Internal resistance analysis is a fundamental diagnostic technique for evaluating the power capability, energy efficiency, thermal behavior, and long-term health of energy storage batteries. By quantifying ohmic, charge transfer, and diffusion resistance components through standardized AC and DC measurement methods, manufacturers, system integrators, and accredited third-party laboratories can screen cells for consistency, predict degradation, and optimize battery management strategies. This article provides a comprehensive technical overview of internal resistance testing principles, sample types, testing items, measurement methods, and practical applications for grid-scale and stationary energy storage batteries.
Technical Overview
Internal resistance is one of the most critical performance and health indicators for energy storage batteries. It represents the opposition to current flow within the cell, directly determining how much energy is dissipated as heat during charge and discharge, how much power the battery can deliver, and how efficiently the storage system operates over its service life. A battery with elevated internal resistance will exhibit reduced round-trip efficiency, increased heat generation, accelerated aging, and premature power derating.
Internal resistance in a lithium-ion or other secondary battery consists of three main components. The first is ohmic resistance, which arises from the electrolyte, electrode materials, separator, current collectors, tabs, and welded connections. The second is charge transfer resistance, an electrochemical polarization term associated with reaction kinetics at the electrode-electrolyte interfaces. The third is diffusion or concentration polarization resistance, which appears when ion transport in the electrolyte and within active material particles becomes rate-limiting, typically at high currents or low frequencies.
In engineering practice, internal resistance is generally characterized in two forms. AC internal resistance (ACIR) is measured by applying a small alternating current signal, conventionally at 1 kHz, and reflects primarily the ohmic component of the cell. DC internal resistance (DCIR) is calculated from the voltage response to a defined current pulse and captures ohmic resistance combined with polarization effects under realistic operating conditions. A more complete picture is obtained through electrochemical impedance spectroscopy (EIS), which separates the individual resistance contributions as a function of frequency.
Because internal resistance correlates strongly with state of health (SOH), temperature response, aging mechanisms such as lithium plating and electrolyte dry-out, and cell-to-cell consistency, its systematic analysis is required by major international and national standards, including IEC 61960, IEC 62660, IEC 62619, GB/T 31486, GB/T 36276, and UL 1973. Accredited third-party laboratories routinely perform internal resistance analysis for quality control, design validation, and safety compliance of energy storage products.
Testing Samples
Lithium-Ion Cells for Stationary Energy Storage
The most common test samples are commercial lithium-ion cells intended for grid and stationary storage, including lithium iron phosphate (LFP), nickel manganese cobalt oxide (NMC), nickel cobalt aluminum oxide (NCA), and lithium titanate oxide (LTO) chemistries. Sample formats range from small cylindrical cells to large prismatic and pouch cells with capacities from a few ampere-hours to several hundred ampere-hours. Single cells are the primary unit for accurate internal resistance characterization.
Modules, Packs, and Racks
Battery modules, packs, and rack-level assemblies can also be evaluated to determine total series resistance, including busbar, welding, and terminal connection resistance. Module-level measurements are essential for verifying manufacturing quality, identifying high-resistance joints, and validating the resistance distribution assumed in thermal and electrical design calculations.
Aged, Cycled, and Second-Life Cells
Cells at various stages of aging, including laboratory-cycled samples, field-retired units, and second-life candidates from electric vehicle applications, are analyzed to quantify resistance growth. Comparing the internal resistance of aged samples against fresh-cell baselines allows degradation mechanisms and remaining useful life to be assessed.
Emerging Chemistry Cells
Sodium-ion cells, flow battery stacks, and advanced lithium-based cells under development can be submitted for internal resistance characterization to benchmark new chemistries, evaluate electrode formulations, and compare electrolyte additives during research and development.
Testing Items
Internal resistance analysis for energy storage batteries typically encompasses the following test items:
- AC internal resistance (ACIR) at 1 kHz, the industry-standard quick screening parameter for cell consistency.
- DC internal resistance (DCIR) under defined charge and discharge pulse profiles, per IEC 61960, GB/T 31486, and hybrid pulse power characterization (HPPC) style protocols.
- Ohmic resistance extracted from the high-frequency intercept of the impedance spectrum.
- Charge transfer resistance and double-layer capacitance derived from the semicircle region of the Nyquist plot.
- Diffusion or Warburg resistance obtained from the low-frequency tail of the impedance response.
- Internal resistance as a function of state of charge (SOC), typically mapped from 0% to 100% in 10% increments.
- Internal resistance as a function of temperature, commonly from -20 °C to 55 °C, to define cold-weather power limits.
- Resistance growth over cycle life and calendar aging, expressed as a percentage increase relative to beginning-of-life values.
- Cell-to-cell consistency, expressed as the resistance dispersion or delta within a production lot or module string.
- Contact and weld resistance at tabs, busbars, and terminal joints in modules and packs.
Testing Methods
AC Internal Resistance (ACIR) Measurement
ACIR measurement applies a small-amplitude alternating current, conventionally at a fixed frequency of 1 kHz, to a fully rested cell held in a four-wire (Kelvin) fixture. The instrument measures the resulting AC voltage response and calculates impedance as the ratio of voltage to current. Because 1 kHz effectively bypasses the slower electrochemical polarization processes, the result closely approximates pure ohmic resistance, making ACIR fast, repeatable, and ideal for 100% production screening. Measurements follow the general procedures described in IEC 61960.
DC Internal Resistance (DCIR) Measurement
DCIR testing measures the voltage change of a cell in response to a defined current pulse. A typical procedure, aligned with IEC 61960 and GB/T 31486, brings the cell to a specified SOC, allows it to rest until the voltage stabilizes, applies a discharge or charge pulse of specified amplitude for a defined duration such as 10 seconds, and records the voltage before and at the end of the pulse. DCIR equals the voltage difference divided by the current difference. Pulse-based DCIR and HPPC-style profiles provide realistic resistance values for power capability estimation and BMS algorithm calibration.
Electrochemical Impedance Spectroscopy (EIS)
EIS measures impedance across a broad frequency range, typically from 10 mHz to 100 kHz, using a small sinusoidal perturbation to keep the cell in a quasi-linear regime. The resulting Nyquist and Bode plots are fitted to an equivalent circuit model, such as a Randles circuit comprising an ohmic resistor in series with a parallel charge transfer resistor and constant phase element, followed by a Warburg diffusion element. EIS is the most informative method, separating ohmic, interfacial, and diffusion contributions and enabling mechanistic diagnosis of aging processes. The technique is also the basis of advanced onboard impedance diagnostics in modern battery management systems.
Test Conditions, Equipment, and Quality Assurance
Accurate internal resistance analysis requires strict control of test conditions. Cells are preconditioned by cycling to a stabilized state, adjusted to the target SOC, and thermally soaked in a climatic chamber so that the cell surface temperature is uniform, typically at 25 ± 2 °C, unless temperature dependence is being studied. Rest periods before measurement ensure electrochemical equilibrium. Four-terminal connections eliminate lead and contact resistance errors, and stable, controlled contact force at terminals is essential for large prismatic cells. Equipment includes precision milliohm meters, programmable battery test systems with pulse capability, and electrochemical workstations with frequency response analyzers. Traceable calibration of instruments and repeat measurements to confirm reproducibility are standard quality assurance practices in accredited third-party laboratories.
Applications
Internal resistance analysis supports a wide range of engineering, quality, and business decisions across the energy storage value chain:
- Production screening and cell sorting, where ACIR distributions are used to group cells into matched strings and reject outliers before module assembly.
- Manufacturing quality control, detecting defective welds, poor tab connections, and contaminated or dry electrolyte conditions.
- State of health estimation, where measured resistance is mapped to capacity fade and remaining useful life in BMS algorithms.
- Power capability prediction, using SOC- and temperature-dependent resistance maps to set real-time charge and discharge current limits.
- Thermal management design, where I²R heat generation values from resistance data feed cooling system sizing and CFD models.
- Aging and failure diagnosis, distinguishing between lithium plating, SEI growth, electrolyte decomposition, loss of active material, and current collector corrosion.
- Second-life battery grading, providing an objective criterion for triaging retired electric vehicle cells into stationary storage applications.
- Compliance and certification testing, demonstrating conformity with IEC 62619, IEC 62660, GB/T 36276, GB/T 31486, and UL 1973 requirements for stationary and grid storage batteries.
- Research and development, benchmarking new chemistries, electrode materials, separators, and electrolyte formulations.
- Warranty and insurance evaluation, where documented resistance growth supports degradation claims and risk assessment for storage assets.
FAQ
What is the difference between ACIR and DCIR?
ACIR is measured with a small 1 kHz alternating signal and reflects mainly the ohmic resistance of the electrolyte, electrodes, and connections. DCIR is derived from the voltage response to a multi-second current pulse and includes ohmic resistance plus electrochemical and concentration polarization. ACIR is fast and ideal for screening, while DCIR better represents real operating conditions and power capability.
Why does internal resistance increase as a battery ages?
Resistance growth results from several degradation mechanisms, including thickening of the solid electrolyte interphase (SEI), lithium plating on the anode, loss of electrolyte and wetting, gas generation, contact loss between active material particles, and corrosion or fatigue of current collectors and welds. Each mechanism adds ohmic or interfacial resistance, which in turn accelerates heat generation and further aging.
What are typical internal resistance values for energy storage cells?
Values depend strongly on format and capacity. Large prismatic LFP cells in the 100 Ah to 300 Ah class commonly exhibit ACIR in the range of roughly 0.2 mΩ to 0.8 mΩ, while smaller cylindrical and pouch cells range from a few milliohms to tens of milliohms. Manufacturers specify limits for each product, and consistency within a lot is often more important than the absolute value.
At what SOC and temperature should internal resistance be measured?
For screening purposes, measurements are commonly taken at a mid-range SOC, such as 50%, and at a controlled room temperature of 25 °C. For design and BMS calibration, resistance should be mapped across the full SOC range and across the expected operating temperature window, because resistance can increase severalfold at low temperature and low SOC.
How is internal resistance used for state of health estimation?
Because resistance rises monotonically with most degradation modes, the ratio of measured resistance to the beginning-of-life baseline provides a practical SOH indicator. Battery management systems can track this ratio using onboard pulse measurements or impedance techniques, supplementing capacity-based SOH estimation and enabling early detection of abnormal cells.
How can measurement errors be minimized?
Best practices include using four-wire (Kelvin) connections, ensuring clean and reproducible terminal contact with controlled force, thermally stabilizing the cell before testing, respecting rest periods after SOC adjustment, using calibrated instruments, and performing multiple repeat measurements. For DCIR, precise synchronization of current pulses and voltage sampling is essential.
Which standards govern internal resistance testing of energy storage batteries?
Key references include IEC 61960 for performance testing of lithium cells, IEC 62660 for lithium-ion cells for electric vehicle applications, IEC 62619 for industrial and stationary lithium batteries, GB/T 31486 and GB/T 36276 for Chinese national requirements, and UL 1973 for batteries used in stationary applications. Testing performed by accredited third-party laboratories to these standards is widely accepted for certification and procurement.