Lithium ion battery internal resistance – measurement and calculation method

What is lithium ion battery internal resistance

The lithium ion battery internal resistance refers to the resistance of the current flowing through the battery when the battery is working, and indicates the degree of obstruction of a circuit element to the transmission of current.

General lithium ion battery internal resistance is divided into AC internal resistance and DC internal resistance. Due to the small internal resistance of the rechargeable battery, the DC internal resistance is measured due to the electrode capacity polarization, resulting in the polarization of the internal resistance, so the true value can not be measured, and the AC internal resistance can be measured to avoid the influence of the polarization of the internal resistance, so as to obtain the true internal value.

Composition of lithium ion battery internal resistance

Ionic-resistance
① Electrolyte inside the diaphragm

Influencing factors: electrolyte conductivity, diaphragm area, thickness, porosity, zigzag coefficient (Gurley)

② Electrolyte inside the cathode

Influencing factors: electrolyte conductivity, cathode thickness, thickness, porosity, zigzag coefficient

③ Electrolyte inside the anode in lithium ion battery

Influencing factors: electrolyte conductivity, cathode thickness, thickness, porosity, zigzag coefficient

Electronic resistance

① The active substance of the two electrodes

Influencing factors: electrode conductivity, thickness, area

② Fluid collection (copper foil and aluminum foil)

Influencing factors: thickness, width, length of fluid collection, number and location of poles

③ Lead (pole, pole cylinder, internal conductive connection element)

Influencing factors: overall dimension, electrical conductivity

Contact resistance between the active substance and the battery current collector

① Cathode material and aluminum foil

② Anode material and copper foil

③ Electrochemical impedance of the battery

The equivalent circuit diagram of the electrode is composed of ohm impedance Rb, double layer capacitance Cd, electrochemical reaction impedance Rct and diffusion resistance Rw. Generally speaking, in the process of lithium ion embedding and removal cycle, the change of Rb value is generally small, while the change of Cd and Rct is more obvious.

The impedance of the battery is mainly embodied in the electrochemical reaction impedance, and the ohmic impedance is relatively small. With the decrease of temperature, the impedance of the battery gradually increased, and after dropping to 0°C, the impedance speed increased, and the cathode impedance showed a similar trend. From the numerical point of view, the impedance of the battery mainly comes from the contribution of the cathode impedance.

Common measurement methods of battery internal resistance

The lithium ion battery internal resistance is one of the important indicators to evaluate the performance of the battery. The internal resistance test includes AC internal resistance and DC internal resistance. For a single battery, the AC internal resistance is generally evaluated, which is usually called the ohmic internal resistance.

However, in the specific application process of large-scale battery pack projects, due to the limitations of test equipment and other aspects, it is not possible or convenient to directly test the AC internal resistance, and the characteristics of the battery pack are generally evaluated through the DC internal resistance.

In practical applications, DC internal resistance is also used to evaluate the health of the battery, predict the life, and estimate the system SOC, SOP, etc. At the same time, the detection of DC internal resistance can judge the connection status in the module, and timely feedback can be made for the loose connection.

1. Density method

Density method is mainly used to measure the density of battery electrolyte to estimate the internal resistance of the battery. It is often used to measure the internal resistance of the open lead-acid battery, and is not suitable for the internal resistance measurement of the sealed lead-acid battery. The application range of this method is narrow.

2. Open circuit voltage method

The open circuit voltage method is to estimate the internal resistance of the battery by measuring the terminal voltage of the battery. This method is not accurate enough and even easy to draw wrong conclusions. Because even if the capacity of a battery has become very small, its terminal voltage may still behave normally in the floating state.

3.DC discharge method

The DC lithium ion battery internal resistance is of great help to the entire balance management, SOP management, and protection logic. Since the SOP and OCV-SOC tested by the battery factory are tested for a single cell, the overall value will be different after the battery group runs for a period of time. At this time, it is necessary to use the measured DC internal resistance to fit and correct the data.

The DC discharge method is to measure the instantaneous voltage drop on the battery (generally 2 ~ 3s) by instant large current discharge on the battery, and calculate the internal resistance of the battery through Ohm’s law (R=V/I). The accuracy of this measurement method is high, if properly controlled, the measurement accuracy error can be controlled within 0.1%, and it has been widely used in practice.

However, this method has obvious shortcomings: it can only measure large-capacity batteries, and small-capacity batteries cannot load 40-80A large current within 2 ~ 3s. When the battery passes through a large current, the electrode inside the battery will be polarized, resulting in an internal resistance to polarization.

Therefore, the measurement time must be very short, otherwise the error of the measured internal resistance value is very large, otherwise the large current passing through the battery will cause certain damage to the electrodes inside the battery.

4. Ac injection method

By injecting a constant AC current signal IS into the battery, the voltage response signal Vo at both ends of the battery is measured, and the phase difference impedance formula of the two is used to determine the internal resistance R of the battery. The battery measurement time of the method is very short, generally about 100ms, almost as soon as the measurement switch is measured, the accuracy of this measurement method is also good, the measurement accuracy error is generally 1% to 2%.

This measurement method can measure almost any battery, including small capacity batteries. However, the measurement accuracy of AC voltage drop measurement method is likely to be affected by ripple current, and harmonic current interference is possible, which is a test for the anti-interference ability of measuring instruments. Measured in this way, the battery itself will not have much damage.

5. Internal resistance tester

The internal resistance tester is used to measure the internal impedance of the battery and the degree of damage of the acidified film of the battery. It is to apply 1KHz AC signal to the tested object, and obtain its internal resistance by measuring its AC pressure drop.

Different from the principle of measuring resistance with a multimeter, the value measured by it is milliohm, while the value measured by a multimeter is ohm. The multimeter can only measure the resistance of the object without power supply, and the internal resistance meter can measure the resistance of both the object without power supply and the object with power supply.

Therefore, the two are not the same. Use the size of the internal resistance value to judge the deterioration of the battery, in general, the smaller the resistance value of the battery, the better the performance. The method of measuring the internal resistance is a good method with high speed and reliability.

Calculation method of lithium ion battery internal resistance

According to the physical formula R=U/I, the test equipment makes the lithium ion battery in a short time (generally 2-3 seconds) to force through a large stable DC current (generally use 40A ~ 80A large current), measure the voltage at both ends of the lithium ion battery at this time, and calculate the lithium ion battery internal resistance at that time according to the formula.

1. Measure the open circuit voltage of the battery: U1

2. Two ends of the battery parallel a fixed resistance value of the resistance: R, discharge;

3. Measure the voltage at both ends of the lithium ion battery during discharge: U2;

4. Calculate the battery internal resistance: r=(U1-U2)/(U2/R)

For example, if the open circuit voltage of a battery is 12V, and the voltage drops to 10V after a 10 ohm resistor is connected in parallel, the internal resistance of the battery is r=(U1-U2)/(U2/R)=(12-10)/(10/10)=2 ohms.

Under normal circumstances, the larger the lithium ion battery internal resistance, the worse the battery load, and the internal resistance r of high-power batteries (such as batteries) is usually very small. The internal resistance of low-power batteries (such as 9V laminated batteries) is usually relatively large.

What is the best lithium ion battery internal resistance

The lithium ion battery internal resistance cannot be greater than the current discharge, and the lower the theoretical internal resistance, the better. The internal resistance and capacity are directly related. However, if the internal resistance is 0, it may be because the battery is bad.

The general lithium ion battery assumes 1 amp, the internal resistance is about 30 to 80 milliohm, and the good power lithium ion battery can be less than 15 milliohm. When the internal resistance increases, the heat loss increases, and the temperature changes in use will worsen the battery performance, and after being used in series with other batteries, there will be a mismatch problem, resulting in a decline in the overall battery pack capacity and performance.

The internal resistance of the lithium ion battery is related to the standby time, taking the 1200MAH mobile phone lithium battery as an example, the typical value of the internal resistance of the finished battery is 110 milliohm.

Sometimes, the internal resistance of the finished product is too low, which may be caused by the failure of the protective MOS tube.

1. If the internal resistance is large, the heat of lithium ion batteries is higher when discharging.

2. If the internal resistance is small, the power consumption is also small.

The key factor affecting the internal resistance

The internal resistance is the most sensitive to temperature, and the internal resistance value can change greatly at different temperatures. The performance of lithium ion batteries decreases at low temperatures, and one of the important reasons is that the lithium ion battery internal resistance at low temperatures is too large. For lithium ion batteries as a power source, the internal resistance is definitely the smaller the better. Especially in power applications, small internal resistance is a necessary condition.

Influence of lithium ion battery internal resistance on power output

In the process of cell selection, we usually have requirements for the internal resistance of the battery, such as 0.54~0.60mΩ, < 1mΩ, 2.0± 0.3mΩ@10s 50%SOC 25℃ and so on. Then how does the internal resistance affect the power output?

The default electromotive force is born with, related to the material, then the existence of internal resistance will be partial voltage, resulting in a drop in terminal voltage, which will affect the increase of power, in addition to the continuous increase of internal resistance, bringing the increase of heat, which is a safety risk to the battery. In line with this principle, the internal resistance value is designed as small as possible.

The power calculation formula is as below (where V0 represents the electromotive force and V represents the terminal voltage) :

When the current increases from 0, the power continues to increase, when I= a certain value, the power can reach the maximum value, and then increase, the power value will gradually decrease. However, under the actual motor power demand, it is far from so large, even if it is 3C fast charging.

In the design of the internal resistance, it is more about heat production and safety considerations, so the smaller internal resistance specifications will be selected as far as possible. The magnitude of the internal resistance only affects the theoretical power value at the maximum discharge current.