Battery internal resistance and its measurement method

　　Every battery has internal resistance. Different types of batteries have different internal resistances. Batteries of the same type have different internal resistances due to inconsistent internal chemical properties. The internal resistance of a battery is very small, and we generally define it in units of micro-ohms or milliohms. 
  
　　Internal resistance is an important technical indicator for measuring battery performance. Under normal circumstances, batteries with small internal resistance have strong high current discharge capabilities, while batteries with large internal resistance have weak discharge capabilities. 
  
　　Take a simple example: an old digital camera that uses AA batteries (such as the power-consuming CANON 210) can take dozens of photos continuously when powered by AA alkaline batteries; but when powered by AA dry batteries, it can only take a few photos before it automatically shuts down, but the dry batteries are not completely dead; if you replace it with AA rechargeable nickel-metal hydride batteries, you can take more photos. After actual measurement, we can know that the internal resistance of nickel-metal hydride batteries is less than that of alkaline batteries and less than that of dry batteries. This example shows that in applications with large current discharge, you must choose batteries with smaller internal resistance. 
  
　　In the schematic diagram of the discharge circuit, we can separate the battery and the internal resistance, and consider it as a battery with no internal resistance and a resistor with a very small resistance in series. At this time, if the external load is light, the voltage distributed on this small resistor will be small. On the contrary, if the external load is very heavy, the voltage distributed on this small resistor will be relatively large, and some power will be consumed on this internal resistance (which may be converted into heat or some complex reverse electrochemical reactions). The internal resistance of a rechargeable battery is relatively small when it leaves the factory, but after long-term use, due to the depletion of the electrolyte inside the battery and the decrease in the activity of the chemical substances inside the battery, this internal resistance will gradually increase until the internal resistance is so large that the power inside the battery cannot be released normally, and the battery will "die". Most aging batteries are useless because of excessive internal resistance and have to be scrapped. 
  
1. Internal resistance is not a fixed value. 
  
　　The troublesome point is that the internal resistance value of the battery is different when it is in different power states; the internal resistance value of the battery is also different when it is in different service life states. 
  
　　From a technical point of view, we generally consider the resistance of the battery in two states: charging state internal resistance and discharging state internal resistance. 
  
　　1. Charging state internal resistance refers to the internal resistance of the battery measured when the battery is fully charged. 
  
　　2. Discharging state internal resistance refers to the internal resistance of the battery measured after the battery is fully discharged (discharged to the standard cut-off voltage). 
  
　　Under normal circumstances, the internal resistance of the discharge state is unstable, and the measurement result is much higher than the normal value, while the charging state internal resistance is relatively stable, and measuring this value has practical comparative significance. Therefore, in the battery measurement process, we use the charging state internal resistance as the measurement standard. 
  
2. The internal resistance cannot be accurately measured by general methods. 
  
　　Perhaps everyone will say that there is a method of calculating the internal resistance of the battery using a simple formula + resistance box in high school physics class. . . . . But the algorithm taught in the physics textbook using the resistance box to calculate is too low in accuracy and can only be used for theoretical teaching, and cannot be used in practical applications. 
  
　　The internal resistance of the battery is very small, and we generally define it in units of micro-ohms or milliohms. In general measurement situations, we require that the internal resistance measurement accuracy error of the battery must be controlled within plus or minus 5%. Such a small resistance value and such precise requirements must be measured with special instruments. 
  
3. Battery internal resistance measurement methods currently used in the industry. 
  
　　In industry applications, the precise measurement of battery internal resistance is carried out by special equipment. Let me talk about the battery internal resistance measurement methods used in the industry. 
  
　　There are currently two main battery internal resistance measurement methods 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 (currently generally using a large current of 40A-80A) in a short period of time (generally 2-3 seconds), measures the voltage at both ends of the battery at this time, and calculates the current battery internal resistance according to the formula. 
  
　　This measurement method has high 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-80A within 2-3 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. 
  
2. AC voltage 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 generally use 1KHZ frequency and 50mA small current), and then sample its voltage. After a series of processing such as rectification and filtering, the internal resistance value 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, and the measurement is almost completed as soon as the measurement switch is pressed. Haha. 
  
　　The accuracy of this measurement method is also good, 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 be used to 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) 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 of online internal resistance monitoring, only the DC discharge measurement method can be used and the AC voltage drop measurement method cannot be used. 
  
3. The component error of the test instrument and the problem 
  
　　of the battery connection line used for testing. Regardless of which of the above methods is used, there are some problems that are easily overlooked by us, that is, the component error of the test instrument itself and the problem of 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 must be taken into consideration. A short connecting wire from the instrument to the battery also has resistance (about micro-ohm level), and the contact surface between the battery and the connecting wire also has contact resistance. These factors must be adjusted for errors in advance inside the instrument. 
  
　　Therefore, regular battery internal resistance testers are generally equipped with special connecting wires and battery fixing racks. 
  
4. Summary. 
  
　　Many aging batteries actually still have a lot of internal power, but the internal resistance is too large to discharge the power, which is a pity. However, once the internal resistance of the battery increases, it is extremely difficult to artificially reduce the internal resistance value. Therefore, for the aging battery, even if we come up with many ways to "activate" it, such as high current shock, low current floating charge, refrigerator... etc., most of them are useless and can't be saved.  
  
　　After understanding the above knowledge, we can basically know that when selecting batteries, we should try to select batteries with smaller internal resistance. In the process of combining battery packs (such as the battery pack combination of laptops), we should try to choose batteries with consistent internal resistance. Another important point is that if the battery is left unused for a long time, its internal resistance will continue to increase. Therefore, I suggest that you should use the battery frequently to keep the chemical substances inside the battery active. Also, do not buy old batteries, such as disassembled batteries.