I. Overview of Internal Resistance in Lithium-Ion Batteries

Historically, batteries were considered ideal voltage sources—meaning their voltage remained constant regardless of the current they delivered. In reality, however, no battery can be regarded as an ideal voltage source due to inherent internal resistance. Key factors affecting a battery's ability to serve as an ideal voltage source include:

Battery lifespan: The older the battery, the higher its internal resistance.

Temperature: High temperatures can affect internal chemical reactions, leading to increased resistance.

SOC: The internal resistance of the battery varies depending on the SOC level.

(1) Composition of Internal Resistance

The internal resistance of a lithium-ion battery primarily consists of electrode materials, electrolyte, separator resistance, and contact resistance. Electrode materials form the core component, where their conductivity and structural properties determine the resistance to current flow; the electrolyte facilitates ion transport, with its ionic conductivity directly influencing internal resistance levels; the separator resistance isolates electrodes to prevent short circuits while also affecting ion diffusion and migration; contact resistance refers to the resistance at interfaces between battery components—including contacts between electrodes and current collectors, as well as electrodes and electrolyte. These components interact synergistically to constitute the battery's internal resistance, each playing distinct roles during operation. For instance, electrode reactions during charging/discharging generate polarization resistance that impacts performance; electrolyte ion conductivity affects both internal resistance and discharge efficiency; separator porosity and pore size distribution influence resistance levels and charge/discharge rates; while contact resistance determines energy loss and heat generation.

(II) Types of Internal Resistance

The internal resistance of lithium batteries is primarily categorized into DC internal resistance and AC internal resistance. DC internal resistance refers to the battery's resistance under direct current conditions, reflecting its internal resistance characteristics in a steady state. During DC resistance testing, it is typically determined by measuring the voltage drop at a specific current level. This parameter is essential for evaluating battery performance under static or steady-state conditions, including open-circuit voltage and charge/discharge capacity.

AC internal resistance refers to the battery's internal resistance under alternating current conditions, reflecting its dynamic resistance characteristics during operation. This measurement typically employs an AC signal source and analyzes the battery's impedance at various frequencies to determine its internal resistance. It is particularly crucial for evaluating battery performance under high-frequency and dynamic operating conditions, such as rapid charging/discharging or load variations.

Different internal resistances are employed in various scenarios to evaluate battery performance. For instance, during routine use and basic testing, DC internal resistance provides straightforward insights into battery performance; whereas in complex electronic devices and systems, AC internal resistance better reflects the battery's performance under dynamic conditions, enabling engineers to optimize design and operation. Additionally, AC internal resistance holds significant value in studying battery charging/discharging processes and predicting battery lifespan.

II. Internal Resistance Analysis Methods

(1) DC Internal Resistance Testing

The principle of DC internal resistance testing is based on Ohm's law, which states that a battery's internal resistance equals the voltage drop across its terminals divided by the current flowing through it. In the constant-current discharge method, the battery is discharged at a constant current, and the DC internal resistance is calculated by measuring the voltage drop during discharge.

The specific testing procedure is as follows: Use a constant-current source to discharge the battery, for example at a current of 5C–10C (for standard battery cells, higher currents can induce a significant voltage drop within a short period); record the voltage changes across the battery during discharge. The internal resistance of the battery can be calculated using the formula (where R_i is the DC internal resistance, ΔV is the voltage drop, and I is the current).

Commonly used testing equipment includes DC power supplies and electronic loads. These devices enable precise control of current magnitude and measurement of voltage across a battery. For example, in laboratories, high-precision digital multimeters are employed to measure the voltage across a battery, ensuring measurement accuracy.

(II) Internal Resistance Measurement of the Circuit

The principle of AC internal resistance testing is based on the impedance characteristics of batteries. When an AC current passes through a battery, an internal impedance is generated, whose magnitude depends on factors such as the battery's internal resistance, capacitance, and inductance. The AC internal resistance is determined by measuring the battery's impedance at different frequencies. The procedure involves applying a low-amplitude AC current signal (e.g., 1 kHz frequency, 50 mA) to the battery and measuring the resulting AC voltage across its terminals. By analyzing the phase difference and amplitude relationship between the voltage and current, the battery's AC internal resistance can be calculated.

AC internal resistance testing offers numerous advantages in practical applications. As it accurately reflects a battery's internal resistance characteristics during dynamic operation, it is particularly crucial for evaluating performance under complex conditions such as high-frequency and rapid charging/discharging cycles. For instance, in applications like electric vehicles where batteries undergo frequent charge-discharge cycles, AC internal resistance provides a more reliable indicator of their performance under these dynamic conditions. Additionally, this test can detect internal battery faults, including electrode aging and electrolyte depletion.

III. The Impact of Internal Resistance on Battery Performance

(1) Discharge Capacity

The internal resistance of a lithium battery significantly impacts its discharge capability. When the internal resistance is high, substantial power loss occurs during discharge because it impedes current flow, causing part of the electrical energy to be dissipated within the battery. For example, in a simple circuit with a battery having internal resistance R and a current I, the power consumed due to internal resistance equals I × R. Increased internal resistance leads to greater heat generation from current flow through the resistance, resulting in reduced actual output power. This is analogous to a car driving: higher internal resistance requires more engine power to overcome resistance, thereby compromising its performance.

From the perspective of battery operation, high internal resistance hinders electron transport and ion migration within the cell. During discharge, electrons generated by electrode reactions must travel through this resistance to reach the external circuit; greater internal resistance increases the resistance to electron flow, thereby slowing down the discharge rate. Additionally, ion diffusion in the electrolyte is also affected by internal resistance, limiting their concentration distribution at electrode surfaces and further compromising the battery's discharge performance.

(II) Charging Efficiency

Internal resistance significantly impacts battery charging efficiency. Higher internal resistance leads to greater energy loss during charging, as the current passing through it generates heat—an additional form of energy consumption. For example, given a battery's internal resistance and charging current, the power dissipated due to internal resistance equals [power value]. This power is converted into heat, causing the battery temperature to rise.

Meanwhile, internal resistance affects the charging speed of a battery. As it impedes current flow, the charging current cannot rapidly reach the desired level, thereby prolonging the charging time. For example, when charging a smartphone battery, higher internal resistance significantly slows down the charging rate, potentially requiring longer duration to achieve full charge.

Furthermore, internal resistance significantly impacts battery charging efficiency. During charging, the chemical reactions within the battery are influenced by this resistance. High internal resistance can lead to incomplete reactions, resulting in reduced charging efficiency. For instance, the electrolyte may not fully participate in the reaction due to resistance, preventing the battery from achieving its theoretical charging capacity.

(3) Battery Life

Internal resistance significantly impacts battery lifespan. High internal resistance leads to heat generation, as the power consumed by this resistance is converted into heat, raising the battery's temperature. Prolonged exposure to elevated temperatures accelerates the degradation of internal chemical components; for instance, the electrolyte may evaporate or decompose due to high temperatures, resulting in reduced performance.

Increased internal resistance also accelerates the chemical reactions within the battery. During charging and discharging, it affects both the rate and stability of electrode reactions. Elevated internal resistance speeds up these reactions, shortening the battery's cycle life. Additionally, it disrupts current distribution inside the battery, causing localized overcurrent that further accelerates aging.

Furthermore, excessive internal resistance can cause voltage drop during battery operation. When the internal resistance is high, the output voltage of the battery decreases, impairing its normal performance. For example, in mobile phone batteries, elevated internal resistance may lead to rapid battery depletion or even automatic shutdown.

The impact of internal resistance on battery performance is multifaceted. During both the design and operational phases of batteries, the factor of internal resistance must be thoroughly considered, and effective measures should be implemented to reduce it, thereby enhancing both battery performance and lifespan.

IV. Factors Affecting Internal Resistance

(1) Temperature

Temperature has a significant impact on the internal resistance of lithium-ion batteries. When the temperature rises, the movement velocity of ions and electrons within the battery increases, resulting in a decrease in internal resistance; conversely, when the temperature decreases, the movement velocity of ions and electrons slows down, leading to an increase in internal resistance.

From the perspective of battery operation, elevated temperatures increase the ionic conductivity of the electrolyte and accelerate ion migration rates, thereby reducing internal resistance. For instance, under high-temperature conditions, lithium ions diffuse more rapidly through the electrolyte, lowering the charge transfer resistance within the battery. Additionally, higher temperatures enhance the activity of electrode materials, promoting electrochemical reactions and further diminishing internal resistance.

Studies have shown that within a certain temperature range, the internal resistance of a battery exhibits a linear relationship with temperature. For example, when the temperature rises from 25°C to 50°C, the internal resistance gradually decreases with increasing temperature. However, at excessively high temperatures, the internal resistance may undergo abrupt changes, potentially leading to a decline in battery performance.

(II) Battery Structure Design

The number, size, and location of tab contacts significantly influence the battery's internal resistance. Within a certain range, increasing the number of tabs effectively reduces the internal resistance, as additional current pathways are provided, leading to more uniform current distribution within the battery and consequently lowering the internal resistance.

The placement of tabs also affects the battery's internal resistance. In wound batteries, where tabs are located at the heads of positive and negative electrodes, the internal resistance is higher; in contrast, laminated batteries—comprising dozens of small cells connected in parallel—exhibit relatively lower internal resistance due to more evenly distributed tabs that enhance current conductivity and reduce internal resistance.

Furthermore, the size of the tabs also affects the battery's internal resistance. Larger tabs provide a greater surface area for current flow, resulting in lower internal resistance. However, excessively large tabs may increase the battery's volume and weight; therefore, this factor must be comprehensively considered during design.

(III) Properties of Raw Materials

The performance of raw materials—including positive and negative electrode active materials, conductive agents, current collectors, and separators—significantly influences a battery's internal resistance. The conductivity and structural characteristics of these materials determine the battery's internal resistance level. For instance, cathode materials undergo coating and doping processes to enhance electron transport between particles; doping with nickel strengthens P–O bonds, stabilizes the LiFePO₄/C structure, and effectively reduces charge transfer impedance. Smaller particle sizes in anode materials result in lower activation polarization and consequently reduced internal resistance.

Conductive agents play a crucial role in batteries. Graphite and carbon black are widely used in lithium-ion batteries due to their excellent performance characteristics. Compared to graphite-based conductive agents, batteries incorporating carbon black-based conductive agents exhibit superior rate performance: graphite-based agents possess a flaky particle structure that leads to significant increases in pore tortuosity at high rates, thereby limiting discharge capacity through restricted Li liquid-phase diffusion. In contrast, batteries with carbon nanotubes (CNTs) demonstrate lower internal resistance because the fibrous CNTs establish linear contact with active materials rather than relying on point contacts with graphite/carbon black, effectively reducing interfacial impedance.

The interfacial resistance between the current collector and active material determines the battery's internal resistance. Applying a conductive carbon coating to aluminum foil and subjecting it to corona treatment significantly reduces the interfacial impedance. Compared to conventional aluminum foil, carbon-coated foil lowers the battery's internal resistance by approximately 65% and mitigates its increase during operation.

The liquid absorption and wettability of the separator significantly influence ion transport within the battery. A separator with higher liquid absorption capacity and a porous structure enhances conductivity, reduces battery impedance, and improves rate performance. For instance, applying a SiO₂ ceramic coating to a PP separator increases its liquid absorption by 17%; coating a PP/PE composite separator with 1 μm of PVDF-HFP raises the absorption rate from 70% to 82%, while reducing cell internal resistance by over 20%.

(IV) Manufacturing Process

Process parameters such as mixing, coating, rolling, and electrode sheet turnover time significantly influence the internal resistance of batteries.

The uniformity of slurry dispersion during mixing determines whether conductive agents can be evenly distributed within the active material and achieve close contact, thereby influencing the battery's internal resistance. Enhancing high-speed dispersion improves slurry homogeneity and reduces internal resistance. Adding surfactants optimizes the distribution of conductive agents in electrodes, minimizes electrochemical polarization, and elevates the median discharge voltage.

Coating surface density is one of the key parameters in battery design. When the battery capacity remains constant, increasing the electrode surface density reduces the total length of current collectors and separators, thereby lowering the battery's ohmic resistance. During coating and drying, the migration and detachment of solvent molecules are closely dependent on oven temperature, directly influencing the distribution of binders and conductive agents within the electrodes and consequently affecting the formation of internal conductive networks. Thus, drying temperature during coating is a critical process parameter for optimizing battery performance.

Within a certain range of rolling, the internal resistance of the battery decreases as the compaction density increases. Higher compaction density reduces the distance between raw material particles, enhances interparticle contact, and creates more conductive bridges and pathways, thereby lowering the battery impedance. However, when the rolling thickness is excessively high, the active material fails to be compacted sufficiently, leading to poor interaction between the active material and the current collector.

V. Practical Applications of Internal Resistance

(1) Black-box testing

In black-box testing, internal resistance measurement of battery modules is a critical method for evaluating battery performance. Typically, customers provide multiple battery modules for testing using various connection methods (e.g., laser welding or screw bonding).

Taking a battery pack with a capacity of 100 Ah and configured according to the 8S4P scheme as an example, two modules employ laser welding (R0–R15), while the remaining module uses screw fastening (R16–R23). During testing, the connection method of the battery pack was not disclosed; the validity of the internal resistance model was evaluated based on the test results.

Test data demonstrate that the internal resistance of laser-welded connections is significantly lower than that of screw-connected ones. This is attributed to the lower contact resistance formed during laser welding, which facilitates smoother current flow. In contrast, screw connections are prone to loosening—such as when a single screw in a module remains untightened—which results in markedly higher internal resistance compared to other batteries. This not only increases overall battery resistance but also generates excessive heat, causing significant corrosion on the battery's cover plate.

If the internal resistance remains excessively high over an extended period, significant safety risks may arise. During testing, upon identifying issues, promptly replace the faulty screws with alternative components for fixation; this results in a noticeable reduction in DC internal resistance. However, since these are not originally compatible screws, the internal resistance still fails to reach an ideal level.

Such tests effectively validate the validity of the battery internal resistance model, identify potential issues during battery connection, and provide insights for optimizing battery performance.

(II) Balanced Testing

In a UPS (Uninterruptible Power Supply) system, the DC internal resistance model plays a critical role in balance testing. Before applying this model, the battery voltage drop is significant and the overall battery pack consistency is poor.

For example, in a UPS system before equilibrium was applied using the DC internal resistance model, the battery voltage difference was 732 mV. By applying the DC internal resistance model, continuous equilibrium was maintained throughout the process. After 2,588 hours, the system's performance became more consistent, with the maximum voltage difference dropping below 10 mV, meeting the specified equilibrium criteria.

During this process, the DC internal resistance model enables real-time monitoring of battery internal resistance variations and adjusts charging/discharging conditions accordingly, ensuring relatively stable internal resistance across all battery cells in the pack. This approach achieves effective battery balancing, enhances consistency among cells, and optimizes overall pack performance.

(III) Bus Applications

High variability in the internal resistance of bus batteries significantly impacts battery performance. In practical applications, a typical bus system consists of 190 battery cells arranged in series. Significant variation in internal resistance directly manifests as reduced battery consistency and decreased range.

Analysis of the battery voltage variations reveals significant voltage variability across the entire battery pack during charging, along with uneven distribution of internal resistance values. Following communication with the customer, it was confirmed that these batteries belong to Batch B and exhibited inherent manufacturing variations at production.

To enhance battery performance, the balancing strategy requires adjustment. By analyzing the causes of significant variations in internal resistance and implementing corresponding measures—such as optimizing charge/discharge management and modifying connection configurations—the entire battery pack undergoes validation. Continuous monitoring of internal resistance and voltage fluctuations enables timely adjustments to the balancing strategy, thereby improving battery consistency and range while ensuring reliable operation of the bus.

Analysis of the internal resistance of lithium-ion batteries is crucial for optimizing battery performance and lifespan. Through research on lithium-ion battery internal resistance, we have gained a comprehensive understanding of its composition, types, analytical methods, and influencing factors.

In terms of internal resistance analysis methods, DC internal resistance testing and AC internal resistance testing each have distinct characteristics. DC internal resistance testing is based on Ohm's law, involves relatively simple procedures, and provides a direct reflection of the battery's internal resistance characteristics under static or steady-state conditions; in contrast, AC internal resistance testing utilizes the battery's impedance characteristics to more accurately represent its internal resistance behavior during dynamic changes, making it particularly important for evaluating battery performance under complex scenarios such as high-frequency operation and rapid charging/discharging.

The impact of internal resistance on battery performance manifests in several aspects. Regarding discharge capability, higher internal resistance increases internal power loss and reduces actual output power, compromising the battery's performance. For charging efficiency, elevated internal resistance leads to greater energy loss, slower charging rates, and lower charging efficiency. As for battery lifespan, increased internal resistance generates heat that accelerates the degradation of internal chemical components, thereby shortening the battery's service life.

Numerous factors influence internal resistance, including temperature, battery structural design, raw material properties, and manufacturing processes. Elevated temperatures reduce internal resistance; the number, dimensions, and placement of tabs in the battery structure affect it; characteristics of raw materials such as active materials for the positive and negative electrodes, conductive agents, current collectors, and separators are closely related to internal resistance; while process parameters like slurry preparation, coating, rolling, and electrode sheet turnover time also impact it.

In practical applications, scenarios such as black-box testing, battery balancing tests, and large-scale vehicle deployments all highlight the critical importance of internal resistance analysis. Black-box testing evaluates the internal resistance of battery modules to validate the effectiveness of internal resistance models; battery balancing tests employ DC internal resistance models to equalize batteries and enhance their performance consistency; in large-scale vehicle applications, significant variations in battery internal resistance can impair performance, which can be mitigated by optimizing balancing strategies.

Lithium-ion battery internal resistance analysis is a complex yet critical field. In practical applications, it is essential to comprehensively consider various factors and employ appropriate testing methods and technical approaches to optimize battery performance and extend its lifespan. Furthermore, with continuous technological advancements, we must persistently explore and develop new methodologies and techniques to meet the growing demands of battery applications.