the increment ΔVco of Vco in two adjacent groups of data in the n groups of Vco sampling value arrays with a length of k and the increment ΔIb of the current Ib in an array of Ib sampling values with a length of n. When ΔIb is not zero, the expression of the dynamic impedance coefficient Krd of each cell can be obtained as follows:

[0053]

[0054] If ΔIb is zero, then Krd is zero. After calculating Krd for each cell in the battery pack, a set of Krd values of length k is obtained; this set of data is sorted to find the maximum value Krdmax and the minimum value Krdmin. If the two are equal, the cell voltage coherence coefficient Kco of the battery pack is obtained as Kco = 0. Otherwise, the expression for Kco can be obtained as follows:

[0055]

[0056] The calculated Kco parameter value is output to the battery pack safety margin calculation process.

[0057] The calculation process for the cell safety factor of the battery pack is as follows. By performing the following operation on the Vco values of the same cell voltage in three adjacent groups in the n sets of Vco sampling value arrays of length k, the expression for the process parameter K1 can be obtained as follows:

[0058]

[0059] Then, by performing the following operation on the n sets of Vco sampling values of length k, the expression for the process parameter K2 can be obtained as follows:

[0060]

[0061] Among them, K2 = f(V) listed in the expression is a pre-built data table, whose values are related to the battery type and are obtained through experimental data measurement; the input of the table is the cell voltage, and the output is the process parameter K2. On this basis, the expression for the cell safety factor Ksc of the battery pack can be obtained as:

[0062]

[0063] The calculated Ksc parameter value is output to the battery pack safety margin calculation process.

[0064] The calculation process for the battery pack safety margin is as follows. By performing the following operation on the previously calculated Kub, Kco, and Ksc, the expression for the battery pack safety margin Ms can be obtained as:

[0065] M s =K c1 *(1 - K ub ) + K c2 *(1 - K co ) + (1 - K c1 - K c2 )*(1 - K sc )

[0066] Among them, Kc1 and Kc2 are weighting coefficients, which are different for different battery types.

[0067] The specific implementation is as follows:

[0068] In this embodiment, the calculation of the safety margin parameter value of the lithium battery pack consists of 6 processes, namely the external excitation loading process of the battery pack, the voltage and current acquisition process of the battery pack, the calculation process of the voltage imbalance degree of the battery pack cells, the calculation process of the voltage coherence coefficient of the battery pack cells, the calculation process of the safety factor of the battery pack cells, and the calculation process of the safety margin of the battery pack; in the above processes, the calculation process of the voltage imbalance degree of the battery pack cells, the calculation process of the voltage coherence coefficient of the battery pack cells, and the calculation process of the safety factor of the battery pack cells are executed in parallel; the external excitation loading process of the battery pack and the voltage and current acquisition process of the battery pack are executed synchronously.

[0069] In this embodiment, the number of cells in the lithium battery pack is 20, the battery type is lithium iron phosphate battery, and the capacity is 50Ah. The excitation loading signal adopted in the external excitation loading process of the battery pack is a square wave signal with a frequency of 100Hz, and the battery is controlled to discharge with a pulsed current of 50A through a PWM switch circuit for a duration of 3 seconds; during this period, the voltage and current acquisition process of the battery pack works synchronously, samples the voltage and current of the battery pack at a sampling rate of 1KHz, and obtains a total of 3000 groups of current data Ib and 3000×20 groups of cell voltage data Vco, and transmits this data in parallel to the calculation process of the voltage imbalance degree of the battery pack cells, the calculation process of the voltage coherence coefficient of the battery pack cells, and the calculation process of the safety factor of the battery pack cells.

[0070] In this embodiment, the calculation process of the voltage imbalance degree of the battery pack cells sorts every 20 cell voltage data in the 3000×20 groups of cell voltage data Vco, and 3000 groups of Vcomax and Vcomin data can be obtained, and on this basis, 3000 groups of deviation absolute value weight coefficient Kvd are obtained according to the foregoing formula, and then the voltage imbalance degree parameter value Kub of the battery pack is obtained.

[0071] In this embodiment, the calculation process of the voltage coherence coefficient of the battery pack cells calculates according to the foregoing formula for the 3000×20 groups of cell voltage data Vco and 3000 groups of battery current data Ib, and obtains an array of dynamic impedance coefficient values Krd with a length of 20; and further calculates the voltage coherence coefficient value Kco of the battery pack according to the formula.

[0072] In this embodiment, the calculation process of the safety factor of the battery pack cells calculates the adjacent three groups of data for the 3000 groups of Vco data of each cell according to the foregoing expression, and the process parameter K1 can be obtained; K2 is obtained by taking the highest cell voltage of the battery pack calculated according to the foregoing formula as the independent variable and looking up the pre-established data table K2 = f(V); the final safety factor Ksc of the battery pack cells is the ratio of the two.

[0073] In this embodiment, the calculation process of the safety margin of the battery pack is obtained by calculating Kub, Kco, and Ksc using the foregoing expressions. The battery pack tested in this embodiment is of the lithium iron phosphate type. According to the experimental data, the weight coefficients in the foregoing expressions are Kc1 = 0.52 and Kc2 = 0.13.

[0074] A new battery safety performance evaluation method based on the safety margin value of the lithium battery pack is designed. Different from the existing technical solutions, it does not rely on the temperature of the battery pack sensed by the temperature sensor at all, nor does it maintain the temperature of the battery pack within a reasonable range through thermal cycle control. Instead, it samples the current of the battery pack and the voltage of each internal battery cell, and combines the designed parameter evaluation algorithm to obtain the quantified safety margin value of each battery pack in real time. The value is between 0 and 1. By comparing this safety margin value, we can judge the safety of the current battery pack. The higher the value, the safer the current battery pack is for use and it can be charged and discharged at full power. On the contrary, depending on the value, different operations such as limiting the input and output power, alarming, and cutting off the battery switch can be performed, so as to actively protect against safety accidents of the lithium battery pack and improve the safety of use of the lithium battery pack.

[0075] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, makes equivalent substitutions or changes, and all should be covered within the protection scope of the present invention.

Claims

1. A method for estimating the safety margin of a lithium battery pack, characterized in that, It includes the following steps: By collecting and recording the changes in the battery pack current and cell voltage during the dynamic change of the external excitation of the battery pack, multiple groups of sampled value arrays of the battery pack current and sampled value arrays of the cell voltage are obtained; Sort the cell voltage data in the sampling value array of each group of cell voltages to obtain the absolute deviation coefficient of the maximum and minimum values of the cell voltages, and calculate the voltage imbalance of the battery pack; the absolute deviation coefficient of the maximum and minimum values of the cell voltages Kvd is as follows: Among them, k The value pairs the number of battery cells in the battery pack, Vco is an array of sampling values of the battery cell voltages with a length of k for n groups, Vavg is the arithmetic mean of all Vco values in this group, Vd max is the absolute value of the deviation between the maximum and minimum values within this group; the voltage imbalance of the battery pack Vco is: Kub is: ； Take the maximum value after performing a ratio operation on the increment of the cell voltage in two sets of adjacent data in the sampled value array of the cell voltage and the absolute value of the current increment in the sampled value array of the current of a battery pack to obtain the dynamic impedance coefficient of each cell, and calculate the coherence coefficient of the cell voltage of the battery pack; the dynamic impedance coefficient of each cell Krd The expression is: Among them, Ib is the battery pack current; Perform an operation on three adjacent sets of data of the same cell voltage in each sampled array of cell voltages, then perform a ratio operation on the maximum value and the minimum value of the operation result to obtain a first process parameter. Then, by performing an operation on the sampled values of the cell voltages and selecting the highest cell voltage of the battery pack as the independent variable, a second process parameter is obtained, and the safety factor of the battery pack cells is calculated; According to the battery pack voltage unbalance degree, the coherence coefficient of the battery pack cell voltages, and the safety factor of the battery pack cells obtained after synchronous calculation, the safety margin value of the current lithium battery pack is calculated.

2. The estimation method of the safety margin of a lithium battery pack according to claim 1, characterized in that, The cell voltage coherence coefficient of the battery pack Kco The expression is as follows: Among them, Krd max and Krd min are the maximum and minimum values of a group with a length of k in Krd respectively.

3. The estimation method of the safety margin of a lithium battery pack according to claim 2, wherein, The first process parameter K The expression of 1 is: The second process parameter K The expression of 2 is: The safety factor of the battery pack cell K sc The expression is: 。 4. The estimation method of the safety margin of a lithium battery pack according to claim 3, characterized in that, The battery pack safety margin Ms is expressed as: Among them, K c1 and K c2 are weight coefficients.

5. The estimation method for the safety margin of a lithium battery pack according to claim 1, wherein The calculation of the battery pack cell voltage unbalance degree, the calculation of the battery pack cell voltage coherence coefficient, and the calculation of the battery pack cell safety factor are executed in parallel, and the loading of the battery pack external excitation and the collection of the battery pack voltage and current are executed synchronously.

Images