You measure internal resistance of lithium batteries by applying methods such as DC, AC, EIS, or using a battery analyzer. Each analyzer offers unique advantages for pack diagnostics. Accurate internal resistance readings ensure compliance with UL 1642, SAE J2464, and IEC 62660-2 standards, supporting safety and reliability in demanding applications.
Key Takeaways
Measuring internal resistance helps monitor lithium battery health, ensuring safety, performance, and longer lifespan.
Use the right method—DC, AC, EIS, or four-wire—based on your battery type and application for accurate results.
Regular testing under stable conditions with proper tools detects aging early and prevents unexpected battery failures.
Part 1: Internal Resistance Basics
1.1 What Is Internal Resistance?
When you discuss battery internal resistance, you refer to the small but critical resistance inside every cell that opposes current flow. This resistance causes a voltage drop when the battery delivers power. You can see the relationship in the table below, which uses a typical NMC Lithium battery as an example:
Parameter | Value/Formula | Explanation |
|---|---|---|
Open Circuit Voltage (OCV) | 3.7 V | Voltage at 50% SOC for a 5Ah NMC cell |
Internal Resistance (Rint) | 0.025 Ω | Typical DC internal resistance |
Current Draw (I) | 10 A | Example load |
Voltage Drop (V_drop) | 0.25 V | V_drop = I × Rint |
Loaded Voltage (V_load) | 3.45 V | V_load = OCV – V_drop |
Maximum Current (I_max) | 48 A | I_max = (OCV – V_min) / Rint |
Cell Capacity | 5 Ah | Standard for this example |
Peak Discharge Rate | ~10 C | 10 × 5 Ah = 50 A |
You use internal resistance measurement to quantify this value, which is usually in milliohms. Measuring internal resistance helps you understand how much energy is lost as heat during operation.
1.2 Why It Matters for Lithium Battery Packs
You need to monitor battery internal resistance because it directly impacts the performance, safety, and lifespan of lithium battery packs. As internal resistance rises, you see reduced power output, increased heat, and faster capacity loss. Empirical studies show that aging, high current, and deep discharge cycles all increase internal resistance, leading to performance degradation and even failure. Battery management systems rely on accurate internal resistance measurement to estimate state of health and prevent unsafe operation. In sectors like medical, robotics, security, infrastructure, consumer electronics, and industrial applications, high internal resistance can cause critical failures or downtime.
Tip: Regularly measuring internal resistance of lithium battery packs helps you detect early signs of aging and avoid unexpected failures.
1.3 Factors Affecting Measurement
You must consider several factors when selecting a measurement method for battery internal resistance:
Battery size and chemistry (NMC, LiFePO4, LCO, LMO, LTO, etc.)
State of charge and temperature
Battery age and cycle history
Calibration accuracy and environmental interference
Factor Category | Specific Factors and Descriptions |
|---|---|
Structure Design | Tab number, size, position; welding quality; compaction density |
Raw Material Performance | Electrode materials, conductive additives, current collector coatings |
Manufacturing Process | Plate turnaround time, liquid injection, infiltration time |
Working Conditions | Temperature, SOC, storage, charge/discharge cycles, cutoff voltage |
You should always use a measurement method that matches your application and pack design. For precise results, minimize noise, calibrate your instruments, and test under stable conditions. If you need custom solutions for your lithium battery packs, consider our OEM/ODM consulting services.
Part 2: How to Measure Internal Resistance of Lithium Batteries
Accurate internal resistance measurement is essential for ensuring the safety, reliability, and performance of lithium battery packs in demanding applications. You can choose from several measurement methods, each with its own advantages and best-use scenarios. Below, you will find step-by-step guides and practical tips for the most widely used techniques.
2.1 DC Method
The DC discharge internal resistance measurement method is straightforward and widely used for field diagnostics and quality control. You apply a known current to the battery and measure the voltage drop before and after the load is applied. This method is especially useful for NMC Lithium battery packs in medical, robotics, and industrial applications.
Step-by-Step Guide:
Prepare the Battery:
Ensure the battery is at a standard state of charge (typically 50%) and at room temperature. This reduces variability in your results.Set Up the Circuit:
Connect a calibrated multimeter across the battery terminals. Use a reliable load resistor or programmable electronic load.Record Open Circuit Voltage (OCV):
Measure and note the voltage with no load applied.Apply the Load:
Switch on the load to draw a constant current (e.g., 1C or 0.5C, depending on the battery’s rating).Measure Loaded Voltage:
After a few seconds, record the voltage under load.Calculate Internal Resistance:
Use Ohm’s Law:R = (OCV - V_load) / I_load
For example, if OCV = 3.7V, V_load = 3.45V, and I_load = 10A, thenR = (3.7 - 3.45) / 10 = 0.025 Ω
Tip: Always use short, thick wires and minimize contact resistance to improve accuracy. For high-precision needs, consider the four-wire technique.
Advantages:
Simple and cost-effective
Suitable for quick diagnostics
Limitations:
Less accurate for low-resistance cells
Sensitive to temperature and state of charge
Advanced protocols, such as pulse power tests and the pulse-multisine method, can further enhance the accuracy of the DC method by simulating real-world battery usage and providing step-by-step verification.
2.2 AC Method
The ac pressure drop internal resistance measurement method uses a small AC signal, typically at 1 kHz, to measure the battery’s impedance. This approach is fast and non-destructive, making it ideal for quality screening in production lines and for large-scale battery packs in infrastructure and security systems.
How It Works:
An impedance meter or battery analyzer applies a small AC current to the battery.
The device measures the resulting voltage response.
The analyzer calculates the internal resistance based on the ratio of voltage to current at the test frequency.
Comparison Table: DC vs. AC Methods
Aspect | DC Method (Pulse/Load) | AC Method (1 kHz) |
|---|---|---|
Speed | Moderate | Fast |
Accuracy | Good for high-resistance cells | High for low-resistance cells |
Suitability | Field diagnostics, SOH checks | Production, quality screening |
5–6 mΩ | ~2.3 ± 0.3 mΩ | |
Frequency Dependence | No | Yes (results vary by frequency) |
Power Loss | Higher | Minimal |
Note: AC methods may underestimate resistance compared to DC methods, especially for power or loss prediction. Always compare results using the same measurement method for consistency.
Simulation studies and technical benchmarks confirm that the AC method provides reliable results, with deviations as low as 1.6% compared to continuous DC circuits. Calibration guides and manufacturer data sheets further support the credibility of this method.
2.3 EIS and Four-Wire Techniques
Electrochemical impedance spectroscopy (EIS) is the gold standard for internal resistance measurement in research and high-end industrial applications. EIS sweeps a wide frequency range, providing a detailed Nyquist plot that reveals not only resistance but also information about electrochemical reactions, diffusion, and separator quality.
EIS Procedure:
Connect the battery to an EIS-capable analyzer.
Sweep frequencies from millihertz to kilohertz.
Analyze the resulting impedance spectrum to extract internal resistance and other parameters.
Four-Wire Technique:
Use separate pairs of wires for current supply and voltage measurement.
This eliminates errors from lead and contact resistance, which is critical for low-resistance lithium battery packs.
Tip: EIS requires careful calibration and metadata management to ensure repeatability and accuracy. Four-wire setups increase precision but may add cost and complexity.
EIS and four-wire techniques are especially valuable for battery R&D, quality assurance in Medical and industrial sectors, and for benchmarking new chemistries like LiFePO4 and lithium-ion.
2.4 Using Testers
Modern battery analyzers and dedicated testers simplify the process of internal resistance measurement. These devices often combine DC, AC, and EIS capabilities, offering automated procedures and data logging.
Key Features to Look For:
High precision and repeatability
Comprehensive database for different battery chemistries (NMC, LiFePO4, LCO, LMO, LTO)
User-friendly interface and software compatibility
Real-time state of charge (SOC) and state of health (SOH) analysis
Load testing and impedance testing modes
Callout: Resistance testing with a battery analyzer quickly identifies failing cells, supporting predictive maintenance in critical applications like security systems and infrastructure.
When selecting a battery analyzer, consider hardware durability, calibration support, and auxiliary features such as temperature measurement. For custom solutions, explore our custom battery consulting services.
2.5 Interpreting Results
Interpreting internal resistance measurement results is crucial for battery health assessment and lifecycle management. Typical values for fresh NMC Lithium battery cells range from 2 to 6 mΩ, depending on the measurement method and cell design. LiFePO4 Lithium battery cells may show slightly higher values due to their chemistry.
What High Internal Resistance Indicates:
Aging and capacity fade
Increased heat generation during operation
Potential safety risks and reduced performance
Case studies show that as battery packs age, internal resistance increases linearly or superlinearly, correlating with capacity loss and voltage inconsistencies. Regular measurement helps you track these trends and make informed decisions about maintenance or replacement.
Tip: Always compare your results to manufacturer specifications and historical data from your own battery packs. Use controlled temperature environments for consistent readings.
Summary Table: Measurement Methods and Applications
Measurement Method | Best For | Key Advantages | Limitations |
|---|---|---|---|
DC Discharge | Field diagnostics, SOH checks | Simple, cost-effective | Less accurate for low R |
AC Pressure Drop (1 kHz) | Production, quality screening | Fast, non-destructive | Frequency-dependent |
EIS | R&D, advanced diagnostics | Detailed, multi-parameter | Complex, costly |
Four-Wire | Low-resistance, high-precision needs | Eliminates lead resistance | More complex setup |
Battery Analyzer/Testers | Routine maintenance, predictive analytics | Automated, user-friendly | Cost varies by features |
By following these best practices, you ensure accurate internal resistance measurement and reliable operation of your lithium battery packs across Medical, robotics, security, infrastructure, consumer electronics, and industrial sectors.
You ensure reliable lithium battery pack performance by following best practices for internal resistance measurement. Regular monitoring and precise interpretation remain essential. Field data highlight how factors like SOC, temperature, and aging impact resistance:
Factor | Key Finding |
|---|---|
State-of-Charge (SOC) | Resistance peaks at empty, lowest at 80–90% SOC |
Temperature | Low temperatures cause significant resistance increase |
Aging | Resistance rises with more charge-discharge cycles |
Cell Differences | Resistance distinguishes cells, even with identical design |
SOC Estimation | Real-time resistance improves SOC accuracy by over 15% |
Control pressure during testing for accurate results.
Use multi-point calibration and sufficient sample sizes to improve reliability.
Select the right fixture design for your battery type.
For custom lithium battery solutions, consult Large Power.
FAQ
1. How often should you measure internal resistance in lithium battery packs?
For best results, check internal resistance every three to six months. This schedule helps you track battery health and prevent unexpected failures in critical applications.
2. What factors can affect the accuracy of internal resistance measurements?
Temperature changes
State of charge
Lead resistance
Calibration errors
You should always test under stable, controlled conditions for reliable results.
3. Where can you get custom solutions for lithium battery pack testing?
You can contact Large Power for tailored battery consulting and advanced testing solutions designed for your specific industry requirements.
