In industrial applications, accurate measurement of battery internal resistance is performed by dedicated equipment. Let me talk about the method of measuring the internal resistance of the battery used in the industry.
At present, there are two main methods for measuring internal resistance of batteries used in the industry:
1. DC discharge internal resistance measurement method
According to the physical formula R=V/I, the test equipment forces the battery to pass a large constant DC current in a short time (generally 2-3 seconds) (currently using a large current of 40A-80A), measuring the battery at this time. The voltage at both ends, and calculate the current internal resistance of the battery according to the formula.
The accuracy of this measurement method is high, and if it is properly controlled, the measurement accuracy error can be controlled within 0.1%.
But this method has obvious shortcomings:
(1) Only large-capacity batteries or batteries can be measured, and small-capacity batteries cannot load 40A-80A in 2-3 seconds;
(2) When the battery passes a large current, the electrode inside the battery will be polarized, resulting in a polarization internal resistance. Therefore, the measurement time must be very short, otherwise the measured internal resistance value is very large;
(3) A large current has a certain damage to the electrodes inside the battery through the battery.
2, AC pressure drop internal resistance measurement method
Because the battery is actually equivalent to an active resistor, we apply a fixed frequency and a fixed current to the battery (currently 1KHZ frequency, 50mA small current), then sample the voltage, and perform a series of processing such as rectification and filtering. The internal resistance of the battery is calculated by an operational amplifier circuit.
The battery measurement time of the AC voltage drop internal resistance measurement method is extremely short, generally about 100 milliseconds, and is almost measured by pressing the measurement switch. The accuracy of this measurement method is also good, and the measurement accuracy error is generally between 1% and 2%.
Advantages and disadvantages:
(1) Almost all batteries, including small-capacity batteries, can be measured using AC voltage drop internal resistance measurement. This method is generally used for the internal resistance measurement of laptop battery cells.
(2) The measurement accuracy of the AC voltage drop measurement method is likely to be affected by the ripple current, and there is also the possibility of harmonic current interference. This is a test of the anti-interference ability in the measuring instrument circuit.
(3) Measurement by this method will not cause 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. In some applications where internal resistance is monitored online, DC discharge measurement can only be used and AC pressure drop measurement cannot be used.
3. The component error of the test instrument and the battery connection line for testing.
Regardless of the above method, there are some problems that are easily overlooked by us, that is, the component error of the test instrument itself and the test cable problem for connecting the battery. Since the internal resistance of the battery to be measured is small, the resistance of the line should be taken into consideration. A short connection from the instrument to the battery itself also has resistance (about the micro-European level), and there is also contact resistance between the contact surface of the battery and the connection line. These factors must be adjusted in advance within the instrument. .
Therefore, regular battery internal resistance testers are generally equipped with a dedicated cable and battery holder.
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