1. introductionThe battery is used as a backup power supply in power system, which has been widely used in industrial production, transportation, communications and other industries. If the battery failure or insufficient capacity is possible, it is possible to cause major accidents, so you must perform a comprehensive online monitoring of the running parameters of the battery. One of the important signs of the battery status is its internal resistance. Whether it is battery failure, insufficient capacity or charge and discharge, it can be embodied in its internal resistance. Therefore, the operating state can be evaluated by measuring the internal resistance of the battery. Common methods for measuring the internal resistance of the battery are:
(1) Density method
The density method mainly estimates the internal resistance of the battery by measuring the density of the battery electrolyte, often used for the internal resistance of the opening lead-acid battery, and is not suitable for the internal resistance of the sealed lead-acid battery. The method is widely applicable.
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(2) Open circuit voltage method
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The open circuit voltage method is to estimate the internal resistance of the battery by measuring the end voltage of the battery, and the accuracy is poor, and even a wrong conclusion. Because even if a capacity has become a small battery, its terminal voltage may still be normal.
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(3) DC discharge method
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The DC discharge method is to measure the instantaneous voltage drop on the battery by instantaneous current to the battery, and measure the internal resistance of the battery through the ohmic law. Although this method has been widely used in practice, it also has some shortcomings. If this method is used to detect the internal resistance of the battery must be carried out in static or offline, online measurement cannot be achieved. Moreover, large current discharge will cause greater damage to the battery, which affects the capacity and life of the battery.
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(4) AC injection method
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The AC method measures the voltage response signal Vo at both ends of the battery by injecting a constant AC current signal IS in the battery, and the phase difference between the two ends is measured.
Phase difference
Impedance formula
Impedance formula
To determine the internal resistance of the battery. This method does not need to discharge the battery, which can realize the internal resistance of the battery in line, so it does not affect the performance of the battery. However, the method needs to measure the alternating current signal IS, the voltage response signal Vo, and the phase difference between the voltage and the current.
Phase difference
It can be seen that this method is not only more interference factors, but also increases the complexity of the system, but also affects the measurement accuracy.
In order to solve the defects of the above methods, this paper uses four terminal measuring methods, and the voltage response signal on both ends of the battery is multiplied by the simulated multiplier by the simulated multiplier, and the simulated multiplier The output voltage signal passes the flow signal to the DC signal, and the DC signal is enlarged after analog-to-digital conversion, and the converted value is sent to the microcontroller for simple processing.
2. Battery internal resistance detection principle
Since the internal resistance of the battery is millilice, the measurement error is measured with a large two-terminal measurement method, and the four-terminal measurement method is used here. Two terminals are applied at the time of measurement
Constant AC excitation current signal
Constant AC excitation current signal, the other two terminals are used to measure. The measurement work schematic diagram is shown in Figure 1, the response signal refers to an alternating current voltage signal measured at both ends of the battery injecting the alternating current constant flow source. The sinusoidal signal is an input signal generated by D / A as a voltage controlled constant flow source.
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Measuring works schematic
Set sinusoidal signals:
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Sinusoidal signal
(1)
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The response voltage signal at both ends of the battery is:
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Response voltage signal
(2)
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Phase difference
In order to inject the alternating current current of the battery and the phase difference of the voltage signal at both ends.
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After simulating the multiplier:
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By simulation multiplier
(3)
K is an amplification factor of analog multiplier.
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Filter off the exchange ingredient after low-pass filtering:
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Filter out the exchange ingredient
(4)
Introduced by the exchange method:
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Internal resistance
(5)
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I is the maximum value of the AC constant stream signal. Comparison (4), (5) available:
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Battery internal resistance
In the above formula, K, A, and I are known, and u is the sample value sent to the single chip microcontroller through A / D, so that a simple division operation is performed in the single chip microcomputer.
3. Design of AC Constant Streaming
The premise of successfully detecting the state of the battery is to provide a required AC constant flow source. The constant current source is a power supply device capable of providing a constant current to the load. It is a power supply that is very large in the internal resistance. In order to ensure that the internal resistance has higher measurement accuracy and better reproducibility, the constant current source is required to have sufficient stability, and the waveform distortion is small. The AC signal here is 40mV, and the frequency is 1 kHz.
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