How to measure the internal resistance of the battery?

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How to measure the internal resistance of the battery? [Copy link]

One of the important indicators of battery status is its internal resistance. Whether the battery is about to fail, has insufficient capacity, or is improperly charged or discharged, it can be reflected from the change in its internal resistance. Therefore, the working status of the battery can be evaluated by measuring the internal resistance of the battery. The question is, how to measure the internal resistance of the battery better? The internal resistance of the battery is very small, and it is generally defined in units of micro-ohms or milliohms. In general measurement situations, we require that the accuracy error of the battery's internal resistance measurement must be controlled within plus or minus 5%. The battery internal resistance measurement methods currently used in the industry mainly include the DC discharge internal resistance measurement method and the AC voltage drop internal resistance measurement method. A customer requested a lithium battery internal resistance test module, and the requirement was that the AC voltage drop internal resistance measurement method be used to design this test module. Customer specific requirements: AC voltage drop internal resistance measurement method: We apply a fixed frequency and fixed current to the battery (currently generally use 1kHz frequency, 50mA small current), then sample its voltage, and after a series of processing such as rectification and filtering, the internal resistance of the battery is calculated through the op amp circuit. The measurement accuracy error is generally between 1% and 2%. Please help analyze the advantages and disadvantages of this method. AC voltage drop internal resistance measurement method: Because the battery is actually equivalent to an active resistor, we apply a fixed frequency and fixed current to the battery (currently generally use 1kHz frequency, 50mA small current), then sample its voltage, and after a series of processing such as rectification and filtering, the internal resistance of the battery is calculated through the op amp circuit. The battery measurement time of the AC voltage drop internal resistance measurement method is extremely short, generally around 100 milliseconds. This measurement method has good accuracy, and the measurement accuracy error is generally between 1% and 2%. Advantages and disadvantages of this method: (1) The AC voltage drop internal resistance measurement method can measure almost all batteries, including small-capacity batteries. This method is generally used to measure the internal resistance of laptop battery cells. (2) The measurement accuracy of the AC voltage drop measurement method is likely to be affected by ripple current, and there is also the possibility of harmonic current interference. This is a test of the anti-interference ability of the measuring instrument circuit. (3) Using this method to measure, there will not be much damage to the battery itself. (4) The measurement accuracy of the AC voltage drop measurement method is not as good as the DC discharge internal resistance measurement method. The DC discharge internal resistance measurement method has higher measurement accuracy. If properly controlled, the measurement accuracy error can be controlled within 0.1%. However, this method has obvious shortcomings: (1) It can only measure large-capacity batteries or storage batteries. Small-capacity batteries cannot carry a large current of 40A to 80A within 2 to 3 seconds; (2) When a large current passes through the battery, the electrodes inside the battery will polarize and produce polarized internal resistance. Therefore, the measurement time must be very short, otherwise the measured internal resistance value will have a large error; (3) The large current passing through the battery will cause certain damage to the electrodes inside the battery. Both of the above methods have drawbacks. In fact, the most reliable measurement method is to use a battery internal resistance tester. A battery internal resistance tester (different from the principle of measuring resistance with a multimeter, the value it measures is in the milliohm level, while the value measured by a multimeter is in the ohm level; and a multimeter can only measure the resistance of an object without a power supply, while an internal resistance tester can measure the resistance of an object without a power supply as well as an object with a power supply, so the two are not equivalent) uses the size of the internal resistance to judge the degradation state of the battery. (Generally speaking) the smaller the resistance, the better the battery performance. Therefore, the method of measuring internal resistance to detect batteries is a good method that is fast and highly reliable. Everyone is welcome to express their opinions

懒猫爱飞

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