Example data table
| Scenario | Voc (V) | Vload (V) | I (A) | R (mΩ) |
|---|---|---|---|---|
| Single 18650 cell | 4.20 | 4.05 | 5.00 | 30.0 |
| Older cell under load | 4.15 | 3.92 | 6.00 | 38.3 |
| High‑drain cell | 4.20 | 4.10 | 10.00 | 10.0 |
Formula used
- Single load method: R = (Voc − Vload) / I
- Two‑point method: R = (V1 − V2) / (I2 − I1)
- Pulse step method: R = ΔV / ΔI
Voltage sag comes from internal resistance plus contact and wiring resistance. Keep probes tight and repeat measurements consistently for reliable trending.
How to use this calculator
- Select the method that matches your test setup.
- Enter voltages in volts and currents in amperes.
- Submit to calculate internal resistance in milliohms.
- Review voltage sag and heat loss at the reference current.
- Download CSV or PDF to store a baseline for comparison.
What internal resistance represents
Internal resistance is the lumped opposition to current inside a battery, combining ionic transport limits, electrode kinetics, current collectors, separators, welds, and busbars. It rises with aging, low temperature, high depth of discharge, and poor external joints. Knowing it helps predict voltage sag, peak power, and heat generation under realistic duty cycles, not just lab conditions.
How the calculator estimates resistance
The single load method uses open circuit voltage and loaded voltage at a measured current, giving R = (Voc − Vload) / I. The two point method uses the slope between two operating points, R = (V1 − V2) / (I2 − I1), reducing sensitivity to one noisy reading and meter offset. The pulse step method uses ΔV / ΔI to approximate fast transient behavior and highlight connection issues.
Temperature, SOC, and rest time
Resistance typically increases as temperature falls because ion mobility and interfacial reaction rates decrease. It may also rise near low state of charge where polarization grows and diffusion limits dominate. For consistent trending, let the battery rest, use the same fixture, record temperature and SOC, and compare only like conditions. Even a few degrees can shift milliohm results enough to mask early degradation.
Linking resistance to design losses
Once R is known, voltage sag at a target current is I×R and heat loss is I²×R. Because current is squared, a modest resistance increase can create large thermal penalties at high load. Engineers use these outputs to size conductors, fuses, contactors, cooling paths, and voltage cutoff margins in converters. For packs, include harness and connector resistance to avoid optimistic power estimates.
Interpreting results for maintenance
Capture a baseline for each cell or pack when new, then monitor percent change over time. Gradual increases often indicate normal wear, while sudden jumps suggest loose terminals, corrosion, cracked welds, or a failing cell group. Pair resistance with capacity checks and temperature rise during load to decide balancing, replacement, or conservative derating. When values drift, retest with the same current level to confirm repeatability before acting. Document test current, clamp pressure, and lead placement; small setup changes can dominate measurements on low‑resistance cells during high-current validation.
FAQs
1) What units should I use for inputs?
Enter voltages in volts and currents in amperes. The calculator outputs resistance in milliohms and ohms. Keep units consistent and use the same meter range across tests for better repeatability.
2) Why can internal resistance look negative?
Negative results usually come from swapped voltage points, sign mistakes in ΔV or ΔI, or readings taken at different states of charge. Recheck polarity, confirm currents differ, and repeat the measurement with stable connections.
3) Should I test at full charge or mid charge?
For trending, pick one state of charge and stick with it, often 50–80%. Near empty, polarization increases and results rise. Record the chosen SOC so future comparisons remain meaningful.
4) How does temperature affect the reading?
Cold temperatures increase resistance and reduce available power. A few degrees can change milliohms noticeably. Measure at a consistent temperature, or record temperature and compare only tests within a narrow range.
5) Does this include cable and connector resistance?
It includes whatever is in your measurement loop. For accurate cell resistance, use short leads and solid clamps. For pack-level assessment, include harness resistance because it impacts real voltage sag under load.
6) How often should I retest?
Retest on a schedule that matches usage, such as monthly for critical packs and quarterly for light duty. Also retest after abnormal heating, storage, or impacts. Use the same method and current level each time.