Why Is Battery Internal Resistance Testing So Crucial?
Batteries have been around for quite some time, but in recent years, we have seen significant traction and new developments. These include broad adoption in the EV sector, as well as equipment and heavy machinery, personal and home devices with long battery life, and an overall improved lifetime. One aspect that helps us broadly understand a battery's quality, current state, and performance is its internal resistance.
As a test and measurement company, our primary interest is in measuring resistance in the most accurate way possible. However, before we delve into the details of measurements and their applications, we need to clarify the basics.
What Is Battery Internal Resistance?
Figure 1. Battery internal resistance
Battery internal resistance (Ri) is an inherent property of a battery that opposes current flow and causes a voltage drop whenever current pass through it. At least this is how it behaves when measured or observed. In reality, it is not as simple as a resistor somewhere in the battery. Instead, the battery's chemistry and properties behave like a resistor under certain conditions.
The internal resistance emerges due to various phenomena inside a battery:
- Ohmic resistance: This is the intrinsic resistance of the conductive components, like the electrolyte, current collectors, and electrode materials.
- Charge transfer resistance: This is due to the electrochemical reactions occurring at the electrode-electrolyte interface and the resistance faced by ions and electrons while crossing them.
- Diffusion resistance: This is the slowness of ions moving through the electrolyte.
- Polarization resistance: This is the sum of charge transfer and diffusion resistance.
- Structural changes: As a battery ages and undergoes many charge-discharge cycles, its active materials degrade. As an unwanted effect, the solid electrolyte interphase (SEI) layer on electrode surfaces could grow rapidly. These structural and chemical changes increase internal resistance over time. As a side note, SEI is very important for the battery and formation processes. It solidifies/supports the growth of an SEI at the beginning of a battery's lifetime. SEI is critical for supporting high current tolerance and longevity. The issue arises when layers grow over time.
- Temperature effects: Lower temperatures increase internal resistance by slowing down ion movement, reducing reaction rates, and increasing electrolyte viscosity. Prolonged exposure to high temperatures increases the resistance due to accelerated degradation.
- State of charge (SoC): A battery's internal resistance is not constant across its charge range. It typically increases at very low and very high states of charge, reflecting changes in the electrode materials' ability to store and release ions.
- Battery chemistry: Different battery chemistries and internal cell designs have different internal resistance characteristics.
To simplify it, internal resistance is defined by chemical properties, internal processes, and changes with the state of charge and aging.
How Do Temperature and Environmental Conditions Affect Battery Internal Resistance Test Results?
Temperature and environmental conditions significantly impact battery internal resistance, rendering it a complex and dynamic parameter.
Lower temperatures significantly increase Ri by slowing ion movement, reducing reaction rates, and increasing electrolyte viscosity. The high Ri makes charging and discharging more difficult, reduces charge acceptance and capacity, and can lead to voltage undershoots during discharge.
Higher temperatures can initially reduce internal resistance by accelerating chemical reactions, which may lead to a reduction in Ri and an increase in battery performance and storage capacity. However, prolonged exposure to high temperatures can increase Ri over the long term due to accelerated aging and degradation. Other risks include reduced lifespan, permanent structural damage, and thermal runaway.
Knowing now what is influencing internal resistance, we can ask what insights measuring it provides.
Why Is Measuring the Internal Resistance of a Battery Important for Evaluating Battery Health?
The internal resistance is a key indicator of a battery's power density, energy dissipation, efficiency, and overall state of health (SoH).
Measuring it could provide a simple way to gain insight into the following aspects:
- Degradation: Internal resistance increases over time due to changes at the electrode surfaces, active material degradation, and byproduct formation.
- Efficiency: Higher internal resistance results in energy loss as heat, less efficient charging and discharging, less battery capacity, and shorter run times.
- Diagnostic information: Measuring internal resistance helps in identifying problems early, assessing cell quality, and optimizing battery management strategies.
It is relatively simple to test the internal resistance, and the battery management system (BMS) can perform the measurement while batteries are operating or being charged.
We learned in the first chapter what influences behavior, but is describing it as a form of resistance sufficient?
What Is the Difference Between Battery Internal Resistance Testing and Battery Impedance Testing?
Internal resistance testing measures the opposition to direct current (DC) signals, which results in a simplified version of a battery's true behavior. Impedance testing measures the opposition to alternating current (AC) signals at various frequencies for a more comprehensive battery profile. Resistance is the first approximation of the battery’s behavior, while impedance with frequency dependence can provide a more accurate picture.
Internal resistance is typically measured using the direct current internal resistance (DC-IR) method. DC-IR measures resistance using a large current pulse, reads the voltage, and calculates the resistance based on this pulsed result.
More comprehensive battery impedance testing is done through electrochemical impedance spectroscopy (EIS). A frequency sweep is performed with a sinusoidal current stimulus (galvanostatic) or voltage stimulus (potentiostatic). It comprehensively profiles a battery's dynamic characteristics and compares them with single-point internal resistance measurements. It helps analyze frequency-dependent electrical and electrochemical phenomena in a battery, including inductive effects, electrode-electrolyte capacitance, and ionic diffusion impedance. The resulting impedance values could be plotted in a Bode Plot, where for each measured frequency point, an impedance point with real and imaginary parts is added. This plotting method allows easy comparison for spotting changes in the battery and also supports matching with equivalent circuits for deeper analysis of different parts of the battery.
To simplify it, a resistance measurement is simpler but does not provide the deep insights that an impedance characterization can.
Regardless of whether we talk about resistance or impedance, the strong recommendation is to use four-wire measurements.
Why Is the Four-Terminal Method Preferred for Accurate Battery Internal Resistance Measurements?
Figure 2. Two-wire and four-wire methods of measuring internal resistance
The four-terminal method (also known as four-wire or Kelvin connection) is preferred for accurately measuring battery internal resistance because it excludes the resistances of the connection wires from the reading.
Internal resistance can be measured in various ways, depending on the instrument and its intended use.
As discussed earlier, in cases of a DCIR measurement, a current pulse is applied, and the voltage is measured before and during the pulse. The internal resistance value is then calculated using Ohm's law.
Figure 3. Internal resistance formula
In a two-wire connection, the tester's leads simultaneously conduct high current and sense the battery voltage. Since the wires themselves have some resistance, the voltage drop across them at high currents will not be negligible. The measured voltage includes this drop across the wires, not just the drop across the battery.
In contrast, in a four-wire connection, one pair of leads just senses the voltage across the battery with a built-in high-resistance voltmeter while the load current flows through the other pair of wires. Due to the high resistance of the voltmeter and sense leads, the voltage drop across them will be extremely negligible, almost zero. The resistance calculated this way will just be that of the battery.
This applies not only to DC-based measurements but also to AC-stimulus impedance measurements, such as EIS.
What’s the Difference Between Handheld Battery Internal Resistance Testers and Lab-Based Systems?
Some differences between lab-based and handheld testers are described below:
- Intended uses: Lab-based systems are built for test labs and production floors. They're used for battery research, quality control, and testing batteries of electric vehicles and medical devices. Handheld testers are for field use and quick checks of standard device batteries.
- Measurement methods: Lab-based systems act as variable loads and draw or supply high currents from the batteries under test. A handheld tester typically supplies a low-voltage AC signal to measure the battery's internal resistance rather than drawing current.
- Capabilities: Lab-based systems are capable of sophisticated DC-IR, AC-IR, and EIS testing. They can characterize Ri at different frequencies and under different conditions. Handheld testers are good for basic Ri measurement.
- Environmental integration: Some lab-based systems integrate with climatic chambers for temperature-controlled testing.
- Accuracy: Lab-based systems have extremely high accuracy and resolution, often down to micro-ohms, which is crucial for characterizing modern low-impedance batteries.
- Form factors: Lab-based systems can be benchtop, rack-mounted, or large freestanding instruments. Handheld testers are compact, lightweight, and portable.
- Scalability: Many lab-based systems support multiple independent testing channels, for parallel testing of dozens to thousands of batteries.
- Automation: Lab-based instruments typically support test automation, while handheld ones typically don't.
Why Is Battery Internal Resistance Testing Critical for Applications Like EVs, UPS Systems, and Energy Storage?
In applications like EVs, uninterruptible power supply (UPS) systems, and energy storage, battery internal resistance testing is crucial for:
- Assessing efficiency: Higher internal resistance wastes energy as heat, reduces EV’s driving range, and lowers the efficiency of power delivery in UPS systems and energy storage.
- Determining SoH: Internal resistance is a key indicator of the SoH and directly affects a battery's lifespan. Ri increases with age, charge/discharge cycles, and exposure to extreme temperatures. By monitoring the Ri, battery life can be accurately predicted. It's crucial for timely maintenance and replacement in these applications.
- Predicting capacity: Increased internal resistance significantly reduces a battery's effective capacity and makes charging and discharging difficult. This directly degrades the system's usable runtime and reliability, making it unsuitable for consistent power delivery.
- Ensuring safety: High internal resistance, combined with high current demands or extreme temperatures, can lead to excessive heat generation, thermal runaways, fires, or explosions. Internal resistance testing can identify batteries with compromised safety characteristics, which is critical for high-voltage EV battery packs and large-scale energy storage systems.
This brings us now to the practical part.
What Equipment Could Be Used for the Different Applications?
We have two categories of systems that could be used: power-supply-based test systems and battery test systems.
The main difference is how the automation and the control of the test environment are executed.
A supply is only executing the measurement by applying the stimulus current and reading back the voltage. The automation, processing, and communication must be done externally on a PC.
A battery test system includes all of this in a larger solution. Taking care of the automation of the whole test bench, including the chamber, external measurement equipment, and safety features. The systems could also perform more complex measurements and tests, run tests for longer periods, and combine different characterization and performance tests.
In addition, a power supply-based system is more mobile, whereas a battery test system focuses on a fixed installation with repeatable results and robust execution.
It could be seen like this: a battery test system is also a power supply, but extend it with a controller, measurement equipment, storage, and automation, and it could run most of the time on its own for long test periods.
As a quick overview, let's start with the benchtop and rackmount devices:
Keysight N6705C (Lower Power)
The Keysight N6705C DC Power Analyzer combines sourcing and precise measurement in a single instrument, enabling dynamic battery characterization without the need for external shunts. Its integrated scope, data logger, and seamless modules make it ideal for analyzing battery and internal resistance under real-world load conditions.
It serves as a great mobile lab device, helping perform quick, simple tests across different devices and battery types. Its modular approach helps shape the right device for the application at hand. Supported modules include basic power supplies for multipurpose applications, precise and advanced modules for fast, accurate transients and measurements, and source measurement units (SMUs) that combine digitizing with source and sink capabilities, including two- and four-quadrant supplies.
Keysight RP5900A (Lower to Mid Power)
The Keysight RP5900A is a compact, bidirectional regenerative DC power supply (2–12 kW) featuring integrated electronic load and regenerative capabilities, serving as a sink. Its integrated digitizer allows you to measure in a one-box solution while performing test profiles. This allows for accurate pricing and capturing of internal resistance, while keeping the setup simple. The voltage range allows not only its use with cells but also with modules and up to smaller packs.
For bigger labs and fixed installations, where precision is key, the battery test systems are a good option:
Keysight SL1035A (Low-Current Precision)
The SL1035A is designed for low-current battery cells, offering up to 96 channels with three current ranges (±5 A to ±20 A) to maximize measurement accuracy. Built-in EIS and reference electrode support make it ideal for detailed internal resistance and electrochemical analysis of small-format cells. With its lower current profile, it is ideal for new chemistries, prototyping, and flexible lab approaches.
Keysight SL1007B (Cell-Level, High Current)
The SL1007B is a bidirectional battery cell test system with a current capacity of up to ±400 A per channel and integrated Electrochemical Impedance Spectroscopy (EIS), enabling precise measurements of capacity, efficiency, and internal resistance. Its regenerative design returns up to 90% of discharge energy to the grid, making it highly efficient for dynamic load profiles and advanced cell characterization. The system offers the highest flexibility, supporting currents from below 25 A to up to 3200 A, allowing testing of the largest batteries on the market and keeping pace with the fast-developing market. The full solution can be combined with a wide range of test chambers for various form factors and applications.
Keysight SL1133A (Compact Combined Solution)
The SL1133A combines power electronics, environmental chamber, and cell fixtures into a compact system for testing up to 24 channels at ±500 A and 0–6 V. With integrated safety features and low-impedance connections, it delivers accurate internal resistance and performance data while saving space in high-throughput labs. It is the right solution when space is limited and simplified commissioning is critical.