Method for estimating maximum allowable power of single lithium ion battery

Abstract

The invention discloses a method for estimating the maximum allowable power of a single lithium ion battery. The method comprises the following steps: S1, calibrating the reference capacity C0 of the single battery; s2, measuring the corresponding relation between the open-circuit voltage and the state of charge of the single battery at the target temperature, and recording the corresponding relation as OCV = f (SOC); s3, at the same target temperature in the step S2, measuring the dynamic DC internal resistance under different charge states and currents, and establishing a mapping relation DCR = R (SOC, I); s4, recording the SOC, needing to be evaluated, of the single battery as SOC1, allowing the single battery to stand at the target temperature to reach heat balance, and calculating the maximum allowable current Im by combining the allowable cut-off voltage Ucut, the pulse duration t and the reference capacity C0 at the target temperature; s5, calculating the maximum allowable power Pm according to the Im and the allowable cut-off voltage Ucut; the method has the advantage that the maximum allowable power of the single lithium ion battery can be obtained more accurately.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, and in particular relates to a method for estimating the maximum allowable power of a single lithium-ion battery cell. Background Technology

[0002] Lithium-ion batteries are the core power component of new energy vehicles, and their power characteristics directly determine the vehicle's acceleration and braking performance. The vehicle controller and battery management system typically need to provide the allowable power value of the power battery system in real time under different states of charge, temperatures, and health conditions to ensure the vehicle's performance and safety.

[0003] In existing technologies, the maximum allowable power is generally obtained through experimentation and calculation. Commonly used methods include:

[0004] 1. Hybrid Pulse Power Test (HPPC): Apply charging and discharging pulses of set amplitude and duration to the battery cell, calculate the dynamic DC internal resistance (DCR) based on the voltage change before and after the pulse, and then calculate the maximum charging / discharging current and corresponding power under a given terminal voltage constraint based on the DCR.

[0005] 2. Trial current method: Preset several current levels, and gradually increase / decrease them in multiple rounds to approximate the battery's voltage / temperature / safety threshold, thereby determining the boundary current and power;

[0006] 3. Trial power method: Preset several power levels and determine the achievable maximum power boundary through multiple rounds of approximation.

[0007] The DCR (Distributed Resistance Rate) relied upon by HPPC (Hypercurrent Pricing Testing) exhibits significant current dependence, meaning that the internal resistance values ​​measured under different pulse currents vary. This leads to a systematic deviation between the power boundary calculated based on a single DCR and the battery's actual capacity. Furthermore, the test current method and test power method can cause a shift in the battery's SOC (State of Charge) during the testing process. Especially under high current or long-term pulse conditions, the SOC changes significantly, causing the battery state at the end of the test to deviate from the initial set point, thus resulting in inaccurate power boundary judgments. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to provide a method for estimating the maximum allowable power of a single lithium-ion battery cell, which can obtain the maximum allowable power of a single lithium-ion battery cell more accurately.

[0009] The technical solution adopted by this invention to solve the above-mentioned technical problems is as follows: a method for estimating the maximum allowable power of a single lithium-ion battery cell, comprising the following steps:

[0010] S1, Calibrate the reference capacity of a single battery cell, C0;

[0011] S2. Measure the relationship between the open-circuit voltage and the state of charge of a single battery cell at the target temperature, denoted as OCV=f(SOC);

[0012] S3. At the same target temperature as in step S2, measure the dynamic DC internal resistance under different states of charge and currents, and establish the mapping relationship DCR=R(SOC,I);

[0013] S4. Denote the SOC of the battery cell to be evaluated as SOC1, and allow it to reach thermal equilibrium at the target temperature. Combine this with the allowable cutoff voltage U at the target temperature. cut Given the pulse duration t and the reference capacity C0, calculate the maximum allowable current I. m ;

[0014] S5, according to I m With the allowable cutoff voltage U cut Calculate the maximum allowable power P m ;

[0015] The target temperature range needs to cover the actual operating temperature range of the power estimation, and temperature points are selected within the range according to a predetermined step size.

[0016] Preferably, step S1 specifically involves calibrating the reference capacity C0 of a single battery cell at an ambient temperature of 25°C.

[0017] Preferably, step S2 specifically involves adjusting the battery's state of charge to multiple preset points by constant current discharge, allowing it to rest at the target temperature until thermal equilibrium is reached, and then measuring its open-circuit voltage. The correspondence between the open-circuit voltage and the state of charge at the target temperature is recorded as a functional relationship OCV=f(SOC).

[0018] Preferably, step S3 specifically includes the following steps:

[0019] S3-1. After each battery cell is adjusted to the preset position, it is left to reach thermal equilibrium.

[0020] S3-2, Applying a current I with a pulse duration of t. a Perform charge / discharge tests and record the pre-pulse voltage U. OCV Pulse termination voltage U end And the pulse end time. If the pulse is completed and the voltage limit threshold is not triggered, proceed to step S3-3. If the voltage limit threshold is triggered before the pulse is completed, proceed to step S3-4.

[0021] S3-3, Calculate I a DC internal resistance DCR a =(U end -U OCV ) / I a Adjusting the current Ia , will current I a +I b Recorded as the new I a Return to step S3-2;

[0022] S3-4, Stop the test, and I a Let I be the set of DCRs. a The set of values ​​is denoted as DCR, and the relationship between DCR, SOC, and I at the target temperature is denoted as DCR=R(SOC,I);

[0023] Wherein, current I a It is positive during charging and negative during discharging. b This is a preset value.

[0024] Preferably, in step S4, the maximum allowable current I m It was calculated as follows:

[0025] ΔSOC=I m ×t÷3600÷C0

[0026] SOC2 = SOC1 + ΔSOC

[0027] Where ΔSOC is the deviation amount, and SOC2 is the state of charge at the end of the pulse;

[0028] To find the OCV2 corresponding to SOC2 and the DCR corresponding to SOC1 at the target temperature, the following formula is established:

[0029] U cut =f(SOC2)+I m ×R(SOC1,I) m )

[0030] Calculate the target temperature and the maximum allowable current I at SOC1. m .

[0031] Preferably, in step S5, the maximum allowable power P m The formula is as follows: P m =U cut ×|I m |

[0032] Preferably, the step size of SOC is 5% or 10%.

[0033] Preferably, the step size of the target temperature range is 10°C or 15°C.

[0034] Compared with the prior art, the advantages of the present invention are as follows:

[0035] 1. By establishing a DCR=R(SOC,I) mapping relationship at the target temperature, the distortion problem caused by HPPC's calculation based on a single DCR is avoided. Compared to iterative approximation using test current / test power, this scheme combines OCV=f(SOC), reference capacity C0, pulse duration t, and voltage constraint U after a single resting period to reach thermal equilibrium. cut I is obtained through parsing / table lookup calculation. m With P m This significantly reduces the impact of SOC offset on the estimation results during the test, and enables more accurate acquisition of the maximum allowable power of a single lithium-ion battery cell.

[0036] 2. Both steps S2 and S3 are calibrated at the same target temperature as the estimation, and the actual working temperature range is covered by a predetermined step size, which reduces temperature cross error. In particular, it shows excellent robustness under extreme low and high temperature conditions, providing an important guarantee for the safe operation of new energy vehicles. Attached Figure Description

[0037] Figure 1 This is a schematic diagram of the process of the present invention;

[0038] Figure 2 The SOC-OCV curve of a single battery cell in Example 1 of this invention at 25°C;

[0039] Figure 3 This is the SOC-OCV curve of a single battery cell at 10°C in Example 2 of this invention. Detailed Implementation

[0040] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0041] Currently, in applications where lithium-ion batteries are the core power unit of new energy vehicles, vehicle controllers and battery management systems need to accurately estimate the maximum allowable charge and discharge power boundary of a single cell or battery pack in real time under different states of charge, ambient temperatures, and battery health conditions, in order to balance power performance and cell safety. This power boundary is usually constrained by multiple factors such as voltage, current, temperature, and state of charge, and its accurate acquisition is crucial for improving the robustness and energy efficiency of the vehicle control strategy. However, existing technologies such as hybrid pulse power characteristic testing, current testing, and power testing suffer from inaccurate power boundary judgments due to the current dependence of dynamic DC internal resistance and continuous state of charge drift, especially under wide temperature range and multiple state of charge conditions. To address these technical problems, this invention proposes a high-precision estimation method that establishes a mapping relationship between dynamic DC internal resistance, state of charge, and current, and couples the state of charge change for iterative calculation of the maximum allowable current, and applies it to a method for estimating the maximum allowable power of a single lithium-ion battery cell.

[0042] Combination Figure 1 A method for estimating the maximum allowable power of a single lithium-ion battery cell includes the following steps:

[0043] S1. Calibrate the reference capacity C0 of a single battery cell: Place the battery cell in a constant temperature environment of 25°C and charge it using a constant current charging mode within the range of 0.1C to 1C until the upper limit voltage U is reached. max Then switch to constant voltage charging mode until the current drops to 0.05C and stop charging. Then let it rest for no less than 30 minutes to ensure that the internal chemical reaction of the battery cells is completely stable. Then discharge the battery to the lower limit voltage U using a constant current discharge mode of 0.1C to 1C. min The discharge capacity was recorded as the reference capacity C0;

[0044] S2. Adjust the battery state of charge to multiple preset points by constant current discharge, and after fully resting in the target temperature until thermal equilibrium is reached, measure its open circuit voltage value. Record the correspondence between the open circuit voltage and the state of charge at the target temperature as a function OCV=f(SOC).

[0045] S3. At the same target temperature as in step S2, measure the dynamic DC internal resistance under different states of charge and currents, and establish the DCR=R(SOC,I) mapping relationship to construct the SOC-I-DCR map, specifically including:

[0046] S3-1. Based on the SOC preset point established in step S2, each battery cell is placed to thermal equilibrium after being adjusted to the preset point.

[0047] S3-2, Applying a current I with a pulse duration of t. a Perform charge / discharge tests and record the pre-pulse voltage U. OCV Pulse termination voltage U end And the pulse end time. If the pulse is completed and the voltage limit threshold is not triggered, proceed to step S3-3. If the voltage limit threshold is triggered before the pulse is completed, proceed to step S3-4.

[0048] S3-3, Calculate I a DC internal resistance DCR a =(U end -U OCV ) / I a Adjusting the current I a , will current I a +I b Recorded as the new I a Return to step S3-2;

[0049] S3-4, Stop the test, and I a Let I be the set of DCRs. aThe set of values ​​is denoted as DCR, and the relationship between DCR, SOC, and I at the target temperature is denoted as DCR=R(SOC,I);

[0050] Wherein, current I a It is positive during charging and negative during discharging. b This is a preset value.

[0051] S4. Denote the SOC of the battery cell to be evaluated as SOC1, and allow it to reach thermal equilibrium at the target temperature. Combine this with the allowable cutoff voltage U at the target temperature. cut Given the pulse duration t and the reference capacity C0, calculate the maximum allowable current I. m The details are as follows:

[0052] ΔSOC=I m ×t÷3600÷C0

[0053] SOC2 = SOC1 + ΔSOC

[0054] Where ΔSOC is the deviation amount, and SOC2 is the state of charge at the end of the pulse;

[0055] To find the OCV2 corresponding to SOC2 and the DCR corresponding to SOC1 at the target temperature, the following formula is established:

[0056] U cut =f(SOC2)+I m ×R(SOC1,I) m )

[0057] Calculate the target temperature and the maximum allowable current I at SOC1. m ;

[0058] S5, according to I m With the allowable cutoff voltage U cut Calculate the maximum allowable power P m The formula is as follows:

[0059] P m =U cut ×|I m |

[0060] In step S1, the battery cell can be charged and discharged at least 3 times. The range of the discharge capacity of the 3 consecutive discharges is less than 1% of the rated capacity. Then, the average value is taken as the reference capacity C0. This can effectively reduce the measurement error caused by the inconsistency of the initial state and can also reflect the actual performance stability of the battery cell to a certain extent.

[0061] In step S2, the SOC step size is selected according to actual needs, preferably 5% or 10%; the target temperature range needs to cover the actual operating temperature range of power estimation, and temperature points are selected within the range according to a predetermined step size. The target temperature step size is selected as 10℃ or 15℃ according to actual needs. For example, if the target temperature range covers -20℃ to 60℃, -20℃, -10℃, 0℃, 10℃, 20℃, 30℃, 40℃, 50℃ and 60℃ can be selected as test temperature points. At each temperature point, the open circuit voltage is measured at different SOC points, and finally a complete OCV=f(SOC) mapping relationship is formed. In addition, the relationship between OCV and SOC at the target temperature can be expressed by table lookup interpolation or polynomial fitting. This embodiment only uses the relationship formula to illustrate and does not limit the specific expression method of the OCV and SOC relationship.

[0062] In step S3, the pulse current I a The initial value is preferably 0.1C, and the adjustment step size is I each time. b The current ranges from 0.05C to 0.2C. For example, at a preset SOC point, if the initial pulse current is 0.1C and the voltage limit threshold is not triggered, the current can be gradually increased to 0.2C, 0.3C, etc., until the trigger condition is met. This process not only quickly approximates the actual power boundary of the battery, but also accurately captures the dynamic characteristics of the battery under different SOCs and temperatures by monitoring voltage changes in real time. The relationship between DCR, SOC, and I at the target temperature can be expressed using table lookup interpolation or polynomial fitting. This embodiment only uses the relationship formula for illustration and does not limit the specific expression method of the relationship between DCR, SOC, and I.

[0063] In step S4, the maximum allowable current I m It is negative when discharging and positive when charging.

[0064] This approach avoids the systematic biases inherent in traditional HPPC single DCR estimation and significantly reduces the impact of SOC offset on test results. Compared to iterative approximation methods such as the test current method or the test power method, this method only requires one static thermal equilibrium period to complete the estimation, greatly improving test efficiency and accuracy. Furthermore, this method is particularly suitable for power boundary determination under extreme temperature conditions, exhibiting excellent robustness under both low and high temperature extreme conditions, providing crucial protection for the safe operation of new energy vehicles.

[0065] Example 1: A 125Ah lithium iron phosphate battery cell, with its reference capacity C0 calibrated to 127Ah and allowable discharge cutoff voltage set to 3V, estimates its maximum discharge power over 30 seconds at 25℃ and 26% SOC.

[0066] The first test obtained the discharge SOC-OCV data of the battery cell at 25℃, with a step size of 5%. The SOC-OCV relationship is denoted as OCV = f(SOC). Figure 2 As shown, the OCV for 26% SOC is 3.2735V obtained by interpolation from the table.

[0067] Then, the DC internal resistance (DCR) of the battery cell was tested at different discharge rates (30% SOC and 25% SOC) in a 25℃ environment. The DCR data for 26% SOC was obtained by interpolation from a table, as shown in the table below. The relationship between the 26% SOC DCR and the discharge current I is denoted as DCR = R(26%, I).

[0068]

[0069] Next, calculate I. m ,3=f(26%+I) m (×30÷3600÷127)+I m ×R(26%, I) m Solving for I, we get m The current is -334A. Finally, the maximum allowable power P is calculated. m It is 1002W.

[0070] Example 2: A 125Ah lithium iron phosphate single cell, with a calibrated capacity C0 of 127Ah and an allowable charging cutoff voltage of 3.55V, estimates its maximum charging power over 30 seconds at 10℃ and 33% SOC.

[0071] First, the SOC-OCV data of the battery cells at 10℃ were obtained by testing, with a step size of 5%. The SOC-OCV relationship was denoted as OCV = f(SOC). Figure 3 As shown, the OCV for 33% SOC is 3.2832V obtained by interpolation from the table.

[0072] Then, the DC internal resistance (DCR) of the battery cell was tested at different charging rates with 30% and 35% SOC in a 10℃ environment. The DCR data for 33% SOC was obtained by interpolation from a table, as shown in the table below. The relationship between the 33% SOC DCR and the charging current I is denoted as DCR = R(33%, I).

[0073]

[0074] Next, calculate I. m 3.55 = f(33% + I) m (×30÷3600÷127)+I m ×R(33%, I) m Solving for I, we get mThe current is 138.4A. Finally, the maximum allowable power P is calculated. m It is 491W.

[0075] The present invention has been described above by way of example with reference to the accompanying drawings. Obviously, the implementation of the present invention is not limited to the above-described manner. Any improvements made using the inventive concept and technical solution of the present invention, or the direct application of the inventive concept and technical solution of the present invention to other occasions without modification, are all within the protection scope of the present invention.

Claims

1. A method of estimating the maximum allowable power of a lithium-ion battery cell, the method comprising: The method comprises the following steps: ​ S1, calibrating the reference capacity C0 of the battery monomer; S2, determining the correspondence between the open circuit voltage and the state of charge of the battery monomer at a target temperature, denoted as OCV=f(SOC); S3, determining the dynamic direct current resistance under different states of charge and currents at the same target temperature in step S2, and establishing a DCR=R(SOC, I) mapping relationship; S4, the SOC to be evaluated of the battery monomer is recorded as SOC1, and it is allowed to stand at the target temperature to reach thermal equilibrium, combined with the allowable cut-off voltage U at the target temperature cut , the pulse length t and the reference capacity Co, the maximum allowable current I is calculated m ; S5. According to I m With the permissible cutoff voltage U cut Calculate the maximum permissible power P m ; Wherein, the target temperature range needs to cover the actual working temperature interval of power estimation, and the temperature points are selected according to the predetermined step length in the interval.

2. The method of claim 1, wherein: The step S1 is specifically calibrating the reference capacity C0 of the battery monomer at an ambient temperature of 25℃.

3. The method of claim 1, wherein: The step S2 is specifically adjusting the state of charge of the battery to a plurality of preset points by constant current discharge, fully standing at the target temperature until the thermal equilibrium state is reached, and then measuring the open circuit voltage value, recording the correspondence between the open circuit voltage and the state of charge at the target temperature as a function relationship OCV=f(SOC).

4. The method of claim 1, wherein: The step S3 specifically comprises the following steps: S3-1, each time the battery monomer is adjusted to a preset point, it is left to thermal equilibrium; S3-2, apply current I with pulse length t a Charge and discharge tests are performed, recording the voltage U before the pulse OCV , the voltage U at the end of the pulse end and the time at the end of the pulse. If the pulse is completed and the voltage limit threshold is not triggered, step S3-3 is performed, if the pulse is not completed and the voltage limit threshold is triggered, step S3-4 is performed; S3-3, calculate I a DCR a = (U end -U OCV ) / I a , adjust the current I a , the current I a + I b is recorded as the new I a , return to step S3-2; S3-4, stop the test, and set I a The set of I is denoted as I, and the DCR a The set of DCR is denoted as DCR, and the relationship of DCR, SOC, and I at the target temperature is denoted as DCR = R(SOC, I); wherein the current I a is positive during charging and negative during discharging, I b is a preset value.

5. The method of claim 1, wherein: In said step S4, the maximum current I m This is calculated as follows: ΔSOC = I m × t ÷ 3600 ÷ Co SOC2=SOC1+ΔSOC Wherein, ΔSOC is the deviation, and SOC2 is the state of charge at the end of the pulse; Query the OCV2 corresponding to the SOC2 at the target temperature, and the DCR corresponding to the SOC1, and establish the following formula: U cut = f (SOC2) + I m x R (SOC1, I m ) The target temperature and the maximum allowable current I at SOC1 are obtained m .

6. The method of claim 1, wherein: In the step S5, the maximum allowable power P m The formula is as follows: P m =U cut ×|I m |。 7. The method of claim 1, wherein: The step length of SOC is 5% or 10%.

8. The method of claim 1, wherein: The step length of the target temperature range is 10℃ or 15℃.

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