Inside the battery: A quick look at internal resistance

Ever wondered why a battery that reads full voltage still struggles to power your device? The answer often lies in its internal resistance. This hidden factor affects how efficiently a battery delivers current, especially under load.

In this post, we will briefly examine the basics of internal resistance—and why it’s a critical factor in real-world performance, from handheld flashlights to high-power EV drivetrains.

What’s internal resistance and why it matters

Every battery has some resistance to the flow of current within itself—this is called internal resistance. It’s not a design flaw, but a natural consequence of the materials and construction. The electrolyte, electrodes, and even the connectors all contribute to it.

Internal resistance causes voltage to drop when the battery delivers current. The higher the current draw, the more noticeable the drop. That is why a battery might read 1.5 V at rest but dip below 1.2 V under load—and why devices sometimes shut off even when the battery seems “full.”

Here is what affects it:

• Battery type: Alkaline, lithium-ion, and NiMH cells all have different internal resistances.
• Age and usage: Resistance increases as the battery wears out.
• Temperature: Cold conditions raise resistance, reducing performance.
• State of charge: A nearly empty battery often shows higher resistance.

Building on that, internal resistance gradually increases as batteries age. This rise is driven by chemical wear, electrode degradation, and the buildup of reaction byproducts. As resistance climbs, the battery becomes less efficient, delivers less current, and shows more voltage drop under load—even when the resting voltage still looks healthy.

Digging a little deeper—focusing on functional behavior under load—internal resistance is not just a single value; it’s often split into two components. Ohmic resistance comes from the physical parts of the battery, like the electrodes and electrolyte, and tends to stay relatively stable.

Polarization resistance, on the other hand, reflects how the battery’s chemical reactions respond to current flow. It’s more dynamic, shifting with temperature, charge level, and discharge rate. Together, these resistances shape how a battery performs under load, which is why two batteries with identical voltage readings might behave very differently in real-world use.

Internal resistance in practice

Internal resistance is a key factor in determining how much current a battery can deliver. When internal resistance is low, the battery can supply a large current. But if the resistance is high, the current it can provide drops significantly. Also, higher the internal resistance, the greater the energy loss—this loss manifests as heat. That heat not only wastes energy but also accelerates the battery’s degradation over time.

The figure below illustrates a simplified electrical model of a battery. Ideally, internal resistance would be zero, enabling maximum current flow without energy loss. In practice, however, internal resistance is always present and affects performance.

Figure 1 Illustration of a battery’s internal configuration highlights the presence of internal resistance. Source: Author

Here is a quick side note regarding resistance breakdown. Focusing on material-level transport mechanisms, battery internal resistance comprises two primary contributors: electronic resistance, driven by electron flow through conductive paths, and ionic resistance, governed by ion transport within the electrolyte.

The total effective resistance reflects their combined influence, along with interfacial and contact resistances. Understanding this layered structure is key to diagnosing performance losses and carrying out design improvements.

As observed nowadays, elevated internal resistance in EV batteries hampers performance by increasing heat generation during acceleration and fast charging, ultimately reducing driving range and accelerating cell degradation.

Fortunately, several techniques are available for measuring a battery’s internal resistance, each suited to different use cases and levels of diagnostic depth. Common methods include direct current internal resistance (DCIR), alternating current internal resistance (ACIR), and electrochemical impedance spectroscopy (EIS).

And there is a two-tier variation of the standard DCIR technique, which applies two sequential discharge loads with distinct current levels and durations. The battery is first discharged at a low current for several seconds, followed by a higher current for a shorter interval. Resistance values are calculated using Ohm’s law, based on the voltage drops observed during each load phase.

Analyzing the voltage response under these conditions can reveal more nuanced resistive behavior, particularly under dynamic loads. However, the results remain strictly ohmic and do not provide direct information about the battery’s state of charge (SoC) or capacity.

Many branded battery testers, such as some product series from Hioki, apply a constant AC current at a measurement frequency of 1 kHz and determine the battery’s internal resistance by measuring the resulting voltage with an AC voltmeter (AC four-terminal method).

Figure 2 The Hioki BT3554-50 employs AC-IR method to achieve high-precision internal resistance measurement. Source: Hioki

The 1,000-hertz (1 kHz) ohm test is a widely used method for measuring internal resistance. In this approach, a small 1-kHz AC signal is applied to the battery, and resistance is calculated using Ohm’s law based on the resulting voltage-to-current ratio.

It’s important to note that AC and DC methods often yield different resistance values due to the battery’s reactive components. Both readings are valid—AC impedance primarily reflects the instantaneous ohmic resistance, while DC measurements capture additional effects such as charge transfer and diffusion.

Notably, the DC load method remains one of the most enduring—and nostalgically favored—approaches for measuring a battery’s internal resistance. Despite the rise of impedance spectroscopy and other advanced techniques, its simplicity and hands-on familiarity continue to resonate with seasoned engineers.

It involves briefly applying a load—typically for a second or longer—while measuring the voltage drop between the open-circuit voltage and the loaded voltage. The internal resistance is then calculated using Ohm’s law by dividing the voltage drop by the applied current.

A quick calculation: To estimate a battery’s internal resistance, you can use a simple voltage-drop method when the open-circuit voltage, loaded voltage, and current draw are known. For example, if a battery reads 9.6 V with no load and drops to 9.4 V under a 100-mA load:

Internal resistance = 9.6 V-9.4 V/0.1 A = 2 Ω

This method is especially useful in field diagnostics, where direct resistance measurements may not be practical, but voltage readings are easily obtained.

In simplified terms, internal resistance can be estimated using several proven techniques. However, the results are influenced by the test method, measurement parameters, and environmental conditions. Therefore, internal resistance should be viewed as a general diagnostic indicator—not a precise predictor of voltage drop in any specific application.

Bonus blueprint: A closing hardware pointer

For internal resistance testing, consider the adaptable e-load concept shown below. It forms a simple, reliable current sink for controlled battery discharge, offering a practical starting point for further refinement. As you know, the DC load test method allows an electronic load to estimate a battery’s internal resistance by observing the voltage drop during a controlled current draw.

Figure 3 The blueprint presents an electronic load concept tailored for internal resistance measurement, pairing a low-R_DS(on) MOSFET with a precision load resistor to form a controlled current sink. Source: Author

Now it’s your turn to build, tweak, and test. If you have got refinements, field results, or alternate load strategies, share them in the comments. Let us keep the circuit conversation flowing.

T. K. Hareendran is a self-taught electronics enthusiast with a strong passion for innovative circuit design and hands-on technology. He develops both experimental and practical electronic projects, documenting and sharing his work to support fellow tinkerers and learners. Beyond the workbench, he dedicates time to technical writing and hardware evaluations to contribute meaningfully to the maker community.

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• Resistivity is the key to measuring electrical resistance
• Cell balancing maximizes the capacity of multi-cell batteries

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