You can measure a battery’s internal resistance using a simple two-step voltage test and Ohm’s law: measure the battery’s voltage with no load, measure it again under a known load, then divide the voltage difference by the current flowing through the load. That gives you the internal resistance in ohms (or more often, milliohms). There are also dedicated instruments that use AC signals to get the measurement faster and more repeatably.
What Internal Resistance Actually Is
Every battery has a small amount of resistance built into its own structure. The electrolyte, the electrodes, and even the metal connectors between cells all resist the flow of current to some degree. This is why a battery’s voltage drops when you connect it to a load: some of that energy is lost as heat inside the battery itself rather than being delivered to your device.
Two types of resistance combine to create the total. Electronic resistance comes from electrons moving through the solid conductive paths (electrodes, tabs, terminals). Ionic resistance comes from charged ions moving through the liquid or gel electrolyte. On top of those, there are small resistances at the interfaces where the electrodes meet the electrolyte and where internal connections meet each other. As a battery ages or degrades, these resistances climb, which is why internal resistance is one of the best indicators of battery health.
The DC Load Test Method
This is the most accessible approach and requires no specialized equipment beyond a multimeter and a resistor (or any known load).
Start by measuring the battery’s open-circuit voltage. This means measuring with nothing connected to the battery, so no current is flowing. Write this number down as V1. Next, connect a load resistor across the battery terminals and, while current is flowing, measure the voltage again. This is V2, and it will be lower than V1. Also measure (or calculate) the current flowing through the load.
The formula is straightforward:
Internal Resistance = (V1 − V2) ÷ Current
For example, if a AA battery reads 1.58 V open-circuit and drops to 1.52 V when delivering 500 mA through a load, the internal resistance is (1.58 − 1.52) ÷ 0.5 = 0.12 ohms, or 120 milliohms.
A few practical tips make this work better. Use a load that draws enough current to produce a measurable voltage drop, but not so much that the battery heats up significantly. For small batteries like AAs, a load drawing 500 mA to 1 A works well. For car batteries, you may need a load of 50 A or more to see a meaningful voltage change. Take your readings quickly, within a few seconds, because the voltage will continue to drift as the battery’s chemistry responds to the sustained load.
The AC Impedance Method
Professional battery analyzers and many handheld testers use a completely different approach. Instead of applying a DC load, they inject a small alternating current signal, typically at 1 kHz, and measure the battery’s response.
The 1 kHz frequency is chosen deliberately. At that frequency, two sources of measurement error essentially disappear: a slow-diffusion effect that shows up at low frequencies and a parasitic inductance that appears at high frequencies. By testing at 1 kHz, the instrument isolates the core resistance of the battery without those complicating factors.
This method is fast (often under a second), doesn’t discharge the battery in any meaningful way, and produces very repeatable results. It’s how most automotive parts stores test your car battery and how battery management systems in electric vehicles monitor cell health in real time. The downside is that you need a purpose-built instrument. Dedicated battery impedance testers range from around $30 for basic handheld models to several hundred dollars for lab-grade equipment.
Why 4-Wire Connections Matter
Internal resistance values are often in the milliohm range, especially for large batteries. A healthy lithium-ion 18650 cell might measure around 70 milliohms. At that scale, the resistance of your own test leads and their contact points with the battery terminals becomes a real problem. A standard multimeter measures all resistance in the circuit loop, including the wires connecting it to the battery.
The solution is a 4-wire (Kelvin) measurement. Two wires carry the test current into and out of the battery, while two separate wires measure only the voltage across the battery’s terminals. Because the voltage-sensing wires carry almost no current, they don’t develop any voltage drop of their own, so they report only the voltage lost inside the battery. Dedicated battery testers use this technique automatically. If you’re building a test setup from scratch, you can use a separate ammeter in series and a voltmeter connected directly at the battery terminals to achieve the same thing.
Factors That Affect Your Reading
Temperature
Temperature has an enormous effect on internal resistance. At low temperatures, ion movement through the electrolyte slows dramatically. Research on lithium-ion traction batteries shows that internal resistance can increase by over 300% at cold temperatures (around −20°C) compared to room temperature. If you’re testing outdoors in winter or on a cold garage floor, your readings will be significantly higher than the battery’s true warm-weather performance. For comparable results, test at or near room temperature, or at least record the temperature alongside your measurement.
State of Charge
A battery’s internal resistance also changes with how full it is. Resistance tends to be lowest in the middle of the charge range and rises at both extremes, particularly when the battery is nearly empty. For the most representative reading, test when the battery is between about 40% and 80% charged.
Battery Age
As batteries cycle through charge and discharge, their internal resistance gradually climbs. Chemical deposits build up on the electrodes, the electrolyte degrades, and contact points corrode. Tracking internal resistance over time is one of the most reliable ways to judge whether a battery is nearing the end of its useful life. A reading that has doubled from the original specification is a strong signal that capacity and performance have meaningfully declined.
Choosing the Right Method
- For a quick health check on a car battery: An inexpensive handheld battery tester that uses the AC method will give you a fast, reliable reading without heavy discharge loads.
- For hobby or DIY projects: The DC load method with a multimeter works well and costs nothing if you already own basic test equipment. Use a known resistor as your load and take readings quickly.
- For sorting or grading cells (e.g., building battery packs from 18650 cells): A dedicated milliohm meter or battery analyzer with 4-wire capability gives you the precision needed to match cells by resistance.
- For ongoing monitoring: Battery management systems in EVs and solar storage use continuous AC impedance sensing to track resistance changes in real time, flagging cells that are degrading faster than their neighbors.
Safety During Testing
DC load testing involves intentionally drawing significant current from a battery, which generates heat in both the load resistor and the battery itself. With lithium-ion cells, this matters more than with other chemistries. Lithium-ion batteries contain flammable electrolyte, and excessive heat or a short circuit can trigger thermal runaway, a chain reaction where one failing cell damages neighboring cells, potentially leading to fire, venting of toxic gas, or explosion.
Keep your load resistor rated well above the power it will dissipate, and limit test duration to a few seconds. Never short-circuit a battery, even briefly, to “see what happens.” Use insulated tools, avoid testing batteries that are visibly swollen or damaged, and work in a well-ventilated area. If you notice a battery becoming hot, venting gas, or producing smoke during testing, move away from it immediately. For large battery banks, wearing safety glasses and insulated gloves is a sensible precaution since the stored energy in a fully charged bank can deliver dangerous fault currents through accidental contact.