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The internal resistance of lithium battery and its measurement method

Different types of batteries have different internal resistances. The same type of battery has different internal resistance due to the inconsistency of internal chemical characteristics. The internal resistance of the battery is very small, we generally use the unit of milliohm 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 capacity, and a battery with a large internal resistance has a weak discharge capacity. In terms of the schematic diagram of the discharge circuit, we can consider the battery and the internal resistance separately, and divide it into a power supply with no internal resistance at all and connect it with a small resistance in series. At this time, if the external load is light, then the voltage distributed to this small resistor is small. On the contrary, if the external load is very heavy, then the voltage distributed to this small resistor is relatively large, and part of the power will be consumed in this Internal resistance (may be converted into heating, or some complex reverse electrochemical reaction). The internal resistance of a rechargeable battery when it leaves the factory is relatively small, but after long-term use, due to the exhaustion of the battery’s internal electrolyte and the decrease in the activity of the internal chemical substances in the battery, this internal resistance will gradually increase until the internal resistance is large enough. The power inside the battery cannot be released normally, and the battery is dead at this time. The vast majority of aging batteries are useless due to excessive internal resistance and have to be scrapped. Therefore, we should pay more attention to the discharged capacity of the battery rather than the charged capacity.

리튬 배터리의 내부 저항


1. The internal resistance is not a fixed value

The troublesome thing is that when the battery is in different power states, its internal resistance is different; when the battery is in a different service life state, its internal resistance is also different. From a technical point of view, we generally consider the resistance of the battery into two states: internal resistance in the charged state and internal resistance in the discharged state.

a). The internal resistance of the charged state refers to the measured internal resistance of the battery when the battery is fully charged.

b). The internal resistance of the discharged state 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 in the discharged state is unstable, and the measured result is much higher than the normal value, while the internal resistance in the charged state is relatively stable. The measurement of this value has practical comparative significance. Therefore, in the battery measurement process, we all use the internal resistance of the charged state as the measurement standard.

2. The internal resistance cannot be accurately measured by general methods

The internal resistance of the battery is very small, we generally use the unit of microohm or milliohm to mean it. In general measurement occasions, we require that the internal resistance measurement accuracy error 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 battery's internal resistance is carried out by special equipment. Let us talk about the battery internal resistance measurement method used in the industry. At present, the battery internal resistance measurement methods used in the industry mainly include the following two:

1). DC discharge internal resistance measurement method

According to the physical formula R=U/I, the test equipment forces the battery to pass a large constant DC current in a short period of time (usually 2 to 3 seconds) (currently a large current of 40A to 80A is generally used), and the battery is measured at this time The voltage at both ends and the current internal resistance of the battery are calculated according to the formula.

The accuracy of this measurement method is relatively high. With proper control, 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 a large current of 40A-80A within 2 to 3 seconds;

(2) When the battery passes a large current, the electrodes inside the battery will be polarized and polarized internal resistance will appear. Therefore, the measurement time must be very short, otherwise the measured internal resistance value will have a large error;

(3) The large current flowing through the battery will damage the internal electrodes of the battery to a certain extent.

2). AC pressure 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 sample its voltage, and go through a series of processing such as rectification and filtering. Then calculate the internal resistance of the battery through the operational amplifier circuit. The battery measurement time of the AC voltage drop internal resistance measurement method is extremely short, generally about 100 milliseconds.

The accuracy of this measurement method is also good, and the measurement accuracy error is generally between 1% and 2%. The advantages and disadvantages of this method:

(1) Almost all batteries can be measured using the AC voltage drop internal resistance measurement method, 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 the ripple current, and there is also the possibility of harmonic current interference. This is a test for the anti-interference ability in 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 that of the DC discharge internal resistance measurement method.

The component error of the test instrument and the problem of the battery connection line used for the test

Regardless of the above method, there are some problems that are easy to be ignored 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 circuit must be taken into consideration. A short connection line from the instrument to the battery also has resistance (about micro-ohm level), and there is also contact resistance on the contact surface between the battery and the connection line. These factors must be adjusted in advance within the instrument. .

Therefore, a regular battery internal resistance tester is generally equipped with a dedicated connection line and a battery fixing shelf.

4.  summary

In fact, many aging batteries still have a lot of internal power, but the internal resistance is too large to be able to discharge electricity, which is really a pity. However, once the internal resistance of the battery is added, it is even more difficult to artificially reduce the internal resistance. Therefore, with regard to the aging battery, even if we think of many ways to activate it, such as high current impact, low current floating charge, and putting in the refrigerator, most of them are of no avail. After understanding the above knowledge, we can basically understand that when choosing a battery, we should choose a battery with a smaller internal resistance as much as possible. Another very important point is that if the battery is left unused for a long time, its internal resistance will continue to increase. It is recommended that you always use the battery to keep the battery's internal chemical substances active.



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