Different types of batteries have different internal resistances. For the same type of battery, due to the inconsistent internal chemical characteristics, the internal resistance is also different. The internal resistance of the battery is very small, we generally use milliohm units to mean it. Internal resistance is an important technical indicator to measure battery performance. Under normal circumstances, a battery with a small internal resistance has a strong high-current discharge capability, and a battery with a large internal resistance has a weak discharge capability. In terms of the schematic diagram of the discharge circuit, we can dissect the battery and the internal resistance and divide it into a power supply with no internal resistance connected in series with a small resistance. At this time, if the external load is light, then the voltage distributed on this small resistor is small, otherwise if the external load is very heavy, then the voltage distributed on this small resistor is relatively large, and part of the power will be consumed in this Internal resistance (may be converted to heat, or some complex reverse electrochemical reaction). The internal resistance of a rechargeable battery at the factory is relatively small, but after long-term use, due to the depletion of the electrolyte inside the battery and the reduction of the activity of the chemical substances inside the battery, this internal resistance will gradually increase until the internal resistance is large The battery's internal power cannot be released normally, and the battery is now dead. Most of the aging batteries have no use value due to excessive internal resistance, so they have to be scrapped. Therefore, we should pay more attention to the battery discharge capacity rather than the charging capacity.
1. The internal resistance is not a fixed value
The trouble is that when the battery is in a different state of charge, its internal resistance value is different; when the battery is in a different service life state, its internal resistance value is also different. From a technical point of view, we generally divide the battery resistance into two states: internal resistance in the charging state and internal resistance in the discharging state.
1). Charged internal resistance refers to the measured battery internal resistance when the battery is fully charged.
2). Discharge internal resistance refers to the internal resistance of the battery measured after the battery is fully discharged (when discharged to the standard cut-off voltage).
In general, the internal resistance of the discharge state is unstable, and the measurement result is much higher than the normal value, while the internal resistance of the charged state is relatively stable, and measuring this value has practical comparison significance. Therefore, in the measurement process of the battery, we take the internal resistance of the charge state as the measurement standard.
2. The internal resistance cannot be accurately measured by a general method
The internal resistance of the battery is very small, we generally use micro-ohms or milliohms to mean it. In general measurement occasions, we require that the accuracy error of the internal resistance measurement of the battery must be controlled within plus or minus 5%. Such small resistance and such precise requirements must be measured with special instruments.
3. The battery internal resistance measurement method currently used in the industry
In industrial applications, the accurate measurement of the internal resistance of the battery is carried out by special equipment. Let me talk about the battery internal resistance measurement method applied in the industry. At present, there are two main methods for measuring battery internal resistance in the industry:
1). DC discharge internal resistance measurement method
According to the physical formula R = U / I, the test equipment allows the battery to force a large constant DC current in a short time (generally 2 to 3 seconds) (currently a large current of 40A to 80A is generally used), and the battery is measured Calculate the current internal resistance of the battery according to the formula.
The accuracy of this measurement method is high. 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 accumulators can be measured, and small-capacity batteries cannot load large currents of 40A to 80A in 2 to 3 seconds;
(2) When the battery passes a large current, the electrode inside the battery will be polarized, and the polarized internal resistance will appear. Therefore, the measurement time must be very short, otherwise the error of the measured internal resistance value is very large;
(3) The large current passing through the battery has 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 resistance, we apply a fixed frequency and a fixed current to the battery (currently 1kHz frequency and 50mA small current are generally used), and then the voltage is sampled and subjected to a series of processes such as rectification and filtering. Then calculate the internal resistance of the battery 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.
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) Using 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, as well as the possibility of harmonic current interference. This is a test of the anti-interference ability of the measuring instrument circuit.
(3) Measuring with this method will not cause too 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.
3). The component error of the test instrument and the problem of the battery cable used for the test
No matter which of the above mentioned methods, there are some problems that are easily overlooked by us, that is, the component errors 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 small, the resistance of the line must be taken into account. A short connection line from the instrument to the battery itself also has resistance (approximately micro-ohm level), and there is contact resistance between the battery and the connection line. These factors must be adjusted in advance within the instrument .
Therefore, the regular battery internal resistance tester is generally equipped with a dedicated connection cable and a battery fixing shelf.
4. Summary
Many aging batteries still have a lot of internal power, but it is a pity that the internal resistance is too large to discharge the electricity. But once the internal resistance of the battery is newly added, it is even more difficult to artificially reduce this internal resistance value. Therefore, even if we think of many ways to activate the aging battery, such as high-current shock, small-current floating charge, and refrigerator, etc., most of them are useless, and they are ineffective. After understanding the above knowledge, we can basically understand that the battery with the lowest internal resistance should be selected as much as possible. Another important point is that the internal resistance will continue to increase as the battery is left unused for a long time. It is recommended that you still use the battery often to maintain the activity of the chemical substances inside the battery.
