Introduction of the internal resistance of lithium ion battery

Internal resistance, static internal resistance and working internal resistance of lithium ion battery tend to be different. In different environments, the internal resistance varies with the temperature. What factors affect the internal resistance of lithium ion battery?

1 the working process of the lithium battery

At present, the mainstream lithium-ion battery, generally in accordance with the type of positive electrode material named, lithium iron phosphate, lithium manganese acid and so on is the type of positive electrode material; Negative electrode graphite material; Positive collector fluid aluminum foil, negative collector fluid for copper foil.

The following discharge, for example, describe the physical process of lithium battery is discharging.

After the external load is switched on, a current path is formed outside the battery body. Because of the potential difference between the positive and negative electrodes, the electrons near the negative electrode first move to the positive electrode by collecting the fluid and external wire. The lithium ion concentration around the negative pole goes up. Electrons that go from the negative electrode to the positive electrode through an external circuit combine with lithium ions near the positive electrode. Embedding the anode material, near the anode of lithium ion concentration decreased. The difference in lithium ion concentration between the positive and negative poles forms. This completes the first push of the battery discharge process.

As the lithium ions leave the negative electrode driven by the difference in ion concentration, vacancies appear near the negative electrode. The lithium ions in the cathode material, detaching from the cathode, enter the electrolyte; A large number of lithium ions from the electrolyte through the diaphragm, the ego pole to the positive pole. At the same time, electrons that were originally bound to lithium ions go to the positive electrode via an external circuit. The battery starts discharging as required by the load.

Charging is the reverse process of discharge. In the same stage of delinking, moving and embedding, the power to promote the development of the process only comes from the charger, while the movement direction of ions is from the positive pole to the negative pole. I won't go into details here.

2 lithium battery internal resistance

Understand the working process of the lithium battery, so in the process of obstacles, then formed the lithium battery internal resistance.

The internal resistance of the battery includes OHMIC resistance and polarization resistance. Under the condition of constant temperature, OHMIC resistance is basically stable, while polarization resistance changes with the factors affecting polarization level.

OHMIC resistance is mainly composed of electrode material, electrolyte and membrane resistance and collection of fluid, such as ear connection of various parts of the contact resistance, and the size of the battery, the structure, connecting method and so on.

Polarization resistance, the resistance created at the moment the current is applied, is the sum of all the trends in the battery that prevent charged ions from reaching their destinations. Polarization resistance can be divided into electrochemical polarization and concentration difference polarization. Electrochemical polarization is caused by the fact that the speed of electrochemical reaction in electrolyte cannot reach the speed of electron movement. Concentration polarization is caused by the fact that lithium ions embed and move out of the anode and cathode materials at a speed lower than the speed at which lithium ions concentrate towards the electrode.

3 lithium battery internal resistance factors

From the above process can deduce the influence factors of battery internal resistance.

3.1; plus factor

Environmental temperature is an important factor affecting the resistance of a variety of factors, especially lithium battery, is because the temperature affects the activity of electrochemical materials, directly determines the speed of electrochemical reaction and the speed of ion movement.

Current or load demand, on the one hand, the size of the current is directly associated with the polarization resistance. This trend is the greater the current, the bigger the polarization resistance. On the other hand, the heating effect of the current, impact on the electrochemical activity of material.

3.2; the battery itself

Cathode materials, anode materials, lithium ion embedding and the ease of embedded, decided to the size of the internal resistance of the material, is part of the concentration polarization resistance.

Electrolyte, lithium ion electrolyte in the moving speed, the influence of electrolyte conductivity, is the main part of electrochemical polarization resistance.

Diaphragm, diaphragm itself resistance, direct constitute part of the ohm internal resistance, at the same time for the lithium ion mobility barriers, and formed a part of electrochemical polarization resistance.

Collection of fluid resistance, parts connected resistance, is the main component of battery ohm internal resistance.

The technological level, the pole piece manufacture technology, whether the coating is uniform, the compaction density, the level of technology in the manufacturing process of these batteries, will also directly effects on the polarization resistance.

4 Measurement of internal resistance of lithium battery

Measurement of internal resistance of lithium battery generally can be divided into dc measurement method and ac measuring method.

4.1; dc resistance measurement

Using a current source, a short pulse is applied to the battery to measure the difference between the terminal voltage and the open circuit voltage. This difference divided by the test current is considered to be the internal dc resistance of the battery.

Lithium battery polarization resistance will be affected by the size of the load current, in order to try to stay away from this factor, dc resistance measurement method of the polarization time is shorter and the load current is larger.

Theoretically, the smaller the measurement current is, the less likely it will cause polarization reaction and reduce the interference of polarization resistance. However, because the internal resistance of the battery itself is very small, which is of milliohm magnitude, the current is too small, and the voltage detection instrument is limited by the measurement accuracy, the measurement error cannot be excluded from the interference of the results. Therefore, people weigh the influence of instrument precision and polarization resistance to find a balance between the two measuring current value.

For ordinary battery monomer, measure the current general in 5 c - around 10 c, very big. With the increment of capacity of batteries, or multiple batteries in parallel, its internal resistance is reduced, therefore, if there is no instrument accuracy improvement, measuring current is difficult to come down.

4.2; ac internal resistance measuring methods

For smaller values of the battery to load a picture ac input as an incentive, monitoring the response of terminal voltage. Using a specific program to analyze data, draw AC internal resistance of the battery. Analysis of resistance, only related to the battery itself characteristic, has nothing to do with the size of the excitation signal.

Because of the capacitance characteristics of the battery, the resistance measured by the excitation signal varies with its frequency. The results of software analysis can be represented by a set of complex Numbers, with the horizontal axis as the real part and the vertical axis as the imaginary part. This creates a pattern called the ac impedance spectrum.

Through further analysis of data, one can get it only from the ac impedance spectroscopy battery ohm resistor, SEI film diffusion resistance, capacitance value of the SEI film, the equivalent capacitance of charge transfer in the electrolyte and charge in the electrolyte diffusion resistance, equivalent model, then map the battery for the further study of the battery performance.

5 resistance application in engineering practice

Internal resistance, as one of the key characteristic of lithium battery, for its research achievements, can be applied in engineering, and other fields.

Resistance have close relationship with the battery charge capacity, therefore be applied to battery SOC estimation of management system;

Resistance directly reflect the battery aging degree, someone put batteries resistance as the SOH battery state of health of the assessment basis;

The uniformity of internal resistance of monomer has a direct influence on the capacity and life of modules after grouping;

Internal resistance is an important indicator of battery failure. It is studied and used in the fault diagnosis system of power battery packs;

Internal resistance and capacity loss can also be used to determine whether lithium analysis exists in batteries, which has been applied in the field of cascade utilization of retired batteries.

Reference:

1Chi-Jian ZHANG, lithium battery SOC prediction methods were reviewed

2 the residual capacity of battery online detection method research

3 battery internal resistance measurement based on ac impedance method

4 Determination of dc internal resistance of lithium-iron phosphate battery

5 Weizhao Huang, based on the open circuit voltage recovery rate and ac impedance with the combination of a lithium ion battery SOH algorithm

6 Xuguo Fan, Study on equivalent circuit modeling and SOC estimation of lithium-ion battery pack 

7 Ran Li, Study on health evaluation and estimation method of lithium power battery 

8 Zhilong Yu, Research on SOC prediction algorithm of lithium battery for electric vehicle based on self-discharge technology 

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