Battery Internal Resistance Calculator

The Battery Internal Resistance Calculator turns a pair of voltmeter readings into the one number that predicts how a battery will actually behave under load: its internal resistance, in milliohms. Give it the open-circuit voltage, the voltage while a known current flows, and the current itself, and it returns the resistance, grades it against a healthy cell of the same type, and works out the power being wasted as heat, the round-trip efficiency, and the highest current the battery can supply before it sags below its cutoff voltage.

What Is Internal Resistance?

No real battery is a perfect voltage source. Current inside a cell has to cross the electrode surfaces, travel through the electrolyte and separator, and pass out through the tabs, welds and terminals. All of that opposition adds up to an internal resistance that sits in series with the cell's ideal electromotive force, or EMF.

The consequence is simple and unavoidable: the moment current flows, some voltage is dropped inside the battery and never reaches the load. That is why a battery reading 4.15 V at rest can read 3.90 V the instant you switch a motor on, and why the missing 0.25 V comes back the moment you switch it off again.

Internal Resistance Formula

Internal resistance is Ohm's law applied to the voltage that goes missing:

Where \( V_{OC} \) is the open-circuit (rested, no-load) voltage in volts, \( V_{load} \) the terminal voltage while current flows, \( I \) the load current in amps, and \( R_{load} \) the value of the resistor you used as a load. A drop of 0.10 V at 2 A gives \( R_{int} = 0.05\,\Omega = 50\ \text{m}\Omega \).

Which Measurement Method Should You Use?

Typical Internal Resistance by Battery Type

There is no single "good" value — it depends entirely on the chemistry and the physical size of the cell. Bigger electrodes mean more area for current to cross, so a large cell has a lower resistance than a small one of the same chemistry. All figures below are per cell.

Why Internal Resistance Matters

• Voltage sag. The drop under load is \( I \times R_{int} \). Double the resistance and you double the sag at the same current.
• Wasted energy and heat. The battery dissipates \( I^2 R_{int} \) internally. At 20 A, a 30 mΩ pack turns 12 W into heat inside itself — that is what makes packs get warm and what limits their continuous rating.
• Usable current. Every device has a minimum voltage it will run at. Internal resistance decides how many amps you can pull before you hit it.
• State of health. Resistance rises measurably as a cell ages, often long before capacity loss is obvious. It is the fastest way to screen a batch of used cells.
• Matching cells. In a parallel or series pack, the cell with the highest resistance runs hottest, sags first and ages fastest. Matching resistance across cells matters as much as matching capacity.

Maximum Power Transfer

A battery delivers its greatest possible power when the load resistance equals the internal resistance. At that point exactly half the energy is wasted inside the battery, so this is a theoretical ceiling, not an operating target:

Real designs sit far to the left of that peak, where efficiency is high and the cell stays cool. The calculator shows both the peak figure and the efficiency at your actual operating point so you can see how much headroom you have.

How to Measure Internal Resistance Accurately

1. Rest the battery first. Let it sit unloaded for a few minutes so the surface charge settles, then take the open-circuit voltage.
2. Use a meaningful load. The drop must be large enough to measure. Aim for a current that produces at least 20–50 mV of sag; too small a load and meter noise dominates.
3. Read quickly. Take the loaded voltage within a second or two. Wait longer and you are also measuring the cell discharging, which inflates the result.
4. Measure at the terminals. Put the voltmeter probes on the battery posts themselves, not at the far end of the load leads, or you will measure your wiring as well as your battery.
5. Note the temperature and state of charge. Both change the answer. Compare readings taken under similar conditions.
6. Prefer the two-load method for anything critical. Subtracting two loaded readings removes the open-circuit voltage, and with it the biggest source of error.

DC Load Testing vs 1 kHz AC Meters

Battery analysers often quote an AC internal resistance measured with a 1 kHz signal. That reading captures mainly the ohmic component, because the alternating current is too fast for the slower electrochemical processes to follow. A DC load test — what this calculator performs — includes those slower effects and therefore returns a somewhat higher figure, typically 10–50% higher on a lithium cell. Neither is wrong; they measure different things. Just compare like with like, and do not judge a DC measurement against a datasheet number quoted at 1 kHz.

How to Use This Calculator

1. Pick a measurement method: voltage drop, two loads, or a known load resistor.
2. Choose the cell type and pack layout so the health grade compares against the right reference, and so a series or parallel pack is normalised back to one cell.
3. Enter your readings — voltages in volts, current in amps, load resistance in ohms.
4. Click Calculate Internal Resistance and read the resistance, the health grade, the wasted power, the maximum usable current and the full voltage-sag ladder.

Frequently Asked Questions

What is battery internal resistance?

Internal resistance is the opposition to current inside the battery itself, caused by the electrodes, the electrolyte, the separator and the tabs and welds that carry current out of the cell. It behaves like a small resistor in series with an ideal voltage source, so every amp you draw drops some voltage before it ever reaches the terminals.

How do you calculate internal resistance from voltage drop?

Measure the open-circuit voltage with no load, then measure the terminal voltage while a known current flows. Internal resistance is the difference between the two voltages divided by the current: R = (V open circuit − V loaded) / I. A drop of 0.10 volts at 2 amps means an internal resistance of 0.05 ohms, or 50 milliohms.

What is a good internal resistance for a battery?

It depends entirely on the cell type. A healthy 18650 lithium-ion cell is roughly 25 to 50 milliohms, a 21700 cell 12 to 25, an RC LiPo pouch 3 to 10, a large LiFePO4 prismatic cell under 1, a NiMH AA 20 to 50, an alkaline AA 100 to 200, and a 12 volt lead-acid car battery about 3 to 5 milliohms across the posts. A CR2032 coin cell is around 10 to 15 ohms, which is a thousand times higher.

Why does internal resistance matter?

It sets how much voltage the battery loses under load, how much energy is wasted as heat inside the cell, how hot the pack runs, and how much current you can draw before the voltage sags below what the device needs. Two cells with identical capacity can behave completely differently if one has twice the internal resistance of the other.

What is the difference between a DC load test and a 1 kHz internal resistance reading?

A DC load test measures the total resistance a real load sees, including the slower electrochemical effects, so it gives a slightly higher number. A 1 kHz AC meter measures mostly the ohmic part, because the alternating signal is too fast for the chemistry to respond, so it reads lower. Both are useful, but only compare a reading against a reference measured the same way.

Does internal resistance increase as a battery ages?

Yes. Growth of the passivation layer on the electrodes, electrolyte loss, corrosion and mechanical damage all raise internal resistance over a battery's life. Rising resistance is usually the first measurable sign of aging, and it often appears well before capacity loss becomes obvious, which is why resistance is the preferred way to screen used cells.

Why does internal resistance change with temperature and state of charge?

Cold electrolyte is more viscous and ions move more slowly, so resistance rises sharply below about 10 degrees Celsius, which is why a car struggles to start on a winter morning. Resistance also rises at very low and very high states of charge. Always compare readings taken at a similar temperature and charge level.

Additional Resources

• Internal Resistance - Wikipedia
• Maximum Power Transfer Theorem - Wikipedia
• Thevenin's Theorem - Wikipedia