Battery internal resistance and its measurement method

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 milliohms. Internal resistance is an important technical indicator for measuring battery performance. Under normal circumstances, a battery with a small internal resistance has a strong discharge capacity for large currents , while a battery with a large internal resistance has a weak discharge capacity.

In the schematic diagram of the discharge circuit , we can separate the battery and the internal resistance, and divide it into a power supply 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. Therefore, we should pay more attention to the capacity discharged by the battery rather than the capacity charged.

1. Internal resistance is not a fixed value

The trouble is that the internal resistance of the battery is different when it is in different power states; the internal resistance 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. Charged internal resistance refers to the internal resistance of the battery measured 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 (discharged to the standard cut-off voltage).

Generally speaking, the internal resistance in the discharge 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, and measuring this value has practical comparative significance. Therefore, in the battery measurement process, we use the internal resistance in the charged state as the measurement standard.

2. Internal resistance cannot be accurately measured using general methods

Maybe you will say that in high school physics class, we learned how to calculate the internal resistance of a battery using a simple formula + a resistance box... But the accuracy of the calculation method using a resistance box taught in physics textbooks is too low, and it can only be used for theoretical teaching, and cannot be used in practical applications. The internal resistance of a battery is very small, and we generally define it in units of micro-ohms or milliohms. In general measurement situations, we require that the measurement accuracy error of the internal resistance of a battery must be controlled within plus or minus 5%. Such a small resistance value and such a precise requirement must be measured with a special instrument.

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. Currently, there are two main battery internal resistance measurement methods used in the industry:

1. DC discharge internal resistance measurement method

According to the physical formula R=U/I, the test equipment forces a large constant DC current (currently 40A to 80A is generally used ) to pass through the battery in a short period of time (usually 2 to 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) Only large-capacity batteries or storage batteries can be measured. Small-capacity batteries cannot carry a large current of 40A to 80A within 2 to 3 seconds.

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

(3) 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 using a 1kHz frequency and a small current of 50mA), then sample its voltage, and after a series of processes such as rectification and filtering, calculate the internal resistance of the battery through an 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) 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 the possibility of harmonic current interference. This is a test for 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 that of the DC discharge internal resistance measurement method.

3. Errors in test instrument components and battery connection wires used for testing

Regardless of which of the above methods is used, there are some issues that are easily overlooked, namely 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 very small, the resistance of the line must be taken into account. A short connecting line from the instrument to the battery also has resistance (about micro-ohm level), and the contact surface between the battery and the connecting line 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.

IV. Conclusion

Many aged batteries actually still have a lot of power inside, 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 aged batteries, even if we come up with many ways to "activate" it, such as high current shock, low current floating charge, and refrigerator, most of them are useless and can't be saved. After understanding the above knowledge, we can basically know that when choosing a battery, we should try to choose a battery with a smaller internal resistance. Another 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 still use the battery frequently to keep the chemical substances inside the battery active.