What is the internal resistance of the battery?
The internal resistance of a battery is the resistance that the current flows through the battery during its operation, including ohmic resistance, electrochemical internal resistance and ion transfer resistance.
Ohmic resistance is mainly composed of electrical * material, electrolyte, diaphragm resistance and contact resistance of parts, which is related to the size, structure, assembly, etc. of the battery.
Electrochemical and ionic transfer * resistance are the internal resistance caused by the positive * and negative * reactions of the battery.
Different types of battery internal resistance are different. The internal resistance of the same type of battery is different due to the inconsistency of internal chemical properties. The internal resistance of the battery is very small. We usually define it in milliohm units. Internal resistance is an important technical index to measure the performance of the battery. Under normal conditions, the high current discharge capacity of the battery with small internal resistance is strong, and the discharge capacity of the battery with high internal resistance is weak.
In the schematic diagram of discharge circuit, we can take the battery and internal resistance apart and divide it into a series of power supply without internal resistance and connect a resistance with small resistance value. At this time, if the external load is light, the voltage distributed on the small resistance is small. Otherwise, if the load is very heavy, the voltage distributed on the small resistance is relatively large, and a part of the power will be consumed on the internal resistance (it may be converted into heat or some complex reverse electrochemical reaction). The internal resistance of a rechargeable battery is relatively small when it is delivered. However, after a long period of use, due to the depletion of electrolyte inside the battery and the decrease of the chemical activity inside the battery, the internal resistance will gradually increase until the internal resistance is large enough to prevent the battery from releasing normally. At this time, the battery will be "dead". Most of the aging batteries are useless because of the internal resistance, so they have to be discarded. So we should pay more attention to the capacity of the battery than the capacity charged.
1、 The internal resistance is not a fixed value
The trouble is that when the battery is in different state of electricity, its internal resistance value is different; The internal resistance of the battery is different when it is in different service life. From the technical point of view, we generally divide the resistance of the battery into two states: the internal resistance in the charge state and the internal resistance in the discharge state.
1. internal resistance in charge refers to the measured internal resistance of the battery when the battery is fully charged.
2. internal resistance in discharge state refers to the internal resistance measured after the battery is fully discharged (when discharged to the standard cut-off voltage).
Generally, the internal resistance of discharge state is unstable, and the measurement results are much higher than the normal value. However, the internal resistance in charging state is relatively stable. It is of practical significance to measure this value. Therefore, in the process of battery measurement, we all take the charge state internal resistance as the measurement standard.
2、 Internal resistance cannot be measured in a general way * * measurement
Maybe we will say that there are methods for calculating the internal resistance of batteries by simple formula + resistance box in physics class in Senior High School... But the algorithm accuracy of calculating by resistance box taught in physics textbooks is too low to be used in theory teaching, which can not be used in practical application. The internal resistance of the battery is very small. We usually define it in micro or milliohm units. In general measurement, we require that the accuracy error of the internal resistance measurement of the battery must be controlled within ± 5%. Such small resistance and such * * requirements must be measured with special instruments.
3、 The measurement method of internal resistance of battery applied in the industry
In industry applications, the * * measurement of battery internal resistance is carried out by special equipment. Let me talk about the measurement method of battery internal resistance applied in the industry. At present, the measurement methods of internal resistance of batteries used in the industry are mainly as follows:
1. internal resistance measurement method of DC discharge
According to the physical formula r = u / I, the test equipment allows the battery to pass a large constant DC current (currently, the current is 40a-80a) in a short period (generally 2-3 seconds). The voltage at both ends of the battery is measured and the current internal resistance of the battery is calculated according to the formula.
The measurement accuracy error can be controlled within 0.1% if the measurement accuracy is high and the control is appropriate.
But the method has obvious shortcomings:
(1) Only large capacity batteries or batteries can be measured, and small capacity batteries cannot load 40a-80a high current within 2-3 seconds;
(2) When the battery passes through a large current, the electric * inside the battery will be * shaped, resulting in * internal resistance. Therefore, the measurement time must be short, otherwise the internal resistance error is very large;
(3) The high current through the battery has certain damage to the electric * inside the battery.
2. measurement method of internal resistance of AC pressure drop
Because the battery is actually equivalent to an active resistance, we apply a fixed frequency and a fixed current (currently, 1kHz frequency and 50mA small current are generally used) to the battery, and then sample its voltage. After a series of processing, such as rectification and filtering, the internal resistance value of the battery is calculated by the discharge circuit. The measurement time of the battery by AC voltage drop internal resistance measurement method is short, generally about 100 ms.
The accuracy of this method is generally 1% - 2%.
The advantages and disadvantages of this method are as follows:
(1) Almost all batteries, including small capacity batteries, can be measured by the method of AC voltage drop and internal resistance measurement. This method is generally used to measure the internal resistance of the battery core in notebook.
(2) The accuracy of AC voltage drop measurement is likely to be affected by ripple current, and harmonic current interference is possible. This is a test of the anti-interference ability of the measuring instrument circuit
(3) The measurement by this method will not cause too much damage to the battery itself.
(4) The accuracy of AC voltage drop measurement is not as good as that of DC discharge internal resistance measurement.
3. component error of test instrument and battery connection wire for test
No matter which of the above methods, there are some problems that we can easily ignore, that is, the component error of the test instrument itself and the test cable used to connect the battery. Because the internal resistance of the battery to be measured is very small, the resistance of the line should be taken into account. A short connecting wire from the instrument to the battery itself also has resistance (about the micro European level), and the contact surface between the battery and the connecting wire also has contact resistance. These factors must be adjusted in advance in the instrument.
Therefore, the normal internal resistance tester is generally equipped with special connecting wire and battery fixing shelf.
Many aging batteries are still very much internal power, but the internal resistance is too big to release electricity, it is a pity. But once the internal resistance of the battery increases, it is difficult to reduce the internal resistance manually. So for the aging battery, we even think of many ways to "activate" it, such as high current shock, small current floating charge, refrigerators, etc., but most of them are useless and will not be able to work. After we have learned the above knowledge, we can basically know that the battery selection should be selected as low as possible. In addition, the battery will be used for a long time, and its internal resistance will increase. It is suggested that you should use batteries frequently to keep the chemical activity inside the battery.
Think: everyone is playing with batteries. How do you choose your power battery? What are their internal resistance? What is the consistency of them? Does your motor and tone match the battery? How many confusing questions are the batteries?