Internal Resistance of a Battery

Internal resistance of a battery is one indicator of a battery’s current-carrying capacity. There is an inverse relationship between the two parameters:

• If the internal resistance of a battery is low, then the battery can deliver higher currents without significant voltage drops.
• If the internal resistance of a battery is high, then the capacity to carry current is low and the battery shows.

In an ideal scenario, a battery would have an internal resistance of zero. In reality, commercial batteries will have some internal resistance. This page outlines the causes, measurement methods, and implications of internal resistance, along with strategies to minimize it.

Why Internal Resistance Matters

Internal resistance is a key indicator of a battery’s performance, closely linked to both its state of charge (SoC) and state of health (SoH). Because a battery’s capacity determines its usefulness and lifespan, internal resistance becomes a critical parameter for monitoring and maintaining battery quality. Measured in milliohms (mΩ), internal resistance can also be used to indirectly estimate battery capacity, especially in systems where direct capacity measurements are impractical.

Measuring a battery cell's internal resistance is also important for identifying potential issues. Different battery chmistries will have different ranges of "normal" internal resistance:

Battery cell manufacturers typically provide internal resistance information in their datasheets. This is important information for designers and engineers who are selecting batteries for specific applications as it helps them understand the cell’s performance characteristics and how it may behave under different conditions.

What Causes Internal Resistance?

Internal resistance results from energy losses within the battery. Instead of all the electrochemical energy being converted into electrical energy, some is lost as heat and other forms of energy. These losses are caused by:

• Ohmic resistance: Caused by the inherent opposition of internal materials (such as electrodes and electrolyte) and electrical connections to the flow of current.
• Charge Transfer / Electrochemical resistance: The resistance of ion transferring from a solvated ionic state in the electrolyte crossing the electrode/electrolyte interface (solid electrolyte interphase - SEI) and inserting into the electrodes.
• Mass transport / Diffusion resistance: From movement of ions in the electrolyte and insertion into the electrode, dependent on material properties.

There are a few factors that affect the various forms of battery internal resistance:

Electrode Materials: The conductivity, potential and surface area of the electrodes significantly impact resistance. The selection of active material and current collector influences ohmic resistance. Lithium corrosion within the anode active materials can decrease battery capacity due to irreversible loss of mobile lithium ions.

Electrolyte: The electrolyte’s concentration, potential, state, and ion diffusion rate significantly affect ohmic resistance, with higher ionic conductivity leading to lower internal resistance. Its reactivity influences SEI formation and the likelihood of lithium plating, impacting electrochemical resistance. Additionally, interfacial resistance between the electrode and electrolyte contributes to overall internal resistance.

Temperature: Generally, lower temperatures increase the internal resistance, while higher temperatures decrease it. However, low temperatures can increase the likelihood of lithium plating as lithium-ion diffusion into graphite is slower due to reduced ionic mobility and sluggish electrode kinetics. High temperatures lead to excessive SEI formation and as a result, the internal resistance increases.

Age and Cycle Life: The number of times a battery has undergone the charge/discharge cycle also impacts the internal resistance. The older and more used a battery is the higher the internal resistance is likely to be due to degradation mechanisms such as electrolyte evaporation or excessive solid electrolyte interphase formation. Changes in diffusion properties may lead to changes in the charge and discharge times, apparent capacity and impedance.

Relationship to Capacity

Internal resistance often correlates more strongly with battery capacity than voltage alone. As batteries age, their capacity decreases and their internal resistance increases. This makes internal resistance a more accessible parameter for capacity estimation, especially within battery management systems (BMS).

Measuring Internal Resistance

Internal resistance of a battery is typically measured by applying a known current and observing the resulting voltage change. Several techniques are used depending on the desired accuracy, battery type, and testing environment.

There are a variety of methods for measuring internal resistance, including:

Direct Current Internal Resistance (DCIR) - This method applies a small DC load to the battery and observes the instantaneous voltage drop. It gives a basic, straightforward measurement of internal resistance.

Hybrid Pulse Power Characterization (HPPC) - HPPC applies short, high-current pulses to the battery and calculates the internal resistance based on the voltage change during those pulses. It is widely used for benchmarking batteries under controlled conditions in EV battery development.

Electrochemical Impedance Spectroscopy (EIS) - A small AC signal is applied at various frequencies. The impedance (including internal resistance) is calculated from the voltage-to-current ratio across frequencies.

Direct Current Short-Pulse Method - This short pulse measurement method can accurately measure the internal resistance of the battery when the battery loads current changes. The capacity calibration is performed by the constant current-constant voltage (CC-CV) charge and discharge test. Proven effective through experimental validation and is used in some embedded battery management systems.

Limitations for Online/Real-Time Testing

Although DCIR, HPPC, and EIS are accurate and well-established, they have complex trigger and operational conditions that make them unsuitable for real-time or “online” testing in electric vehicles.

Online testing refers to the continuous monitoring and assessment of battery parameters, such as internal resistance, in real-time, while the battery is in use, without interrupting its operation. For EVs, this is essential to ensure safety, efficiency, and predictive maintenance during driving and charging cycles.

However, both DCIR and HPPC typically require:

• Controlled load pulses or rest periods.
• Specific battery states (e.g. fully rested or stable temperatures).
• Temporarily taking the battery out of normal operational mode.

As such, these methods are better suited for lab environments or scheduled diagnostics, not for in-the-loop monitoring required by battery management systems in EVs during real-world operation.

Modeling Internal Resistance

Models simplify complex electrochemical behavior into electrical components that can be easily analyzed and computed. Model-based methods include empirical models, electrochemical models , and equivalent circuit models (ECM). Empirical models use linear, exponential, or polynomial modeal to fit the battery capacity decay process. These models are usually only applicable to specific aging patterns and battery types, and difficult to capture complex aging processes due to the unknown internal variations. Electrochemical models use partial differential equations to describe the electrochemical processes inside the battery with complex calculations. The complexity, which limits their engineering applications, is reduced by ignoring certain factors which in turn limits the accuracy.

The ECM equates battery electrochemistry with circuit elements to track the battery voltage dynamically. Among these, the 1 RC (single resistor-capacitor) equivalent circuit model, also known as the Thévenin equivalent circuit, strikes a balance between accuracy, computational efficiency, and real-time applicability. This approach is particularly useful in battery management systems (BMS), where quick and reliable estimation of battery parameters is essential for performance, safety, and longevity.

The battery’s voltage response across its internal resistance can be analyzed using an equivalent circuit model during pulsed discharge or charging at a given current. When a discharge current (I) is applied, there is an immediate and sharp drop in the battery’s voltage due to its internal resistance (R_b). This instantaneous change in voltage is referred to as the V_edge value:

V_edge = R_b × ΔI = (R_b × I) − (R_b × I₀)

Where I₀ is the baseline current load which includes the current drawn from the battery by the device even in sleep mode. If the load remains the same, the battery voltage decreases almost linearly after the initial sharp drop. This is due to the battery’s double-layer capacitance (C₀) and the polarization resistance. The length of the linearity depends on the duration of the discharge load.

The internal resistance as a result of a charging current can be calculated using the same equation. It follows that instead of current load, the current is injected into the battery. Therefore, there is an increase in battery voltage as the current is injected due to internal resistance. The V_edge calculation remains the same where I is charge current instead of discharge current.

The internal resistance also depends on the amount of charging or discharging current applied to a battery in a pulse. Voltage drop across battery internal resistance increases linearly with the pulse discharging loads for a battery. However, the resistance is inversely proportional to the applied current. Therefore, the resistance decreases exponentially as the pulse current increases.

Improving the Internal Resistance of a Battery

Minimizing the internal resistance of a battery is crucial for enhancing its overall electrochemical performance. Lower internal resistance directly contributes to increased capacity, improved power delivery, reduced heat generation, and ultimately, extended battery lifetime. Therefore, the reduction of internal resistance is a core strategy in advancing battery technology.

Materials Optimization

The selection and engineering of battery materials are fundamental to minimizing internal resistance. Key approaches include:

• High-Quality Electrode Materials: Materials with high intrinsic electrical conductivity and chemical stability are essential to maintain low resistance throughout the battery's lifespan.
• High Ionic Conductivity Electrolytes: Liquid, gel, or solid-state electrolytes with superior ionic mobility help reduce electrolyte-related resistive losses. Ionic conductivity is directly correlated with the efficiency of ion transport across the cell.
• Conductive Additives: Incorporating highly conductive nanomaterials such as graphene, carbon black and carbon nanotubes (CNTs) into the electrode matrix significantly enhances the formation of conductive networks. These networks improve electronic transport pathways and reduce interparticle resistance within electrodes.

Structural Design Enhancements

Battery architecture plays a decisive role in controlling internal resistance. Several structural innovations include:

• Optimization of Current Collector Connections: The design and positioning of current collector connection tabs welded to the electrodes must be carefully controlled to minimize contact resistance and ensure efficient electron flow.
• Electrode Densification: Increasing the electrode density, particularly for the cathode, through mechanical compression or reduction of thickness, optimizes the electrode microstructure. Densification enhances electronic pathways and reduces void spaces, thereby lowering overall resistance.
• Ultrathin Electrodes: Employing thinner electrode layers shortens the ionic diffusion distance, accelerating charge/discharge rates and reducing ionic resistance across the active material.
• Nanostructuring: Designing electrodes with nanoscale features can provide large surface areas, shorten ion diffusion paths, and enhance electrode–electrolyte interaction, all of which contribute to minimizing internal impedance.

Processing and Manufacturing Techniques

The processing phase critically affects the final internal resistance characteristics of a battery:

• Mixing and Dispersion Control: The preparation of homogeneous electrode slurries involves complex mixing, dissolution, and dispersion processes between liquid and solid phases. Proper control during slurry preparation ensures that conductive additives are uniformly distributed, facilitating continuous conductive pathways.
• Mitigating Particle Agglomeration: Solid particles in slurry are prone to settling and agglomeration, which can disrupt the uniformity of the electrode microstructure. Poor mixing can damage particle morphology and inhibit the formation of stable, well-connected conductive networks, resulting in higher internal resistance.
• Mechanical Stability and Bond Formation: Ensuring the mechanical integrity of the electrode through optimized processing conditions supports the establishment of robust particle-to-particle and particle-to-binder connections, essential for maintaining low electronic and ionic resistance throughout battery cycling.

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