Measuring Battery Internal Resistance with Ohm's Law

Measuring Battery Internal Resistance with Ohm's Law

Every battery has internal resistance that causes terminal voltage to sag under load. Open-circuit voltage tells you the state of charge at rest, but internal resistance determines how much voltage the battery can deliver when the load actually draws current. Applying Ohm's Law to the difference between open-circuit voltage and loaded terminal voltage gives the internal resistance directly: R = (OCV - Vterm) / I.¹ Fresh AA alkaline cells typically measure below 0.3 Ω; aged or cold cells rise above 3 Ω, causing significant voltage sag at even modest currents. A 12 V lead-acid battery in good condition stays below 0.05 Ω, while a 18650 lithium-ion cell in good health measures below 0.1 Ω. These benchmarks help decide whether a battery is fit for service before deployment.

The measurement procedure step by step

Measuring internal resistance requires three readings: open-circuit voltage, loaded terminal voltage, and load current. First, disconnect all loads and let the battery rest for at least 30 seconds, then measure the open-circuit voltage (OCV) with a multimeter. Next, connect a known resistive load that draws a significant fraction of the battery's rated current. Immediately read the terminal voltage under load (Vterm) and the current (I) with a series ammeter or by dividing the terminal voltage by the known load resistance.

The internal resistance is R = (OCV - Vterm) / I. For a 12 V lead-acid battery with OCV = 12.6 V, terminal voltage = 12.3 V at 10 A: R = (12.6 - 12.3) / 10 = 0.03 Ω, which represents a healthy cell that can still deliver strong cranking current without excessive voltage sag at the starter motor terminals.

Taking the reading immediately after applying the load

Take the voltage reading immediately after applying the load to avoid thermal drift in the measurement. Long measurement windows allow the battery terminal voltage to drift as charge redistribution occurs within the cell. A 12 V lead-acid battery at 25°C settles to a stable loaded voltage within about one second, but a lithium-ion cell may take several seconds as the electrolyte concentration equalises near the electrode surfaces. Use the reading at the one-second mark for lead-acid and the three-second mark for lithium-ion to get a consistent, repeatable result that you can compare across cells and over time.

Interpreting results against chemistry-specific benchmarks

Internal resistance benchmarks vary by chemistry and cell size.² Fresh AA alkaline cells: below 0.3 Ω. Aged or deeply discharged AA alkaline: above 2 Ω. NiMH AA: 0.02 to 0.05 Ω. 18650 lithium-ion in good health: below 0.1 Ω; end-of-life: above 0.3 Ω. 12 V lead-acid in good health: 0.01 to 0.05 Ω; ready-for-replacement: above 0.1 Ω.

A high internal resistance reading relative to the chemistry benchmark indicates one of three conditions: the cell is aged and has lost capacity, the cell is cold (internal resistance rises sharply below 0°C for most chemistries), or the cell is deeply discharged. Repeating the measurement at room temperature after a partial charge helps distinguish aging from temperature or state-of-charge effects. Furthermore, cells with abnormally high internal resistance in a pack cause voltage imbalance during charge and discharge, which accelerates degradation of the remaining healthy cells.

Building a trend across battery service life

For capacity-critical deployments, record internal resistance at the same state of charge and temperature on each maintenance cycle to build a trend across battery service life. A cell degrading faster than the chemistry specification predicts should be replaced before it forces the remaining cells into imbalanced operation. Track at least five successive measurement points at the same conditions before drawing conclusions about the degradation rate. Plotting internal resistance against cycle number reveals a characteristic curve: flat for most of the cell's life, then rising steeply as the active material degrades and contact resistance increases. The inflection point on this curve is your predictor for end of life, and scheduling replacement just before it arrives prevents unexpected failures in the field.

A practical logging approach is to store each measurement as a timestamped record alongside the ambient temperature and the state of charge at the time, so you can normalise future readings against the same reference conditions. Without this context, a measurement taken at 0 degrees Celsius looks identical to a measurement of a failed cell at 25 degrees Celsius, and you would replace batteries that are merely cold while missing the ones that are genuinely degrading. Building the log takes minimal effort when you already have the meter in hand for the load test.

Comparing measurement methods and their accuracy

The DC load method described here gives a practical internal resistance value that includes both ohmic and polarisation contributions, which is the number that matters for predicting real-world performance. AC impedance spectroscopy at 1 kHz gives a lower value that represents only the ohmic resistance and ignores the slower electrochemical processes. For field testing, the DC load method is more representative of actual operating conditions. Dedicated battery impedance meters use AC injection at multiple frequencies to separate ohmic, charge-transfer, and diffusion impedances, which gives deeper diagnostic insight but requires specialised equipment. For most maintenance purposes, a consistent DC load test at a known current gives you a reliable trend that you can compare over time, which is what matters for predicting when a battery needs replacement.

How internal resistance limits available power and causes voltage sag

Internal resistance reduces both terminal voltage and available power in a way that becomes increasingly severe as the battery ages and its internal resistance climbs. As load current rises, the internal voltage drop rises proportionally, so the terminals deliver progressively less voltage to the external circuit at the exact moment when the load needs it most. The battery also dissipates I²R heat inside itself, which wastes stored energy and raises cell temperature, creating a feedback loop where higher temperature further increases internal resistance and accelerates the degradation process.

For starting motors and high-drain devices, internal resistance caps the peak current the cell can deliver, which is why a battery that powers headlights and a radio perfectly well may still fail to crank an engine whose starter motor demands hundreds of amperes for a few seconds. The starter motor's low impedance means the inrush current is limited almost entirely by the battery's own internal resistance, so any increase in that resistance directly reduces the cranking power available at the starter solenoid.

Peak current limits from internal resistance

At a given terminal voltage, the maximum useful current is (OCV - V_cutoff) / R_internal, where V_cutoff is the discharge cutoff voltage.³ An aged lead-acid battery with 0.6 Ω internal resistance cannot deliver the 100 A cranking current a starter motor may demand, even if the open-circuit voltage still reads 12 V. Measuring internal resistance with a load test reveals this limitation before a field failure.

A fresh 12 V automotive battery with 0.02 Ω internal resistance can theoretically deliver (12.6 - 7.2) / 0.02 = 270 A, which covers the 150 to 200 A that a typical starter motor demands. The same battery at end of life with 0.1 Ω internal resistance can only deliver 54 A, which is insufficient for starting and explains why the engine cranks slowly or not at all even though the headlights still work. track a cell's internal resistance trend to catch that end-of-life drop early.

When to use this

Use this guide when testing a battery pack before a field deployment, after long storage, or when a device shows weak performance that a state-of-charge check cannot explain. Reference it when diagnosing early cutoff in a battery-powered device where the battery still reads significant open-circuit voltage but the device resets under load.

Examples