The calculation, without hidden assumptions
A simple DC source model treats the real battery or supply as an ideal EMF in series with an internal resistance. Under load, current creates an internal voltage drop and heat loss. MAXScanner supports the common open-circuit/load-current test, a two-load-point method that does not require a separate no-load reading, and a forward model once r is known.
How to use this calculator
Choose the measurement set you actually have.
Enter voltages and currents from the same source state; avoid mixing readings taken at very different charge, temperature or rest conditions.
Review internal resistance, voltage sag, internal heat loss and efficiency context, then compare with the device or cell manufacturer’s test method where available.
Where people use it
- •Battery load-test calculations
- •Comparing voltage sag at different currents
- •Teaching Thevenin-style source models
- •Checking internal I²R loss in a simple DC model
Example: 12.8 V open circuit, 12.2 V at 30 A
r = (12.8 − 12.2) / 30 = 0.02 Ω = 20 mΩ. Internal loss at that test current is I²r = 18 W.
What the result does not assume
- •Battery internal resistance changes with state of charge, temperature, age, chemistry, frequency/test duration and measurement method. A single DC load test is an estimate, not a universal cell specification.
- •High-current battery tests can be hazardous. Use suitable equipment, wiring, protection and manufacturer procedures; this calculator does not tell you how to perform a load test safely.
- •The one-resistor source model does not capture electrochemical dynamics, polarization, recovery or frequency-dependent impedance.
Frequently asked questions
Why does loaded voltage fall below open-circuit voltage?+
In the simple source model, load current produces an internal drop I×r, so Vterminal = E − Ir.
Why show milliohms?+
Healthy batteries and power sources can have very small internal resistance, so mΩ is often easier to read than a small decimal number of ohms.
Can I calculate r without an open-circuit reading?+
Yes. If two terminal-voltage readings are taken at two different known currents, the slope ΔV/ΔI gives the internal resistance under the simple model.
Is lower internal resistance always better?+
Lower r generally means less voltage sag and internal I²R loss for a given current, but acceptable values depend on chemistry, size, state, test method and application.
Semantic next steps
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MAXScanner keeps the formula and assumptions beside the result so you can verify the number before using it in a drawing, estimate, specification, worksheet or document. Where a supplier, manufacturer, drawing or applicable standard owns a requirement, that source remains authoritative.