Lithium-Ion Battery Internal Resistance Guide: ACIR vs DCIR vs EIS

Internal resistance is one of the key metrics for evaluating lithium-ion battery consistency, power capability and aging. In battery production QC and R&D testing, two mainstream resistance testing methods are widely adopted: ACIR (AC Internal Resistance) and DCIR / DCR (DC Internal Resistance).

ACIR, DCIR and EIS are sometimes confused because all are related to battery impedance. In fact, ACIR, DCIR, and EIS probe different battery internal physical and electrochemical processes.

This article systematically clarifies their definitions, test principles, core differences, applicable scenarios, and influencing factors, providing a standardized testing and data interpretation guide for battery engineers.

1 Root Causes of Lithium-Ion Battery Internal Resistance

Lithium-ion battery operation relies on dual transmission systems: electron conduction in solid structures and lithium-ion migration in liquid/interface environments. All defective or resistive links contribute to total internal resistance.

Battery impedance mainly falls into six core sources:

• Electronic resistance: Active materials, conductive networks, current collectors, tabs, and welding points
• Ionic resistance: Electrolyte, separator, and porous electrode channels
• Contact resistance: Electrode-current collector fitting and tab connection interfaces
• Interfacial resistanceSEI/CEI structural resistance film
• Charge-transfer resistance: Electrochemical reaction kinetic impedance
• Diffusion polarization: Lithium-ion diffusion resistance inside active materials

Different testing methods capture different resistance components, which explains why ACIR and DCIR values are never interchangeable.

2 What Is ACIR (AC Internal Resistance)?

Definition & Measurement Principle

ACIR is tested by injecting a small-amplitude fixed-frequency AC sinusoidal signal into the battery and calculating resistance via the real-time voltage response.

Single-frequency ACIR is commonly measured in the kHz range, often at 1 kHz, while the excitation amplitude depends on the instrument and test protocol. All ACIR results must be marked with test temperature, SOC, frequency, and signal amplitude for validity.

Core Test Characteristics

• Ultra-fast testing with negligible battery disturbance
• Highly adaptable for full-capacity and all-format cells
• Focuses on fast-response ohmic resistance and contact resistance

What ACIR Primarily Reflects

High-frequency ACIR mainly captures instantaneous resistive components: electrolyte conductivity, electronic conduction, tab welding quality, and fixture contact status.

It can help screen cells with abnormal resistance potentially related to contacts, welding, electrolyte condition or other manufacturing variations.

3 What Is DCIR / DCR (DC Internal Resistance)?

Definition & Measurement Principle

DCIR (DCR) is calculated by the classic formula: \(R_{DC}=\frac{\Delta V}{\Delta I}\)

Under fixed temperature and SOC, apply a constant DC charge/discharge pulse current, then calculate resistance via the voltage difference before and after the pulse.

Unlike fixed ACIR parameters, DCIR test conditions are cell-dependent: pulse current, duration, and sampling time must match cell capacity and chemistry.

Key Feature: Time-Dependent Resistance

DCIR values change with pulse duration due to sequential activation of internal battery processes: At very short time scales, the response is dominated by fast ohmic components. As the pulse continues, interface polarization, charge transfer, and lithium-ion diffusion polarization gradually contribute to the measured voltage change.

This provides a practical representation of the cell's response under DC load conditions.

What DCIR Primarily Reflects

DCIR reflects a time-dependent combination of ohmic and polarization contributions under the defined pulse protocol. It is a widely used indicator for evaluating battery power capability.

4 Core Differences Between ACIR and DCIR

Item	ACIR	DCIR / DCR
Excitation Signal	Small fixed-frequency AC signal	DC current step/pulse
Test Duration	Extremely fast	Depends on pulse protocol
Detected Components	Fast ohmic & contact resistance	Ohmic resistance polarization contributions (time-dependent)
Battery Disturbance	Minimal	Moderate
Core Application	Mass production batch screening & consistency check	Power performance verification & load simulation
Power Relevance	Limited / Indirect	High
Electrochemical Analysis	Limited	Basic evaluation only

5 Critical Distinction: ACIR vs EIS (Most Common Misunderstanding)

Many engineers confuse single-frequency ACIR for EIS, but the two are fundamentally different:

Single-Frequency ACIR

• Outputs only one fixed resistance value
• For rapid screening, has limited electrochemical decomposition capability
• Cannot separately quantify interface, charge transfer, and diffusion resistance

EIS (Electrochemical Impedance Spectroscopy)

• Scans full-range frequency spectrum to obtain complete impedance curves
• Can help distinguish contributions associated with Rs (ohmic resistance), RSEI (surface film resistance), Rct (charge-transfer resistance), Warburg diffusion impedance when interpreted with an appropriate model
• Supports in-depth analysis of SEI growth, interface aging, and ion diffusion failure

CANRD's impedance diagnostic framework notes that the correlation between EIS frequency bands and physical processes is affected by material system, SOC, temperature and model selection. EIS results should be cross-referenced with charge-discharge data, DCIR and electrode or post-mortem analysis.

Conclusion: ACIR is a screening tool, while EIS is a professional electrochemical diagnosis tool.

6 Key Factors Affecting ACIR & DCIR

1. Temperature

Low temperature increases electrolyte viscosity, slows ion migration and reaction kinetics, and significantly raises both ACIR and DCIR. Low-temperature DCIR surge is the main cause of poor low-temperature power performance of batteries.

2. SOC (State of Charge)

The relationship between SOC and internal resistance depends strongly on cell chemistry, temperature, aging state and measurement method. Some cells show relatively stable resistance over part of the mid-SOC range and higher resistance near one or both SOC extremes, but this behavior should be characterized experimentally for each cell system.

3. Charge/Discharge Direction

Charge and discharge DCIR should be evaluated separately because their voltage responses and polarization behavior can differ. Discharge DCIR guides peak power design, while charge DCIR supports fast-charging performance optimization.

4. Battery Aging

Different aging mechanisms may affect ACIR, DCIR and EIS to different degrees. Contact or welding degradation can increase high-frequency resistance; interface-film growth may be more clearly resolved through EIS; and transport or diffusion limitations can increase both pulse resistance and low-frequency impedance. These indicators should be interpreted together rather than used as one-to-one failure signatures.

7 Engineering Selection Guide: ACIR, DCIR or EIS

1. Production Quality Control & Batch Screening

✅ Choose ACIR Fast, low-disturbance, and high-efficiency for sorting inconsistent cells and eliminating manufacturing defects.

2. Power Performance & Application Simulation

✅ Choose DCIR Evaluates voltage sag, heat generation, peak power, and low-temperature performance for EVs and energy storage batteries.

3. R&D Mechanism Research & Failure Analysis

✅ Choose EIS Helps decompose internal electrochemical processes to locate aging, wetting failure, and interface defect root causes with supporting validation data.

8 Possible Diagnostic Directions from Combined ACIR & DCIR Trends

Dual-indicator joint judgment provides clues for battery state diagnosis:

1. ACIR ↑ DCIR ↑: may indicate a broad increase in cell resistance; further investigation is required.
2. ACIR stable DCIR ↑: may suggest increased slower polarization or transport limitations.
3. ACIR ↑ with limited DCIR change: may point toward high-frequency/contact-related changes, but should be confirmed.
4. Resistance stable Capacity ↓: capacity-loss mechanisms may dominate, although resistance-independent degradation cannot be confirmed from these two measurements alone.

These trends are diagnostic clues rather than unique root-cause signatures.

9 Recommended ACIR–DCIR–EIS Evaluation Workflow

1. Stabilize cell temperature and SOC, perform fixed-time rest
2. Complete batch ACIR testing with unified frequency and parameters
3. Conduct directional (charge/discharge) DCIR pulse testing
4. Perform EIS scanning for abnormal cells to locate root causes
5. Correlate data with capacity, cycle life, and rate performance for comprehensive evaluation

Conclusion

• ACIR is a high-efficiency screening indicator for batch consistency and manufacturing quality, reflecting fast ohmic contact resistance.
• DCIR is a practical performance indicator for battery power and actual load performance, covering time-dependent polarization effects.
• EIS is an in-depth research tool for electrochemical mechanism and failure diagnosis.

Depending on the project objective, ACIR, DCIR and EIS can be used individually or in combination to provide different levels of information about cell resistance and electrochemical behavior.

FAQ

Q1: Is DCIR equal to battery ohmic resistance?

A: No. DCIR includes ohmic resistance, plus interface polarization, charge transfer, and diffusion polarization related to pulse duration.

Q2: Why Can DCIR Be Higher Than ACIR?

A: DCIR can include slower polarization contributions that may not appear strongly in a high-frequency ACIR measurement. However, there is no universal rule requiring DCIR to be numerically higher than ACIR because the result depends on frequency, pulse current, duration, SOC, temperature and measurement definition.

Q3: Can ACIR replace EIS for battery aging analysis?

A: No. Single-frequency ACIR cannot decompose electrochemical processes, while EIS can help distinguish changes associated with interfacial, charge-transfer and diffusion-related processes, but reliable failure diagnosis usually requires complementary electrochemical or post-mortem analysis.

Q4: Why must temperature and SOC be fixed for resistance testing?

A: Temperature and SOC directly affect ion migration and reaction kinetics. Unfixed conditions lead to incomparable test data.

Q5: Which indicator is suitable for battery cycle aging testing?

A: For cycle-aging studies, capacity retention and resistance growth should be tracked together. ACIR can provide fast consistency trends, DCIR can track changes in load response and power capability, while EIS can provide deeper information about evolving interfacial and transport processes. The appropriate combination depends on the research objective.