Battery Internal Resistance Calculator

Calculate Internal Resistance of Battery

Calculation method

Open circuit voltage, Voc

Loaded terminal voltage

Load current, A

Load resistance, Ω

Loaded voltage 1

Load current 1, A

Loaded voltage 2

Load current 2, A

Lead resistance, mΩ

Number of cells

Battery capacity, Ah

Test temperature, °C

Reference temperature, °C

Temperature coefficient, % per °C

Test notes

Formula Used

Single load method:

Internal resistance = (Open circuit voltage − Loaded voltage) ÷ Load current

Load resistor current:

Load current = Loaded voltage ÷ Load resistance

Two load method:

Internal resistance = (Voltage 1 − Voltage 2) ÷ (Current 2 − Current 1)

Power lost inside battery:

Power loss = Current² × Internal resistance

Estimated short current:

Short current = Open circuit voltage ÷ Internal resistance

How to Use This Calculator

Select the calculation method.
Enter open circuit voltage and loaded voltage.
Enter load current, or enter load resistance.
Use two loaded readings for comparative testing.
Add lead resistance if test leads affect readings.
Enter cell count for per cell resistance.
Press Calculate to view the result above the form.
Use CSV or PDF buttons to save the report.

Example Data Table

Battery Type	Voc	Loaded Voltage	Load Current	Resistance	Comment
12 V lead acid	12.72 V	12.10 V	20 A	31 mΩ	Normal bench check
Single Li-ion cell	4.15 V	3.95 V	5 A	40 mΩ	Small cell test
Battery pack	24.8 V	23.9 V	30 A	30 mΩ	Pack level reading

Battery Internal Resistance Guide

Internal resistance is the hidden opposition inside a battery. It limits current. It also creates heat when a load is connected. A healthy battery usually has low resistance. An aged, sulfated, cold, or damaged battery often shows a higher value.

Why Internal Resistance Matters

Every battery can look normal at open circuit. The voltage may seem fine. The real test starts when current flows. Internal resistance causes terminal voltage to fall under load. That drop affects motors, inverters, radios, sensors, and backup systems. High resistance wastes energy as heat. It can also reduce usable capacity during heavy discharge.

Main Measurement Method

The common method uses open circuit voltage, loaded voltage, and loaded current. First measure the battery with no load. Then connect a known load and measure terminal voltage again. The difference between those readings is the internal voltage drop. Divide that drop by the load current. The calculator also accepts a load resistor. In that mode, current is estimated from loaded voltage divided by resistance.

Advanced Review

Two loaded readings can improve practical checks. It compares two operating points and estimates resistance from the voltage change and current change. Lead resistance can be subtracted when test cables are known. This is useful for high current tests, where small cable losses matter. Temperature correction can normalize results to a reference temperature. Use this only when you have a trusted coefficient.

Interpreting Results

Results are shown in ohms and milliohms. Small cells may have values in tens or hundreds of milliohms. Large lead acid or lithium packs may show very low values. Always compare results with the same battery type, size, chemistry, age, and state of charge. A single number should not be used alone for safety decisions.

Good Testing Practice

Use short, thick leads for load tests. Avoid loose clips. Do not exceed the battery discharge rating. Stop if the battery heats, swells, vents, or smells unusual. Record voltage, current, temperature, and load time. Repeat the test after charging and resting the battery. Consistent data makes trends clearer and helps you find weak cells before field failure. Record tester model during each session. Different meters can vary slightly. Use stable clamps and repeat readings after load has settled fully again.

FAQs

What is battery internal resistance?

It is the resistance inside the battery. It causes voltage drop when current flows. It also creates heat and limits high current performance.

Which voltage values should I enter?

Enter open circuit voltage before loading the battery. Then enter terminal voltage while the load is connected. Use stable readings for better results.

Can I use load resistance instead of current?

Yes. Enter loaded voltage and load resistance. The calculator estimates current by dividing loaded voltage by load resistance.

Why is the result shown in milliohms?

Battery internal resistance is often very small. Milliohms make small values easier to read, compare, and record during bench testing.

What does lead resistance mean?

Lead resistance is the resistance of test wires and clips. It can affect high current tests. Enter it if you know the value.

Is short current exact?

No. It is only an estimate from voltage and resistance. Real short current depends on chemistry, protection circuits, temperature, and battery condition.

Why use two loaded readings?

Two loaded readings can reduce reliance on open circuit voltage. They are useful when testing batteries under two controlled load levels.

Can high internal resistance mean battery failure?

It can indicate aging, poor charge, cold temperature, sulfation, weak cells, or damage. Compare with similar batteries before making a final decision.