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Internal Resistance of A Lithium Battery - AC Resistance And DC Resistance

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Internal Resistance of A Lithium Battery - AC Resistance And DC Resistance

Resistance is a physical quantity that describes the degree to which circuit elements obstruct current transmission. The internal resistance (internal resistance) of lithium batteries is an important indicator for evaluating battery performance. The internal resistance of lithium batteries plays three important roles in practical applications:


1. It can be used to assess battery health and forecast battery life.


2. It can be used to calculate the battery's state of charge (SOC).


3. The internal resistance can be used to determine the connection status of the circuit in the battery module, and timely judgment can be made when the connection is loose.


When current flows through an electrode, the phenomenon in which the electrode deviates from the equilibrium electrode potential is referred to as polarization of the battery, and the polarization causes the overpotential. Understanding polarization is critical for understanding the internal resistance of the battery and the relationships that exist between them. Polarization in lithium batteries can be classified into three types based on the cause of polarization:


1. Ohmic polarization: the battery is made up of electrode materials, electrolytes, a diaphragm, and various parts; ohmic polarization is caused by the resistance of the battery connected to each part; the voltage drop value follows the Ohmic law, and the current is reduced; polarization is immediately reduced, and the current disappears.


2. Electrochemical polarization occurs when the electrode surface produces an electrochemical reaction after the battery is turned on. At this point, the charge transfer rate of a step in the electrochemical reaction process does not reach the impedance of the external discharge rate, and the battery must allocate a certain voltage to meet the activation energy of its transfer rate. Polarization decreases significantly in microseconds as the current decreases. Electrochemical polarization, on the other hand, generates electrochemical internal resistance, also known as charge transfer impedance. Click here for 51.2V 50Ah Lithium Iron Battery.


3. Concentration polarization: Because the electrode surface consumes reactants that cannot be replenished in time, ion concentration differences on the reaction surface occur as a result of material transfer, resulting in concentration polarization. On the microsecond scale (a few seconds to tens of seconds), this polarization decreases or disappears as the current increases. Concentration polarization, on the other hand, causes concentration internal resistance, also known as lithium-ion migration impedance.


On the time scale, ohmic polarization occurs instantly, electrochemical polarization occurs at the microsecond level, and concentration polarization occurs at the second level.


Several concepts are related:

1. Ohmic internal resistance: Ohmic internal resistance is produced by ohmic polarization.


2. Polarization internal resistance: the resistance caused by polarization during an electrochemical reaction, including electrochemical polarization and concentration polarization, and the polarization capacitor connected in parallel to form a resistance loop, used to simulate the dynamic characteristics of the battery polarization generation and elimination process.


Thevenin equivalent circuit model, also known as the first-order model, can approximate batteries, and their connection relationships are depicted in the figure below. Where OCV denotes the battery's open circuit voltage, Ro denotes the ohm internal resistance, Rp denotes the equivalent polarization internal resistance, and Cp denotes the equivalent polarization capacitance.

Cp is the equivalent polarization capacitance

In general, the test results that are commonly used by businesses are divided into two categories: 1. internal resistance to communication; 2. internal resistance to direct current


The internal resistance of Ac: The AC internal resistance is calculated by injecting a sinusoidal current signal I=Imaxsin(2ft) into the positive and negative electrodes of the battery and detecting the voltage drop U=Umaxsin(2ft+) at both ends of the battery. In general, a 1kHz sinusoidal AC signal is applied to the battery's positive and negative terminals, and the parallel value of the Rp and Cp of the battery at this frequency is generally small (note: because the capacitor is approximately short-circuited under the high-frequency signal), which can be ignored. As a result, the resistance detected by the AC signal is relatively close to the value of the ohm internal resistance Ro, and the AC internal resistance can be thought of as the battery's ohm internal resistance. The internal resistance meter is frequently used in the battery production line to measure the internal resistance of the battery, as well as the AC resistance, which is primarily used to evaluate the battery core production process. The coating effect of positive and negative electrode materials can be evaluated using the voltage waveform, and the electrode welding effect can be improved.


DC internal resistance is the application of a DC signal to the battery to test its internal resistance, typically a constant current pulse current. The DC internal resistance is generally defined as the battery's ohm internal resistance + charge transfer impedance + lithium ion migration impedance (the absence of concentration polarization due to differences in test methods may result in only the ohm internal resistance + charge transfer impedance).



The ohm internal resistance of a battery is related to its size, structure, and assembly, and its resistance value has nothing to do with the charge and discharge states and is almost unaffected by the SOC state.


The polarization internal resistance of the battery occurs only during the charge and discharge process, and it is affected by the state of SOC. When the battery's SOC is close to 0% or 100%, its internal resistance to polarization is high, and when the SOC is between 20% and 80%, it has a low internal resistance to polarization. And as the number of battery cycles increases, this phenomenon will become more prevalent. The interface between the electrode active substance and the electrolyte of the lithium-ion battery gradually degrades after many cycles, increasing electrochemical impedance.


DC internal resistance testing procedure:

Because of the presence of polarization, the voltage of the battery will rebound after the discharge process is completed. The DC impedance measurement is used to calculate the internal resistance of the battery by using the voltage difference between the voltage before and after the end of discharge. The battery is specifically discharged with a constant current of size I, as shown below:

DC internal resistance

As shown in the figure below, record and draw the curve of battery terminal voltage over time, as well as collect the voltage drop and voltage rise of the battery: The initial stage of discharge begins at time t0. The battery terminal voltage drops from point A to point B due to the presence of ohm internal resistance and then enters the discharge stabilization stage until the voltage drops to point C (time t1). The ohm internal resistance voltage drop disappears at this point due to the interruption of the current, and the voltage rises to point D. At the same time, the capacitor voltage cannot change due to the presence of the polarized capacitor, and the battery voltage gradually recovers and enters the discharge recovery stage, until the polarized capacitor at point E is discharged and the battery terminal voltage does not change.

the existence of ohm internal resistance

The DC internal resistance is equal to the difference in battery terminal voltage between C and Phase E divided by the discharge current I.


Polarization resistance testing method:

In the discharge recovery stage, the voltage at both ends of the polarization capacitor Cp does not change sharply and is equal to the polarization resistance Rp voltage, its value is the value of the battery voltage recovery stage, and the current flowing through the polarization resistance Rp before stopping discharge is the discharge current I, as shown in the figure above. As a result, D- can pass the polarization resistance Rp. The terminal voltage change calculation formula in phase E is as follows: I is the discharge current divided by the change in terminal voltage.

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