Battery resistance, often called internal resistance, is the opposition to current flow that exists inside a battery itself. Every battery has it, and it directly affects how much usable power you actually get. A battery with low internal resistance delivers voltage efficiently, while one with high resistance wastes energy as heat and experiences noticeable voltage drops under load.
How Internal Resistance Works
When you connect a battery to a device, current flows through the external circuit (your device) and through the battery’s own internal materials. Those materials aren’t perfect conductors. The electrodes, the electrolyte solution between them, and the chemical interfaces all resist the flow of charged particles to some degree. That cumulative resistance inside the battery is its internal resistance.
The relationship follows Ohm’s Law: voltage equals current times resistance (V = I × R). Applied to a battery, this means the voltage lost inside the battery equals the current drawn multiplied by the internal resistance. If a battery has an internal resistance of 50 milliohms and you draw 2 amps, 0.1 volts is lost inside the battery before the current ever reaches your device. The higher the current demand, the larger that internal voltage drop becomes. This is why batteries that seem fine at rest can sag dramatically when you put them under heavy load.
Three Sources of Resistance
Internal resistance isn’t a single phenomenon. It has three distinct components working together.
Ohmic resistance is the simplest: it’s the physical resistance of the battery’s materials. The metal contacts, electrode coatings, and electrolyte all resist electron and ion flow the same way a wire resists current. This component is constant at a given temperature and doesn’t change with how fast you discharge the battery.
Charge transfer resistance occurs at the boundary where electrode surfaces meet the electrolyte. Chemical reactions happen at these interfaces, and they don’t occur instantaneously. The energy needed to drive those reactions at a given rate shows up as resistance. When you demand more current, this resistance becomes more significant because the reactions need to happen faster.
Diffusion resistance comes from the movement of ions through the electrolyte. Ions need to physically travel between electrodes, and at high discharge rates, they can’t migrate fast enough to keep up with demand. This creates a bottleneck that looks, electrically, like additional resistance. It’s most noticeable during sustained high-current draws.
What Typical Values Look Like
Internal resistance is measured in milliohms (thousandths of an ohm), and the numbers vary widely by battery type and size. A healthy 18650 lithium-ion cell, the standard rechargeable cell used in everything from laptops to electric vehicles, typically measures between 5 and 50 milliohms. Cells designed for high-drain applications (like power tools) land on the lower end, while standard-capacity cells sit higher.
Lead-acid batteries used in cars generally have internal resistance in the single-digit milliohm range because of their large electrode surface area. Small coin cells and alkaline AAs, by contrast, can have internal resistance in the hundreds of milliohms or higher, which is part of why they can’t deliver large currents without significant voltage sag.
If you’re testing 18650 cells and see readings of 300 to 500 milliohms, those cells are degraded and likely past their useful life.
Why Resistance Increases Over Time
Every rechargeable battery develops higher internal resistance as it ages, and this is the main reason old batteries perform worse than new ones even when they show a full charge.
In lithium-ion batteries, the primary culprit is a thin film that grows on the negative electrode called the solid electrolyte interphase. This layer forms naturally as the electrolyte reacts with the electrode surface. It’s actually necessary for the battery to function, but it keeps getting thicker with every charge cycle. Since ions have to pass through this layer to enter and exit the electrode, a thicker film means more resistance. Research from the University of Michigan found that after just 10 charge cycles, this layer’s resistance can reach about 60 milliohms per square centimeter, causing roughly 4% capacity loss from the resulting voltage drop alone.
The growth rate slows over time because the layer itself makes it harder for the electrolyte to reach the electrode surface and keep reacting. This is why lithium-ion batteries lose capacity fastest in their first few months and then degrade more gradually. But the process never fully stops, and after hundreds of cycles, the accumulated resistance increase is significant enough that you’ll notice shorter run times and reduced power output.
Other aging mechanisms contribute too. Electrode materials can crack from repeated expansion and contraction during charging. Electrolyte slowly decomposes. Metal current collectors can corrode. All of these add to the total resistance over the battery’s lifetime.
Temperature Changes Everything
Temperature has a dramatic effect on internal resistance. In lithium-ion batteries, resistance decreases almost exponentially as temperature rises. This happens because warmer electrolyte is less viscous, allowing ions to move more freely, and because the chemical reactions at the electrodes speed up.
Cold temperatures create the opposite problem. Below 0°C (32°F), lithium-ion batteries lose much of their ability to deliver power. The electrolyte becomes sluggish, ion mobility drops sharply, and in extreme cold the electrolyte can partially freeze. This is why your phone dies quickly on a cold winter day even though the battery had plenty of charge. The energy is still there, but the internal resistance is so high that the battery can’t push it out fast enough to maintain the required voltage.
Charging in cold conditions is even more problematic. High internal resistance during charging can cause lithium metal to plate onto the electrode surface instead of being absorbed into it properly, which permanently damages the cell and creates safety risks.
How Internal Resistance Is Measured
There are two main approaches to measuring internal resistance, and they give slightly different information.
The DC method applies a brief pulse of current to the battery and measures how much the voltage drops. Dividing that voltage drop by the known current gives you a resistance value. This is straightforward and uses inexpensive equipment, but it captures a somewhat blurred picture because the three resistance components all respond at different speeds.
The AC method, called electrochemical impedance spectroscopy, sends a small alternating current through the battery at many different frequencies. Because ohmic resistance, charge transfer resistance, and diffusion resistance each respond to different frequencies, this technique can separate them out individually. Testing on lithium iron phosphate batteries has shown that both methods yield values in the same range (roughly 25 to 50 milliohms for the cells tested), but the AC method provides a much more detailed breakdown of what’s happening inside the cell.
For most practical purposes, a simple DC pulse test gives you a good enough number to assess battery health. Many battery chargers and analyzers marketed to hobbyists use this approach. The full AC impedance analysis is mainly useful for researchers and engineers designing battery systems.
Why It Matters in Practice
Internal resistance determines how much power a battery can actually deliver. A battery with very low resistance can supply high currents without its voltage collapsing, which is why high-performance applications like power tools, drones, and electric vehicles use cells specifically engineered for minimal resistance.
It also determines how much heat a battery generates. The power wasted inside the battery equals the current squared times the internal resistance. At high discharge rates, a battery with elevated resistance can heat up significantly. This is both an efficiency problem (wasted energy) and a safety concern, since excessive heat accelerates degradation and, in extreme cases, can trigger thermal runaway in lithium-ion cells.
For anyone managing a collection of rechargeable cells, whether for a flashlight, an e-bike pack, or a home solar system, tracking internal resistance over time is the most reliable way to gauge battery health. A cell whose resistance has doubled from its original value is nearing the end of its useful life, even if it still holds a charge. The voltage sag under load will make it perform poorly and cause it to heat up more than the healthier cells around it.