Internal Resistance Testing of Energy Storage Batteries: Principles, Methods, and Standards

Internal resistance is one of the most critical indicators of energy storage battery performance, directly governing power capability, energy efficiency, heat generation, and state of health. This article provides a comprehensive overview of energy storage battery internal resistance testing, covering measurement principles, AC and DC test methods, applicable international standards, and typical applications in quality control, system commissioning, and lifecycle maintenance.

Technical Overview

Internal resistance is the inherent opposition of a battery to current flow, and it fundamentally determines how efficiently a cell can store and deliver electrical energy. It consists of two principal components: ohmic resistance, arising from the electrolyte, electrode materials, current collectors, tabs, and terminal connections, and polarization resistance, arising from charge-transfer kinetics and ion diffusion at the electrode interfaces. Together, these components cause a voltage drop under load and irreversible energy dissipation in the form of heat, which follows the relationship P = I²R.

For energy storage batteries, elevated internal resistance reduces round-trip efficiency, limits charge and discharge power capability, accelerates thermal aging, and can contribute to cell imbalance across large series-parallel strings. Because internal resistance increases progressively as a battery degrades, it is widely used as a quantitative indicator of state of health (SOH) and as a screening parameter for safety risks. Measurement is based on Ohm's law, R = ΔV/ΔI, and is performed in two principal domains: AC internal resistance, measured with a small-amplitude alternating signal, and DC internal resistance, measured with a defined current pulse. Relevant reference frameworks include IEC 62660-1, IEC 61960, IEC 62619, IEC 60896, UL 1973, and GB/T 36276, and testing is commonly conducted by accredited third-party laboratories or manufacturer laboratories equipped with precision impedance instrumentation.

Testing Samples

Lithium-Ion Cells

The most common test specimens are large-format lithium-ion cells used in grid-scale and commercial energy storage systems, including lithium iron phosphate (LFP) prismatic and pouch cells, ternary (NMC/NCA) cells, and lithium titanate (LTO) cells with capacities ranging from several ampere-hours to several hundred ampere-hours. Both fresh production cells and aged cells withdrawn from cycle-life testing may be evaluated.

Battery Modules and Packs

Modules and packs assembled from series-parallel cell configurations are tested to verify the cumulative resistance of intercell busbars, welds, and connectors, and to assess the resistance consistency of individual cells within the assembly. These measurements are essential during BESS commissioning and periodic maintenance.

Lead-Acid and Other Chemistries

Stationary lead-acid batteries, including flooded and valve-regulated (VRLA) types, remain widely deployed in backup and renewable energy storage applications and are routinely evaluated for internal resistance or conductance in accordance with IEC 60896 and IEEE 1188. Emerging chemistries such as sodium-ion cells may also be characterized using the same general methodology.

Special and Comparative Samples

Comparative testing may be performed on cells conditioned at different temperatures, cells at different states of charge, cells after long-term cycling or calendar aging, and cells returned from the field for failure analysis, in order to quantify resistance growth and degradation mechanisms.

Testing Items

Typical internal resistance test items for energy storage batteries include:

• AC internal resistance at the standard 1 kHz frequency (also referred to as ohmic or impedance resistance)
• DC internal resistance determined from short-duration and long-duration current pulses (for example, 0.1 s, 1 s, and 10 s discharges)
• Ohmic resistance component, isolating the purely resistive contribution of electrolyte and conductive parts
• Charge-transfer resistance and diffusion (Warburg) resistance derived from electrochemical impedance spectroscopy
• Resistance consistency analysis across all cells within a module or rack
• Internal resistance growth rate over cycle life and calendar life aging
• Temperature-dependence and SOC-dependence characterization of internal resistance

Testing Methods

AC Internal Resistance Method

The AC method applies a small-amplitude sinusoidal current, typically at 1 kHz, to the cell and measures the in-phase voltage response; the ratio of voltage amplitude to current amplitude yields the AC internal resistance. A four-terminal (Kelvin) connection is used to eliminate lead and contact resistance errors. This method is fast, non-intrusive, and highly repeatable, making it the standard approach for 100% production screening and incoming inspection. It primarily reflects the ohmic resistance of the current path and suppresses the slower polarization processes.

DC Internal Resistance (Pulse) Method

The DC method determines resistance from the voltage response to a controlled current step. After the battery is rested and stabilized, a defined discharge or charge pulse, such as a 10-second pulse at a specified current, is applied, and the internal resistance is calculated as the change in terminal voltage divided by the change in current. Because the pulse duration allows polarization to develop, DC internal resistance is normally higher than AC internal resistance and better represents real operating conditions, including power capability and heat generation during sustained discharge. This approach is emphasized in IEC 62660-1 and GB/T 36276 test protocols.

Electrochemical Impedance Spectroscopy (EIS)

EIS sweeps a small AC perturbation across a wide frequency range, typically from millihertz to several kilohertz, and produces a Nyquist plot that separates ohmic resistance, interfacial film resistance, charge-transfer resistance, and diffusion impedance. By fitting the spectrum to an equivalent circuit model, engineers can diagnose degradation mechanisms such as electrolyte dry-out, lithium plating, or loss of active material. EIS is the most information-rich method and is widely used in research, development, and advanced diagnostics.

Test Conditions and Preconditioning

Accurate determination requires strict environmental and electrical conditioning. Tests are normally conducted at 25 ± 2 °C after the battery has reached thermal equilibrium, with the state of charge standardized, commonly at 50% SOC or as specified by the relevant standard, and an adequate rest period, often one hour or more, following prior charging or discharging. Instruments must be calibrated against certified reference resistors, and fixture contact resistance must be controlled through proper probe force and contact area. All measurements should be documented with temperature, SOC, and current conditions to ensure comparability.

Applications

Internal resistance measurement supports every stage of the energy storage battery lifecycle, including:

• Production quality control and 100% end-of-line screening of cells and modules
• Cell grading, binning, and consistency sorting to optimize pack assembly and prolong system life
• Incoming inspection by battery system integrators and energy storage project developers
• Commissioning verification and acceptance testing of battery energy storage systems
• Periodic maintenance testing and SOH trending for operational storage plants
• Thermal risk and failure screening for aged or abused cells
• Comparative benchmarking and materials research in battery development programs
• Failure analysis and warranty assessment of field-returned products

FAQ

What is the difference between AC and DC internal resistance?

AC internal resistance is measured with a small 1 kHz signal and mainly captures the ohmic resistance of the current path, whereas DC internal resistance is measured with a current pulse lasting from fractions of a second to tens of seconds and additionally includes polarization effects. As a result, DC internal resistance values are typically higher than AC values and better reflect real discharge performance.

Which standards apply to energy storage battery internal resistance testing?

Commonly referenced frameworks include IEC 62660-1 and IEC 61960 for lithium-ion cell performance testing, IEC 62619 and UL 1973 for industrial and stationary lithium batteries, IEC 60896 for stationary lead-acid batteries, IEEE 1188 for VRLA maintenance testing, and GB/T 36276 for lithium-ion batteries used in electrical energy storage. The applicable standard depends on cell chemistry, application, and market requirements.

Why is 1 kHz the standard frequency for AC measurement?

At approximately 1 kHz, the capacitive reactance of the electrochemical double layer is minimal, so the measurement is dominated by the purely resistive elements of the electrolyte, electrodes, and connections. This frequency also offers excellent repeatability with simple instrumentation, which is why it has become the global industry convention.

How do temperature and state of charge affect internal resistance?

Internal resistance rises significantly at low temperature due to slower ion transport and sluggish charge-transfer kinetics, and it varies with SOC, particularly near full charge and deep discharge. For valid comparisons, measurements must therefore be performed under controlled temperature and standardized SOC conditions.

What internal resistance values are considered acceptable?

Acceptance limits are cell-specific and defined by the manufacturer's datasheet. Large-format LFP storage cells typically exhibit internal resistance in the milliohm to sub-milliohm range. As a general maintenance guideline, a measured value that has grown 20% or more above the baseline or fleet average indicates notable degradation and warrants further evaluation.

Can internal resistance be tested on-site in an operating storage plant?

Yes. Portable handheld AC internal resistance testers enable on-site checks of individual cells and modules, and modern battery management systems continuously estimate internal resistance from voltage and current data during operation, supporting online SOH monitoring without taking the system offline.