How to test the internal resistance of lithium battery?

Different types of batteries have different internal resistances. The same type of battery has different internal resistance due to inconsistent internal chemical characteristics. The internal resistance of the battery is very small. We usually define it in milliohm units. Internal resistance is an important technical indicator for measuring battery performance. Under normal circumstances, a battery with a small internal resistance has a large current discharge capability, and a battery with a large internal resistance has a weak discharge capability.

In the schematic diagram of the discharge circuit, we can consider the battery and internal resistance to be taken apart, and divide it into a power supply with no internal resistance and connect a resistor with a small resistance. At this time, if the external load is light, then the voltage distributed on this small resistor is small. If the external load is very heavy, then the voltage distributed on this small resistor is relatively large, and some power is 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 internal electrolyte of the battery and the decrease of the chemical activity inside the battery, the internal resistance will gradually increase until the internal resistance is as large as the battery. The internal power can not be released normally, and the battery will be "end to sleep". Most of the aging batteries are useless because of the internal resistance is too large, so they have to be scrapped. Therefore, we should pay more attention to the capacity of the battery to discharge rather than the capacity.

First, the internal resistance is not a fixed value

The trouble is that when the battery is in different state of charge, its internal resistance value is different; when the battery is in different service life, its internal resistance value is also different. From a technical point of view, we generally divide the resistance of the battery into two states: the internal resistance of the charge state and the internal resistance of the discharge state.

1. Charge state internal resistance refers to the measured internal resistance of the battery when the battery is fully charged.

2. Discharge state 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).

Under normal circumstances, the internal resistance of the discharge state is unstable, and the measurement result is also much higher than the normal value, while the internal resistance of the charge state is relatively stable, and measuring this value has practical comparative significance. Therefore, in the measurement process of the battery, we all use the internal resistance of the charge as the measurement standard.

Second, the internal resistance can not be accurately measured by the general method

Perhaps everyone will say that the high school physics class has a method of teaching the internal resistance of the battery using a simple formula + resistance box... But the algorithm used in the physics book to calculate the resistance box is too low, and can only be used for theoretical teaching. It can't be used in practice. The internal resistance of the battery is very small. We usually define it in units of micro-ohms or milliohms. In the general measurement occasion, 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 dedicated instruments.

Third, the current internal resistance measurement method used in the industry

In industrial applications, accurate measurement of battery internal resistance is performed by dedicated equipment. Let me talk about the method of measuring the internal resistance of the battery used in the industry. At present, there are two main methods for measuring internal resistance of batteries used in the industry:

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 time (generally 2 to 3 seconds) (currently a large current of 40A to 80A is used), and the battery is measured at this time. The voltage at both ends and calculate the current internal resistance of the battery according to the formula.

The accuracy of this measurement method is high, and 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 batteries 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, resulting in a polarization internal resistance. Therefore, the measurement time must be very short. Otherwise the measured internal resistance value is very large;

(3) A large current has certain damage to the electrodes inside the battery through the battery.

2. AC pressure 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 using a 1kHz frequency, 50mA low current), then sampling its voltage, and performing a series of processing such as rectification and filtering. The internal resistance of the battery is calculated by an 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, including small-capacity batteries, can be measured using AC voltage drop internal resistance measurement. This method is generally used for the internal resistance measurement 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 of the anti-interference ability in the measuring instrument circuit.

(3) Measurement by 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.

3. Test component error and battery connection problem for testing

Regardless of the above method, there are some problems that are easily overlooked by us, that is, the component error of the test instrument itself and the test cable problem for connecting the battery. Since the internal resistance of the battery to be measured is small, the resistance of the line should be taken into consideration. A short connection from the instrument to the battery itself also has resistance (about the micro-European level), and there is also contact resistance between the contact surface of the battery and the connection line. These factors must be adjusted in advance within the instrument. .

Therefore, regular battery internal resistance testers are generally equipped with a dedicated cable and battery holder.

Fourth, summary

Many aging batteries actually have a lot of internal power, but it is a pity that the internal resistance is too large to put out electricity. However, once the internal resistance of the battery is increased, it is more difficult to artificially reduce the internal resistance. Therefore, for the aging battery, we even think of many ways to "activate" it, such as high current impact, small current floating charge, put the refrigerator, etc., but most of them do nothing to help. After understanding the above knowledge, we can basically know that the battery should be selected as much as possible to select the battery with less internal resistance. In addition, it is very important that the battery will be used for a long time and its internal resistance will increase. It is recommended that you always use batteries to keep the chemical activity inside the battery.

The page contains the contents of the machine translation.