Analysis of Internal Resistance Changes in Energy Storage Batteries

Internal resistance is one of the most critical health and performance indicators for energy storage batteries, directly determining energy efficiency, heat generation, power capability, and remaining service life. This article presents a professional technical analysis of internal resistance change mechanisms in lithium-ion energy storage cells, covering resistance composition, aging drivers, laboratory measurement methods, applicable standards such as IEC 62660-1, IEC 62619, GB/T 36276 and UL 1973, and practical applications in battery management and lifecycle assessment.

Technical Overview

Internal resistance is the opposition to current flow within a battery, and its evolution over time is widely regarded as a direct reflection of the state of health (SOH) of an energy storage cell. As internal resistance rises, more energy is dissipated as heat (I²R losses), round-trip efficiency declines, usable capacity under load is reduced, and the ability of the system to deliver or absorb high power is degraded. For grid-scale and commercial energy storage systems, internal resistance growth is therefore one of the primary criteria used to define end of life, often alongside capacity fade.

Composition of Internal Resistance

Total internal resistance is generally decomposed into two components:

• Ohmic (impedance-independent) resistance: the pure resistive contribution from electrode foils, current collectors, tabs, terminals, busbars, welds, and the ionic conductivity of the electrolyte and separator. This component responds instantaneously to a current step.
• Polarization resistance: the dynamic contribution from charge-transfer kinetics at the electrode/electrolyte interfaces and solid-state or diffusional mass transport of lithium ions. This component is time- and frequency-dependent.

Primary Degradation Mechanisms Driving Resistance Growth

• Thickening of the solid electrolyte interphase (SEI) on the negative electrode, consuming lithium inventory and increasing interfacial impedance.
• Loss of active material due to particle cracking, structural transition, and contact loss between active material and conductive networks.
• Electrolyte decomposition and dry-out, reducing ionic conductivity and wetting of electrodes.
• Lithium plating, particularly at low temperature or high charge rate, which permanently raises impedance and creates safety risk.
• Corrosion or loosening of welds, tabs, and busbar joints, increasing contact resistance in modules and packs.
• Current collector corrosion and binder degradation at extreme states of charge or elevated temperature.

Representative test programs referenced in this article align with IEC 62660-1, IEC 61960, IEC 62619, IEC 62933 series, GB/T 36276, GB/T 34131, and UL 1973, which collectively define measurement conditions, cycle life protocols, and performance requirements for lithium-ion cells and battery systems used in stationary and propulsion applications.

Testing Samples

Cell-Level Samples

Lithium iron phosphate (LFP) prismatic cells with capacities in the range of 100 Ah to 320 Ah, ternary (NMC/NCA) prismatic and pouch cells, and cylindrical cells intended for energy storage or hybrid applications. Cell-level testing isolates the intrinsic electrochemical behavior of the chemistry and is the basis for resistance growth modeling.

Module-Level Samples

Series-parallel configured modules (for example, 1P16S to 1P48S) with busbars, cell interconnects, and integrated sampling harnesses. Module testing evaluates the additional contribution of weld quality, busbar joints, and contact resistance to overall pack impedance.

Pack-Level Samples

Complete battery packs including the battery management system (BMS), contactors, fuses, and cabling. Pack-level resistance characterization supports DC-IR validation, thermal modeling, and verification against warranty performance targets.

Alternative Chemistries

Lead-carbon and valve-regulated lead-acid (VRLA) cells per IEC 60896, sodium-ion cells, and flow battery stacks, where internal resistance change analysis is used comparatively to benchmark lithium-ion performance or to assess stack health.

Testing Items

A comprehensive internal resistance change analysis program typically includes the following measurable items:

• AC internal resistance at 1 kHz (milliohm-level ohmic resistance) at beginning of life (BOL) and at defined intervals.
• DC internal resistance derived from current pulses of specified amplitude and duration at defined SOC and temperature.
• Full electrochemical impedance spectroscopy (EIS) spectra, typically from 10 kHz down to 0.01 Hz, for separation of ohmic, charge-transfer, and diffusion contributions.
• Internal resistance versus state of charge (10% to 100% SOC) mapping.
• Internal resistance versus temperature characterization, typically from -20 °C to +55 °C.
• Resistance growth curves as a function of cycle number and calendar age, obtained under standardized cycling protocols.
• Charge-direction versus discharge-direction resistance asymmetry, an indicator of lithium plating or anode limitation.
• Contact and weld resistance of interconnects at module and pack level.
• Statistical dispersion of cell resistance within a batch, supporting consistency grading and module matching.

Testing Methods

AC Internal Resistance Method

A small-amplitude alternating current at 1 kHz is applied using a four-wire (Kelvin) connection, and the internal resistance is calculated from the voltage response, following the general requirements of IEC 61960. This method is fast, non-invasive, and well suited to production screening and periodic health checks, though it primarily captures ohmic resistance and underrepresents slow polarization processes.

DC Internal Resistance (Pulse) Method

A defined current step or pulse, for example at 0.5C to 2C for 10 to 30 seconds, is applied at a controlled SOC and temperature, and the DC-IR is calculated as the ratio of the voltage change to the current change, consistent with the pulse procedures described in IEC 61960 and IEC 62660-1. Hybrid pulse power characterization, combining discharge and regenerative pulses, further separates charge and discharge direction resistance. This method best reflects real operating conditions in energy storage systems.

Electrochemical Impedance Spectroscopy and Equivalent Circuit Analysis

Galvanostatic or potentiostatic EIS is performed across a wide frequency window in a climate-controlled chamber. The resulting Nyquist spectra are fitted to equivalent circuit models, such as an ohmic resistor in series with parallel RC elements and a Warburg diffusion element, or analyzed using distribution of relaxation times (DRT) to deconvolute overlapped time constants. This allows degradation to be attributed to specific physical processes, for example SEI growth versus electrolyte depletion.

Aging Protocol and Trend Analysis

Reference performance tests (capacity check plus AC-IR, DC-IR, and EIS) are executed at BOL and then at fixed intervals, for example every 100 to 500 equivalent full cycles, under cycle life protocols such as those in GB/T 36276 and UL 1973, including elevated temperature stress at approximately 45 °C. Resistance growth is expressed as a percentage increase relative to BOL, fitted with empirical or semi-empirical aging models, and extrapolated to define the cycle at which the resistance reaches the end-of-life threshold, commonly 150% to 200% of the initial value.

Measurement Quality Control

• Four-wire Kelvin fixturing to eliminate lead and contact errors at sub-milliohm levels.
• Thermal soak periods to guarantee uniform cell temperature before each measurement.
• Calibration against certified low-value reference resistors and traceable metrology equipment.
• Repeatability and reproducibility verification, with typical acceptance of less than 2% variation between consecutive measurements.

Applications

• State of health estimation: online DC-IR or impedance features are used by BMS algorithms to estimate SOH and predict remaining useful life (RUL), supporting warranty tracking and dispatch planning.
• Cell screening and binning: incoming inspection at cell manufacturers and system integrators sorts cells by initial resistance to build electrically consistent modules and packs.
• Aging and cycle life validation: resistance growth curves under standardized cycling demonstrate compliance with product specifications and certification requirements.
• Field diagnostics and root cause analysis: abnormal resistance signatures identify weld failure, electrolyte dry-out, lithium plating, or cell reversal before catastrophic escalation.
• Thermal management design: accurate resistance data feeds heat generation models used to size cooling systems and prevent localized hot spots.
• Second-life assessment: retired electric vehicle and storage batteries are graded by resistance and capacity for redeployment in lower-duty stationary applications.
• Safety evaluation: accelerated resistance growth serves as an early warning indicator in abuse and long-term reliability studies.

FAQ

Why does internal resistance increase as an energy storage battery ages?

Aging mechanisms such as SEI thickening, loss of cyclable lithium, loss of active material, electrolyte decomposition, and contact degradation at welds and current collectors all add barriers to charge and ion transport. The cumulative effect is a gradual rise in both ohmic and polarization resistance, which accelerates in later life as multiple mechanisms interact.

What is the difference between AC internal resistance and DC internal resistance?

AC resistance measured at 1 kHz mainly reflects the ohmic component and is fast, stable, and ideal for screening. DC resistance from current pulses includes ohmic plus charge-transfer and short-term diffusion polarization, so it is typically higher and better represents actual voltage drop under real load conditions in energy storage duty.

How much internal resistance growth is considered end of life?

Industry practice commonly defines end of life at 150% to 200% of the beginning-of-life resistance, or at a fixed percentage increase such as 50% to 100%, depending on the power capability required by the application. Many stationary storage warranties instead use 20% to 30% capacity fade as the primary EOL criterion, with resistance growth tracked as a supporting parameter.

How does temperature affect internal resistance measurements?

Internal resistance rises steeply at low temperature due to sluggish charge-transfer kinetics and reduced electrolyte ionic conductivity, following approximately Arrhenius-type behavior. All comparative measurements should therefore be performed after full thermal stabilization at a reference temperature, typically 25 °C ± 2 °C.

Which standards govern internal resistance measurement for storage batteries?

Key references include IEC 61960 and IEC 62660-1 for cell test procedures, IEC 62619 for industrial lithium batteries, IEC 62933 for electrical energy storage systems, GB/T 36276 and GB/T 34131 for lithium storage cells and battery management systems, and UL 1973 for stationary battery safety and performance. Testing should be performed by accredited third-party laboratories or qualified in-house facilities with traceable calibration.

Can internal resistance analysis detect potential safety risks?

Yes. Abrupt or asymmetric resistance growth, particularly in the charge direction, can indicate lithium plating, electrolyte dry-out, or internal connection failure, all of which are precursors to overheating or thermal events. Periodic impedance characterization is therefore a valuable preventive maintenance tool for large storage plants.

How often should internal resistance be measured in service?

Grid and commercial storage operators typically perform a reference performance test, including capacity verification and DC-IR measurement, every 6 to 12 months, while the BMS continuously tracks online impedance indicators. Accelerated intervals are recommended after abnormal events such as deep discharge, overtemperature, or extended standby at high SOC.