A method for joint calibration and test of start-stop lithium battery SOC and SOH under cold start condition

Abstract

This invention discloses a joint calibration test method for the SOC and SOH of a start-stop lithium battery under cold start conditions, comprising: placing the battery at low temperature until thermal equilibrium and fully charging it, and recording the initial open-circuit voltage OCV0 and the corresponding SOC0; applying a simulated cold start current pulse to the battery, including a peak start-up segment and a steady-state drag segment, and simultaneously acquiring the terminal voltage and current; recording the open-circuit relaxation voltage rebound curve after discharge; performing multi-parameter synchronous identification based on the acquired data: calculating the ohmic internal resistance R0 from the voltage step; fitting the relaxation curve using a second-order RC equivalent circuit model to identify the polarization parameters and the fully relaxed open-circuit voltage OCV0. final Integrating the current yields the discharge charge Q, which is then combined with SOC0 and OCV. final The mapped SOC1 is used to calculate the current cryogenic usable capacity and converted to the standard temperature capacity; the SOC is calibrated using OCV_final and the temperature-compensated OCV-SOC curve; the internal resistance health status SOH is calculated from R0 and the standard capacity respectively. R and volume health SOH C Weighted fusion yields the joint SOH, achieving simultaneous correction of SOC and SOH.

Description

Technical Field

[0001] This invention relates to the field of lithium battery management technology, specifically a method for joint calibration and testing of SOC and SOH of lithium batteries under cold start conditions. Background Technology

[0002] In vehicles equipped with start-stop systems, lithium batteries need to output starting currents of up to 10C-15C within milliseconds at low temperatures. Because the electrolyte's ionic conductivity decays exponentially with decreasing temperature, the battery's ohmic and polarization resistances increase sharply, causing the terminal voltage to drop vertically by several volts at startup, compounded by significant concentration and electrochemical polarization. Traditional SOC estimation strategies rely on a combination of ampere-hour integration and periodic calibration of open-circuit voltage (OCV). However, the polarization overpotential introduced by large current pulses can last for several minutes or even tens of minutes. During this period, OCV cannot be directly measured, forcing the interruption of terminal voltage-based calibration, leading to accumulated ampere-hour integration errors and severely distorted SOC estimations. Furthermore, SOH is typically obtained through offline capacity calibration or AC impedance spectroscopy, making it impossible to update synchronously with SOC and even more difficult to achieve self-calibration under normal operating conditions.

[0003] Existing technologies include methods for identifying RC parameters using voltage relaxation data after discharge, but these are mostly applied to room temperature constant current pulse conditions and do not address the extreme variable current waveforms of cold starts at low temperatures. Furthermore, there is no systematic method that simultaneously uses the large current process of cold starts as a combined information source for internal resistance identification, capacity estimation, and OCV recovery, thereby outputting SOC and SOH calibration values ​​in one go. In addition, the usable capacity of batteries decreases sharply at low temperatures. Relying solely on capacity or internal resistance to determine SOH cannot fully reflect the complex aging characteristics caused by low-temperature shocks, such as lithium deposition and active material stripping. Therefore, there is an urgent need for a method that can utilize the inherent cold-start conditions of vehicles to simultaneously achieve high-precision online correction of SOC and SOH at extremely low temperatures. Summary of the Invention

[0004] The purpose of this invention is to provide a joint calibration and testing method for the SOC and SOH of lithium batteries under cold start conditions, so as to solve the problems in the prior art.

[0005] The objective of this invention can be achieved through the following technical solutions: A joint calibration and testing method for start-stop lithium batteries based on SOC and SOH under cold start conditions includes the following steps: S1. Allow the battery to stand at low temperature until thermal equilibrium is reached and fully charge it. Record the initial open circuit voltage OCV0 and the corresponding SOC0. S2. Apply a simulated cold start current pulse to the battery, including a peak start-up segment and a steady-state drag segment, and simultaneously collect terminal voltage and current. S3. Record the open-circuit relaxation voltage rebound curve after the discharge is completed. S4. Multi-parameter synchronous identification based on acquired data: Calculate the ohmic internal resistance R0 from the voltage step; fit the relaxation curve using a second-order RC equivalent circuit model to identify the polarization parameters and the open-circuit voltage OCV after complete relaxation. final Integrating the current yields the discharge charge Q, which is then combined with SOC0 and OCV. final The mapped SOC1 is used to calculate the current available low-temperature capacity and convert it to the standard temperature capacity. S5. Calibrate the SOC using OCV_final and the temperature-compensated OCV-SOC curve; calculate the internal resistance health status SOH using R0 and standard capacitance respectively. R and volume health SOH C Weighted fusion yields the joint SOH, achieving simultaneous correction of SOC and SOH.

[0006] Furthermore, the simulated cold start current waveform in S2 is as follows: the current linearly increases from 0 to 600A (10C) from 0 to 0.2s, the current linearly decreases to 200A (3.3C) from 0.2 to 3s, and cuts off at 3s; the sampling frequency is ≥100Hz.

[0007] Furthermore, in S4, the voltage drop at the instant the current jumps from zero to its peak value is extracted. ΔU and the corresponding current change ΔI Calculate the ohmic internal resistance: R 0 =ΔU / ΔI .

[0008] Furthermore, in S4, a second-order RC equivalent circuit model is adopted, and the relaxation voltage is expressed as:

[0009] OCV, V1(0), τ1, V2(0), and τ2 are obtained by nonlinear least squares fitting.

[0010] Furthermore, the calculation of polarization internal resistance and capacitance in S4 is based on equivalent constant current I. eq With pulse duration T pulse Implemented according to the formula: .

[0011] Furthermore, the cryogenic available capacity C_available in S4 is calculated as follows:

[0012] Standard capacity conversion uses C std = C available / k T kT is the capacity retention factor at temperature T.

[0013] Furthermore, the internal resistance health in S5 SOH R Based on the obtained ohmic internal resistance R 0 Combined with the battery's initial internal resistance at the factory R 0,initial and the criterion for termination of life R 0,EOL Calculate the internal resistance health status using the following formula: .

[0014] Furthermore, volume health status (SOH) C Standard capacity based on S4.4 conversion C std and battery rated capacity C rated Calculate capacity health: .

[0015] Furthermore, according to the method of claim 1, the SOH in step S5 is characterized by dynamic weighted fusion:

[0016] Among them, SOH R = (R0 EOL - R0) / (R0 EOL - R0 initial )×100%, SOH C = (C std / C rated )×100%, α and β are calibrated according to the aging stage.

[0017] Furthermore, this method can be executed offline or embedded in the vehicle battery management system, automatically triggered by each cold start event of the vehicle to achieve online joint calibration of SOC and SOH.

[0018] The beneficial effects of this invention are: 1. This invention is based on a joint calibration test method for SOC and SOH of lithium batteries under cold start conditions. It simultaneously utilizes voltage step (ohmic information), discharge integral (capacity information), and relaxation rebound (polarization and OCV information) to obtain R0, polarization parameters, OCV, and capacity at one time, thereby achieving coordinated calibration of SOC and SOH and breaking through the limitation of isolated evaluation of SOC and SOH in traditional methods.

[0019] 2. Based on the second-order RC equivalent circuit model, the low-temperature polarization process is accurately analyzed. The time constant and polarization voltage are extracted by double exponential fitting. The parameter inversion problem under the cold start variable current waveform is solved by constant current equivalence and temperature conversion. The theory is rigorous.

[0020] 3. No additional testing equipment or specific charging and discharging procedures are required. The process can be completed online using only the vehicle's own cold start process. The identification algorithm has low computational load and fast convergence, and can be seamlessly embedded into the vehicle. BMS .

[0021] 4. Direct recovery at low temperatures OCV Eliminated high current polarization SOC Interference, two-factor fusion SOH It better reflects the decoupling aging characteristics of internal resistance and capacitance caused by low-temperature shock. Experiments have shown that at -20°C... SOC The calibration error is less than 2%. SOH The deviation between the assessment and the standard test is less than 3%. Attached Figure Description

[0022] The invention will now be further described with reference to the accompanying drawings.

[0023] Figure 1 This is an overall flowchart of the method of the present invention; Figure 2 A schematic diagram of the simulated cold start current waveform (a) and the corresponding battery terminal voltage response curve (b); Figure 3 This is a structural diagram of the second-order RC equivalent circuit model. Detailed Implementation

[0024] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Example

[0025] A joint calibration and testing method for start-stop lithium batteries based on SOC and SOH under cold start conditions includes the following steps: S1, Low-temperature initial state standardization The lithium-ion battery under test was placed in a constant temperature test environment (25℃±2℃) until the cell temperature and the ambient temperature reached thermal equilibrium. Discharge continuously at a constant current of 1C as specified by the manufacturer until the discharge termination voltage specified in the product technical document is reached. After the discharge is completed, leave the circuit open and stand for 1 hour. The battery cell is then fully charged using the standard constant current-constant voltage (CC-CV) method: in the constant current stage, the battery is charged at 1C to the rated upper limit voltage, and then switched to constant voltage to maintain this voltage until the charging current drops to 0.05C (unless otherwise specified by the company), at which point charging is stopped. After full charge, the battery is left to stand for 1 hour with the circuit open. Then, the battery is discharged again at the standard reference constant current of 1C to the specified discharge termination voltage. The total amount of charge generated during the complete discharge is collected and integrated in real time, and this value is the initial measured capacity of the battery cell.

[0026] The target battery is placed in the target low-temperature environment and allowed to stand until thermal equilibrium is reached; it is charged to full capacity using standard constant current-constant voltage and the charging capacity is recorded; after charging, it is allowed to stand for a preset period of time (1 hour) again, and the initial open circuit voltage OCV0 is measured and recorded. The initial state of charge SOC0 is obtained through the preset OCV-SOC mapping table.

[0027] S2, Apply simulated cold start current pulse Apply a predefined simulated cold start current waveform to the battery. I(t) .

[0028] The waveforms are calculated from the actual vehicle starting current and include: peak starting segment: a high-rate discharge current lasting 0.1s to 0.5s with an amplitude of 5C to 15C; steady-state drag segment: then a medium-rate discharge current lasting 2s to 5s with an amplitude of 3C to 5C; the total discharge time does not exceed 8s.

[0029] Throughout the discharge process, at the sampling frequency f ≥100Hz synchronous acquisition terminal voltage V(t), Current I(t) and temperature T(t) Discharge voltage-current time series were obtained.

[0030] S3, Relaxation Voltage Springback Record The load is disconnected immediately after the discharge ends, leaving the battery in an open-circuit state. The terminal voltage rebound process is recorded at the same sampling frequency until the voltage change rate is less than 0.1mV / s or the preset relaxation time (e.g., 10-15 minutes) is met, and the relaxation voltage curve is obtained. V rel(t) .

[0031] S4, Multi-parameter synchronous identification Based on the recorded data, perform the following sub-steps: S4.1 Ohmic internal resistance identification Extract the voltage drop at the moment the current jumps from zero to its peak value. ΔU and the corresponding current change ΔI Calculate the ohmic internal resistance: R 0 =ΔU / ΔI .

[0032] S4.2 Identification of Polarization Parameters and Open Circuit Voltage Second order RC The equivalent circuit model describes the battery relaxation behavior.

[0033] The open-circuit voltage of the model OCV Ohmic internal resistance R 0 and two RC Steps ( R 1 / / C 1 , R 2 / / C 2 The relaxation stage model equation is expressed as follows: ………………(1) in, τ 1 =R 1 C 1 , τ 2 =R 2 C 2 , V 1 (0) and V 2 (0) Two at the moment the discharge ends RC Polarization voltage on the circuit.

[0034] By least squares curve fitting, from V rel(t) Identify the parameter vector θ= [ OCV, V 1 (0), τ 1 ，V 2 (0), τ 2 ].

[0035] The fitting problem can be expressed as: ………………(2) The solution algorithm can be the Levenberg-Marquardt algorithm or recursive least squares (RLS).

[0036] The open-circuit voltage after full relaxation is directly obtained from the identification results. OCV final =OCV , and time constants τ1 and τ2.

[0037] S4.3 Calculation of Polarization Resistance and Capacitance Based on the excitation current during pulse discharge I(t) and pulse duration T pulse Calculate the polarization resistance and capacitance.

[0038] If equivalent constant current I eq Approximately (e.g., taking the average of the peak current and the final current), then: ………………(3) When the pulse duration is much greater than τ, the denominator approaches 1, which can be simplified to R=V(0) / I.

[0039] S4.4 Current Available Capacity Calculation The current recorded in S2 I(t) By performing numerical integration, we obtain the cumulative charge Q = ∫I(t)dt released during the pulse.

[0040] Before discharge SOC 0 After discharge OCV final Mapped SOC 1. Calculate the actual usable capacity under the current low-temperature environment: ………………(4) Simultaneously, using a temperature-capacity retention curve pre-calibrated experimentally, C available Discharge capacity converted to standard temperature (25°C) C std ,in, k T For temperature T The capacity retention factor is below.

[0041] ………………(5) S5, SOC and SOH joint correction calibration S5.1, SOC Correction The results identified by S4.2 OCV final Input temperature compensation OCV-SOC Mapping Model f_ T(OCV) The model was established through multi-temperature static experiments and outputs the calibrated state of charge. SOC cal = fT(OCV) .

[0042] Replace the current estimated SOC value of the battery management system with this calibration value.

[0043] S5.2, Two-factor fusion SOH estimation Internal resistance health SOH R Ohmic resistance obtained based on S4.1 R 0 Combined with the battery's initial internal resistance at the factory R 0,initial and the criterion for termination of life R 0,EOL Calculate the internal resistance health status using the following formula: ………………(6) SOH (Self-Health) C Standard capacity based on S4.4 conversion C std and battery rated capacity C rated Calculate capacity health: ………………(7) Joint SOH: Employs a dynamic weighted fusion strategy, with weights... α and β Determined based on the current aging stage of the battery or the internal resistance-capacity correlation calibration function: ………………(8) For example, in the early stages of aging α Greater emphasis on capacity, later stages of aging β It has a relatively large internal resistance.

[0044] S5.3, Update Write the SOC_cal and combined SOH values ​​into the battery management system to complete the joint calibration for this cold start event.

[0045] Simulated cold start current waveform I(t) It can be the envelope average curve of the actual vehicle starting current measured multiple times at different temperatures, stored in the form of a lookup table.

[0046] The parameter identification in S4.2 can be performed online in real time using the recursive least squares (RLS) algorithm.

[0047] Temperature compensation OCV-SOC The family of curves is obtained in advance through constant-flow intermittent titration experiments at different temperatures and stored as a two-dimensional table or a polynomial model. Example

[0048] The method was implemented using a lithium iron phosphate (LiFePO4) start-stop battery with nominal parameters of 12V / 60Ah as the test object in a low temperature chamber of -20°C±2°C.

[0049] S1: Allow the battery to stand at -20°C for 12 hours to ensure electrochemical and thermal equilibrium.

[0050] Charge at a constant current of 0.5C (30A) to 14.4V, then switch to constant voltage charging until the current decays to below 0.05C (3A), at which point it is considered fully charged.

[0051] The initial open-circuit voltage was measured after 1 hour of settling. OCV 0 =13.35V, based on -20°C OCV-SOC Table, corresponding SOC 0 =100%.

[0052] S2: Electronic load programming output simulates cold start current waveform, the current waveform is as follows: Figure 2 As shown in (a): The current linearly increases from 0A to 600A (10C) within 0 to 0.2s; it linearly decreases from 600A to 200A (3.3C) within 0.2s to 3s; and the current is instantaneously cut off at 3s.

[0053] The terminal voltage and current are synchronously acquired at a frequency of 200Hz. The voltage response is as follows: Figure 2 As shown in (b), at the beginning of 0.2s, the voltage drops instantaneously from 13.35V to approximately 9.5V. ΔU =3.85V, ΔI =600A, calculate the internal resistance in ohms according to S4.1. R 0 =3.85 / 600=6.417mΩ.

[0054] S3: After 3 seconds, the battery enters open-circuit relaxation. The terminal voltage is continuously recorded for 15 minutes to obtain the relaxation voltage curve. V rel(t) The curve shows a double exponential upward trend, and the voltage stabilizes at 13.08V after 15 minutes. OCV final .

[0055] S4: Execute S4.2: Use the second-order RC model to... V rel(t) Perform nonlinear least squares fitting.

[0056] Automatic initial value estimation uses the tail logarithmic stripping method, and the parameters are obtained after iterative convergence. OCV final =13.08V, V1 (0) =0.48V, τ1=1.44s, V 2 (0) =0.30V, τ2=9.75s. The root mean square of the fitting residual is less than 3mV.

[0057] Execute S4.3: with equivalent constant current I eq =(600+200) / 2=400A (approximately), pulse duration T pulse =3s, calculate the polarization internal resistance and capacitance: R 1 = 0.48 / (400×(1-e^(-3 / 1.44))) ≈ 0.80mΩ, C 1 = 1.44 / 0.80m ≈ 1800F; R 2 = 0.30 / (400×(1-e^(-3 / 9.75))) ≈ 1.50mΩ, C 2 = 9.75 / 1.50m ≈ 6500F.

[0058] Execute S4.4: For Figure 2 (a) Numerical integration of the current waveform yields the amount of charge released. Q =0.155Ah. (From) OCV final =13.08V, from the table, SOC1=91.5%.

[0059] Low-temperature usable capacity C available =0.155 / (1-0.915)=1.824Ah. The capacity retention factor k of this battery at -20°C is... T The value is calibrated to 0.35, therefore the standard capacity C converted to 25°C is... std =1.824 / 0.35≈5.211Ah.

[0060] S5: SOC calibration: Adjust OCV_ final Substituting 13.08V into the OCV-SOC function at -20°C, we directly obtain SOC_ cal =91.5%.

[0061] Assuming the BMS pure ampere-hour integral estimate before this discharge was 95% with an error of +3.5%, it is now updated to 91.5%.

[0062] SOH estimation: It is known that this type of battery was manufactured... R 0,initial = 2.5mΩ, end-of-life criterion R 0,EOL = 10mΩ. Calculate. SOH R =(10-6.42) / (10-2.5)×100%=47.7%. Rated capacity C rated =60Ah, calculate SOH C =5.211 / 60×100%≈8.7% (This battery is in a severely aged stage and is only an example). Selecting dynamic weights α=0.4, β=0.6, the fused SOH = 0.4×8.7%+0.6×47.7%≈32.1%.

[0063] Update: Write SOC=91.5% and SOH=32.1% into BMS to complete joint calibration.

[0064] Comparative verification: After this calibration, subsequent standard charge and discharge tests showed that the SOC reference value was 91.8%, with an error of only -0.3%; the SOH, evaluated by the combination of standard capacity test and internal resistance test, was 32.5%, with a deviation of 0.4%, both of which are far better than the traditional independent estimation method.

[0065] The above steps are all completed automatically by the battery testing system and PC-based algorithm, taking approximately half an hour in total. By porting this algorithm to the vehicle's BMS, fully automatic online calibration can be achieved using only the voltage and current data triggered by each cold start, significantly improving the robustness and accuracy of battery state estimation under low-temperature conditions.

[0066] This invention can be widely applied to lithium battery management systems for vehicles, ships, drones, and other equipment equipped with start-stop systems. It is especially suitable for vehicles operating in extremely cold regions. It can achieve self-calibration of battery status through daily cold starts, significantly improving system safety, reliability, and battery life management accuracy, and has extremely high commercial application value.

[0067] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A method for joint calibration and testing of SOC and SOH of lithium batteries under cold start conditions, characterized in that, Specifically, the steps include the following: S1. Allow the battery to stand at low temperature until thermal equilibrium is reached and fully charge it. Record the initial open circuit voltage OCV0 and the corresponding SOC0. S2. Apply a simulated cold start current pulse to the battery, including a peak start-up segment and a steady-state drag segment, and simultaneously collect terminal voltage and current. S3. Record the open-circuit relaxation voltage rebound curve after the discharge is completed. S4. Multi-parameter synchronous identification based on acquired data: Calculate the ohmic internal resistance R0 from the voltage step; fit the relaxation curve using a second-order RC equivalent circuit model to identify the polarization parameters and the open-circuit voltage OCV after complete relaxation. final Integrating the current yields the discharge charge Q, which is then combined with SOC0 and OCV. final The mapped SOC1 is used to calculate the current available low-temperature capacity and convert it to the standard temperature capacity. S5. Calibrate the SOC using OCV_final and the temperature-compensated OCV-SOC curve; calculate the internal resistance health status SOH using R0 and standard capacitance respectively. R and volume health SOH C Weighted fusion yields the joint SOH, achieving simultaneous correction of SOC and SOH.

2. The method for joint calibration and testing of SOC and SOH of lithium batteries under cold start conditions as described in claim 1, characterized in that, The simulated cold start current waveform in S2 is as follows: from 0 to 0.2s, the current linearly increases from 0 to 600A (10C), from 0.2 to 3s, the current linearly decreases to 200A (3.3C), and cuts off at 3s; the sampling frequency is ≥100Hz.

3. The method for joint calibration and testing of SOC and SOH of lithium batteries under cold start conditions as described in claim 1, characterized in that, The voltage drop at the instant the current jumps from zero to its peak value is extracted in S4. ΔU and the corresponding current change ΔI Calculate the ohmic internal resistance: R 0 =ΔU / ΔI .

4. The method for joint calibration and testing of SOC and SOH of lithium batteries under cold start conditions as described in claim 3, characterized in that, The second-order RC equivalent circuit model is used in S4, and the relaxation voltage is expressed as follows: OCV, V1(0), τ1, V2(0), and τ2 are obtained by nonlinear least squares fitting.

5. The method for joint calibration and testing of SOC and SOH of lithium batteries under cold start conditions as described in claim 1, characterized in that, The calculation of polarization resistance and capacitance in S4 is based on the equivalent constant current I. eq With pulse duration T pulse Implemented according to the formula: 。 6. The method for joint calibration and testing of SOC and SOH of lithium batteries under cold start conditions as described in claim 1, characterized in that, The available low-temperature capacity C_available in S4 is calculated as follows: Standard capacity conversion uses C std = C available / k T k T is the capacity retention factor at temperature T.

7. The method for joint calibration and testing of SOC and SOH of lithium batteries under cold start conditions according to claim 1, characterized in that, Internal resistance health in S5 SOH R Based on the obtained ohmic internal resistance R 0 Combined with the battery's initial internal resistance at the factory R 0,initial and the end-of-life criterion R 0,EOL Calculate the internal resistance health status using the following formula: 。 8. The method for joint calibration and testing of SOC and SOH of lithium batteries under cold start conditions according to claim 7, characterized in that, SOH (Self-Health) C Standard capacity based on S4.4 conversion C std and battery rated capacity C rated Calculate capacity health: 。 9. The method for joint calibration and testing of SOC and SOH of lithium batteries under cold start conditions as described in claim 8, characterized in that, According to the method described in claim 1, the SOH combination in step S5 employs dynamic weighted fusion: Among them, SOH R = (R0 EOL - R0) / (R0 EOL -R0 initial )×100%, SOH C = (C std / C rated )×100%, α and β are calibrated according to the aging stage.

10. The method for joint calibration and testing of SOC and SOH of lithium batteries under cold start conditions according to any one of claims 1-9, characterized in that, This method can be executed offline or embedded in the vehicle battery management system, automatically triggered by each cold start event of the vehicle to achieve online joint calibration of SOC and SOH.

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