A method for testing internal resistance of lead-acid battery

Abstract

The present invention discloses a method for testing the internal resistance of a lead-acid battery, and relates to the technical field of lead-acid battery detection. The present invention improves the principle of the direct current discharge method. First, the transient process of the VRLA battery equivalent circuit model is analyzed. Secondly, an improved internal resistance test equivalent circuit model is established, and the three influencing factors of sampling time, resistance and voltage values, and current and voltage sensor measurement errors are analyzed, and corresponding solutions are proposed. Finally, a direct current discharge method internal resistance test model is constructed using the Multisim simulation platform and experiments, and the proposed scheme is compared with the traditional test method. The present invention reduces the size of the battery discharge current and the sampling error of the battery terminal voltage, reduces the complexity of the internal resistance test system, and improves the accuracy of the internal resistance test method.

Description

Technical Field

[0001] The invention belongs to the technical field of lead-acid battery detection, and in particular relates to a method for testing the internal resistance of a lead-acid battery. Background Art

[0002] my country's power distribution terminals use valve-controlled lead-acid batteries as backup power sources. When the city power is interrupted, the lead-acid battery provides DC power to the system and supplies power to important loads such as circuit breaker opening and closing mechanisms, disconnectors, and relay protection devices in an emergency. It is an important guarantee for maintaining the safe and reliable operation of power equipment. Through online monitoring of the internal resistance of lead-acid batteries, real-time evaluation of the battery's operating conditions and health status can be achieved. Therefore, the research on the internal resistance test method of lead-acid batteries is of great significance.

[0003] At present, the commonly used methods for studying the internal resistance test of batteries are mainly divided into: (1) DC discharge method; (2) AC impedance method. (1) DC discharge method, usually discharges the battery with a current of tens to hundreds of amperes for 2-3 seconds, and the measurement accuracy error can be controlled within 0.1%, which can eliminate the interference of the internal inductance and capacitance of the battery on the internal resistance test results. However, large current discharge will cause small-capacity batteries to over-discharge, heat up the battery internally, cause expansion, and cause the plate to deform and vulcanize too quickly, thereby reducing the battery capacity. At the same time, there are errors in collecting the battery terminal voltage at different times, which affects the internal resistance measurement results. (2) AC impedance method, inject a constant current frequency into the battery, and the test time is usually 100 milliseconds, but during the test process, it is necessary to measure the voltage and current signals, as well as the phase difference between the two, which increases the complexity of the system. When measuring the internal resistance at different frequencies, the measurement error of the internal resistance is positively correlated with the frequency, and the error is the smallest when the frequency is 0. Compared with the DC discharge method, the measurement accuracy error of the AC impedance method is usually between 1-2%.

[0004] Therefore, how to effectively improve the testing accuracy of lead-acid batteries has become an important issue to be solved in engineering applications. Summary of the invention

[0005] In order to solve the problems in the prior art of DC discharge method in the process of testing internal resistance, the use of large current discharge will damage the battery electrodes, there are transient effects before and after the circuit switch is closed, and the sampling accuracy of the battery terminal voltage is affected by different sampling moments. A lead-acid battery internal resistance test method proposed in the present invention reduces the size of the battery discharge current, reduces the sampling error of the battery terminal voltage, reduces the complexity of the test system and improves its speed, thereby improving the accuracy of the internal resistance test method.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] The present invention provides a method for testing the internal resistance of a lead-acid battery. The circuit used in the internal resistance testing method includes a DC voltage source U, a precision resistor R, a switch S1 and a switch S2, a voltmeter V, and a battery equivalent circuit model. The DC voltage source U, the precision resistor R, the battery equivalent circuit model, and the switch S1 are sequentially connected in series. The voltmeter V and the switch S2 are connected in series and then connected in parallel with the battery equivalent circuit model. The voltmeter V is used to measure the open circuit voltage U0 of the battery. The battery equivalent circuit model includes a battery internal inductance L, an ohmic internal resistance r Ω , polarization internal resistance r c and polarization capacitance C, the polarization internal resistance r c After being connected in parallel with the polarized capacitor C, it is connected to the battery internal inductance L and ohmic internal resistance r Ω The internal resistance test method specifically comprises the following steps:

[0008] Test step 1: Open switch S1 and close switch S2, and measure the open circuit voltage E of the battery at this time. Assume the loop current is i, and the transient process is as follows:

[0009]

[0010] Where A is a constant and the attenuation coefficient τ1 is:

[0011]

[0012] Test step 2: Close the switch S1 and inject a DC source signal U across the battery. The transient process is as follows:

[0013]

[0014] Where B is a constant and the attenuation coefficient τ2 is:

[0015]

[0016] Test step 3: Since the internal resistance of the voltmeter is very large, the current flowing through the voltage measurement circuit can be considered to be 0, and the voltage U0 across the battery is as follows:

[0017]

[0018] Test step 4: When the circuit to be tested enters a steady state, the inductance inside the battery is short-circuited and the capacitor is open-circuited. At this time, the internal resistance r of the battery is equivalent to the sum of the ohmic resistance and the polarization resistance, as shown below:

[0019] r=r Ω +r c

[0020] Test step 5: At this time, collect the battery discharge current I, and the internal resistance r is calculated by the following formula:

[0021]

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] This method establishes an improved internal resistance test model, derives the calculation formula of the internal resistance, comprehensively considers the transient process before and after the battery discharge, reduces the size of the battery discharge current and the sampling error of the battery terminal voltage, reduces the complexity of the internal resistance test system, and improves the accuracy of the internal resistance test method. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is the equivalent circuit model diagram of lead-acid battery;

[0025] Figure 2 This is a schematic diagram of the improved DC discharge method for testing internal resistance of the present invention;

[0026] Figure 3 It is a diagram of the evolution process of the genetic algorithm of the present invention;

[0027] Figure 4 is the iteration graph of the genetic algorithm of the present invention;

[0028] Figure 5 is a curve diagram of the battery terminal voltage at the instant when the switch S2 of the present invention is closed;

[0029] Figure 6 is a battery discharge voltage curve diagram when the switch S1 of the present invention is closed;

[0030] Figure 7 is a battery discharge current curve diagram when the switch S1 of the present invention is closed;

[0031] Figure 8 It is a curve diagram of the sampling resistor voltage at the instant when the switch of the present invention is closed;

[0032] Fig. 9 It is a load current curve diagram when the switch of the present invention is closed. DETAILED DESCRIPTION

[0033] The technical solutions in the embodiments of the present invention will be described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0034] This embodiment provides a method for testing the internal resistance of a lead-acid battery. First, the transient process of the VRLA battery equivalent circuit model is analyzed. Secondly, an improved internal resistance test equivalent circuit model is established, and the three influencing factors of sampling time, resistance and voltage values, and current and voltage sensor measurement errors are analyzed, and corresponding solutions are proposed. Finally, a DC discharge method internal resistance test model is constructed using the Multisim simulation platform and experiments, and the proposed scheme is compared with the traditional test method. The internal resistance of the Yuasa battery (12V, 18AH) is tested using a Fluke internal resistance meter. The results prove the accuracy of the proposed improved internal resistance test method. The details are as follows: 1. Analysis of the equivalent circuit model of the internal resistance of lead-acid batteries

[0035] Figure 1 is the internal equivalent circuit model of the lead-acid battery, R1 is the ohmic resistance, R2 is the polarization resistance, C2 is the polarization capacitance, L1 is the internal inductance of the battery, E is the open circuit voltage, U(t) is the battery terminal voltage, i(t) is the discharge current, and its equation is as follows:

[0036]

[0037] 2. Improvement of lead-acid battery internal resistance test method

[0038] In order to improve the measurement accuracy and speed of VRLA battery internal resistance, Figure 1 Based on the battery model, Figure 2 In order to improve the internal resistance equivalent circuit model of DC discharge test, a DC voltage source U, a precision resistor R, a switch S1 and a switch S2, a voltmeter V and a battery equivalent circuit model are included. The DC voltage source U, the precision resistor R, the battery equivalent circuit model and the switch S1 are connected in series. The voltmeter V and the switch S2 are connected in series and then connected in parallel with the battery equivalent circuit model. The voltmeter V is used to measure the open circuit voltage U0 of the battery. The battery equivalent circuit model includes the battery internal inductance L, the ohmic internal resistance r Ω , polarization internal resistance r c and polarization capacitance C, polarization internal resistance r c After being connected in parallel with the polarized capacitor C, it is then connected to the battery internal inductance L and ohmic internal resistance r Ω The internal resistance test method specifically includes the following steps:

[0039] 1) Test step 1: Open switch S1 and close switch S2, measure the open circuit voltage E of the battery at this time, and set the loop current to i. The transient process is as follows:

[0040]

[0041] Where A is a constant and the attenuation coefficient τ1 is:

[0042]

[0043] 2) Test step 2: Close the switch S1 and inject a DC source signal U across the battery. The transient process is as follows:

[0044]

[0045]

[0046] Where B is a constant and the attenuation coefficient τ2 is:

[0047]

[0048] 3) Test step 3: Since the internal resistance of the voltmeter is very large, the current flowing through the voltage measurement circuit can be considered to be 0, and the voltage U0 across the battery is as follows:

[0049]

[0050] 4) Test step 4: When the circuit to be tested enters a steady state, the inductance inside the battery is short-circuited and the capacitor is open-circuited. At this time, the internal resistance r of the battery is equivalent to the sum of the ohmic resistance and the polarization resistance, as shown below:

[0051] r=r Ω +r c

[0052] 5) Test step 5: At this time, the battery discharge current I is collected and the internal resistance r is calculated by the following formula:

[0053]

[0054] From the above analysis, it can be seen that the measurement error of the internal resistance of the lead-acid battery depends on the values ​​of the resistor R and the injected DC source U. At the same time, during the test, the sampling time and accuracy of the battery discharge current I and the open-circuit voltage E also determine the accuracy of the internal resistance test.

[0055] 1) Sampling time

[0056] The sampling time of the battery open circuit voltage and discharge current is an important factor affecting the accuracy of internal resistance measurement. The decay rate of τ1 depends on the inductance L, capacitance C and the internal resistance of the voltmeter. The internal resistance of the voltmeter can reach 10KΩ per volt, which has little effect on the circuit. The decay rate of τ2 is related to L, C and resistance R. It takes 3-5τ for sampling when the circuit reaches a steady state. However, L and C cannot be obtained online, so it is necessary to collect the battery open circuit voltage and discharge current in real time and compare them with the sampling value of the previous moment. When the difference between the two is equal to ε (ε tends to 0), the decay process ends, and this is the best sampling time for the open circuit voltage and discharge current.

[0057] 2) Analysis of the value of resistance R and voltage U

[0058] The internal resistance of a lead-acid battery is usually tens of milliohms, so the value of the sampling resistor must be in the milliohm level to ensure its test accuracy. Since the current of the trace on the PCB board is proportional to its line width, when the copper platinum thickness is 35μm and the line width is 2.5mm, the maximum withstand current is 4.5A. The value of the voltage source must be close to the open-circuit voltage of the battery, otherwise a discharge current of tens of amperes will be generated, exceeding the maximum current carrying capacity of the circuit board. Therefore, in order to select suitable R and U, taking the Yuasa battery (NP18-12) with a nominal voltage of 12V and a rated capacity of 18AH as an example, a genetic algorithm (GA) is used to achieve single-objective optimization. The objective function and mathematical constraints are as follows:

[0059]

[0060] Based on MATLAB simulation, the initial population of the genetic algorithm is set to 40, the mutation probability is 0.8, the crossover probability is 0.7, the number of iterations is set to 200, and the solution process is as follows: Figure 3 shown.

[0061] The optimal solution is concentrated in the voltage range of 0-20V and the resistance range of 100-140mΩ. The current corresponding to different values ​​is shown in Table 1:

[0062] Table 1 Distribution of optimal values ​​of voltage U and resistance R

[0063]

[0064] The final iteration result is as follows Figure 4 As shown in the figure, the battery discharge current stabilizes at 2.26A after 200 iterations, so the optimal solution is:

[0065]

[0066] 3) Current and voltage sensor measurement errors

[0067] In the process of measuring the internal resistance of lead-acid batteries, the battery discharge current and its terminal voltage need to be collected. The accuracy of the sensor determines its measurement accuracy. The accuracy levels of current and voltage sensors are 0.1, 0.2, 0.5, etc.

[0068] Current sensors are mainly divided into resistor shunts, current transformers and Hall current sensors. Resistor shunts have high accuracy and low cost, but consume more power when measuring large currents. The advantage of current transformers is that they have electrical isolation, but they require a changing magnetic field and can only measure AC circuits, while Hall current sensors can be constant magnetic fields and are suitable for AC and DC occasions. The measurement accuracy of Hall sensors is affected by the following six factors: sensitivity error e s , nonlinear error e NL , offset voltage error e IOS , power supply rejection ratio error e PSRR , Common mode rejection ratio error e CMRR , external magnetic field error e Bext .

[0069] Sensitivity error e s , nonlinear error e NL Given by the electrical characteristics of the sensor, the offset voltage error e IOS is the deviation from the ideal output voltage when zero input current flows through the device, as shown below, I OS is the current offset error, I OS,drift is the specified device temperature drift relative to the input, ΔT is relative to the temperature range of 25°C, I IN is the input current.

[0070]

[0071] Power Supply Rejection Ratio Error PSRR It is the change in device offset voltage caused by a change in the power supply voltage relative to the nominal 5V voltage, as shown below, where VS is the operating power supply voltage and S is the device sensitivity.

[0072]

[0073] Common mode rejection ratio error e CMRR The effective input current error caused by the change in the device's isolated input voltage is quantified as follows: V CM is the maximum operating voltage at the device input.

[0074]

[0075] External magnetic field error e Bext Refers to the output measurement error caused by external magnetic fields from adjacent high current leads or nearby magnets, as shown below, B EXT is the external magnetic field, and G is the magnetic coupling coefficient.

[0076]

[0077] In summary, the total measurement error RSS is calculated by the following formula:

[0078]

[0079] 3 Simulation and Experiment

[0080] In order to verify the effectiveness of the improved DC discharge method proposed in the present invention and the improvement effect of its measurement accuracy, a circuit model was built using Multisim and compared with the simulation effect of the traditional internal resistance test method. Figure 5 As shown, the switch S2 is closed, and after a 10μs transient process, the battery terminal voltage is collected to be 12.35V.

[0081] Then close the switch S1, as Figure 6 As shown in the figure, the battery terminal voltage stabilizes at 12.32V after a 2.69ms transient process. Figure 7 As shown in the figure, the battery discharge current reaches 2.258A after a 3.87ms transient process. Based on this, the VRLA internal resistance is 14.9mΩ. In the traditional internal resistance test method, the internal resistance r calculation formula is as follows:

[0082]

[0083] Where: U0 is the sampling resistor voltage; I0 is the load current.

[0084] At the moment of switch closing in the traditional test circuit, Figure 8 As shown in the figure, the sampling resistor voltage stabilizes to 603.29mv after a 3.75ms transient process. Fig. 9 As shown in the figure, the load current reaches 40.8A after a 2.84ms transient process. Therefore, the internal resistance is 14.787mΩ.

[0085] In order to verify the accuracy of the method proposed in the present invention, an internal resistance test experimental platform was constructed, which consists of an oscilloscope, a multimeter, a control power supply, a chip resistor, and a Yuasa battery (NP12-18). The internal resistance of the lead-acid battery was tested using a Fluke tester and compared with the traditional method and the improved method, as shown in Table 2.

[0086] Table 2 Comparison of internal resistance test methods for NP12-18 lead-acid batteries

[0087]

[0088] The lead-acid battery internal resistance testing method provided in this embodiment establishes an improved internal resistance testing equivalent circuit model, derives the calculation formula of the internal resistance, reduces the size of the battery discharge current and the sampling error of the battery terminal voltage, reduces the complexity of the internal resistance testing system, and improves the accuracy of the internal resistance testing method.

[0089] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A method for testing the internal resistance of a lead-acid battery, characterized in that: The circuit used in the internal resistance test method includes a DC voltage source U, a precision resistor R, a switch S1 and a switch S2, a voltmeter V and a battery equivalent circuit model. The DC voltage source U, the precision resistor R, the battery equivalent circuit model and the switch S1 are connected in series in sequence. The voltmeter V and the switch S2 are connected in series and then connected in parallel with the battery equivalent circuit model. The voltmeter V is used to measure the open circuit voltage U0 of the battery. The battery equivalent circuit model includes the battery internal inductance L, the ohmic internal resistance r Ω , polarization internal resistance r c and polarization capacitance C, the polarization internal resistance r c After being connected in parallel with the polarized capacitor C, it is connected to the battery internal inductance L and ohmic internal resistance r Ω The internal resistance test method specifically comprises the following steps: Test step 1: Open switch S1 and close switch S2, and measure the open circuit voltage E of the battery at this time. Assume the loop current is i, and the transient process is as follows: Where A is a constant and the attenuation coefficient τ1 is: Test step 2: Close the switch S1 and inject a DC source signal U across the battery. The transient process is as follows: Where B is a constant and the attenuation coefficient τ2 is: Test step 3: Since the internal resistance of the voltmeter is very large, the current flowing through the voltage measurement circuit can be considered to be 0, and the voltage U0 across the battery is as follows: Test step 4: When the circuit to be tested enters a steady state, the inductance inside the battery is short-circuited and the capacitor is open-circuited. At this time, the internal resistance r of the battery is equivalent to the sum of the ohmic resistance and the polarization resistance, as shown below: r＝r Ω +r c Test step 5: At this time, collect the battery discharge current I, and the internal resistance r is calculated by the following formula:

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