Battery Internal Resistance Testing
Measuring DC internal resistance and AC impedance to quantify power capability, diagnose aging mechanisms, and predict battery behavior under real-world high-rate loads.
Understanding Battery Internal Resistance Testing
From DCIR pulse methods to AC impedance spectroscopy — a practical guide to measuring, tracking, and interpreting battery internal resistance.
Why Internal Resistance Matters
Internal resistance is a fundamental indicator of battery power capability and state of health. A low-resistance cell delivers more voltage to the load at high currents and wastes less energy as heat. As a battery ages, resistance grows — first at interfaces (SEI, cathode surface), then through electrolyte depletion and active material loss. Tracking resistance over a battery's life provides an early warning of degradation before capacity fade becomes significant.
DCIR vs. AC Impedance
Two complementary methods are used in battery testing, each revealing different aspects of resistance:
- DC Internal Resistance (DCIR): A short current pulse is applied and the voltage response is measured. DCIR = ΔV ÷ ΔI. This captures ohmic resistance plus short-timescale polarization. It is the most common in-cycle resistance metric.
- AC Impedance (EIS): A small sinusoidal current is swept across a frequency range. The impedance spectrum separates ohmic resistance, charge-transfer resistance, and diffusion contributions. Maccor supports EIS via the ACImp and EDA/FRA step types.
- DCIR from discharge step: Resistance can also be estimated from the voltage drop at the start of a discharge step at a known current.
- Pulse Resistance: MIMS Client cycle-based charts can plot pulse resistance directly from embedded pulse steps.
Measuring DCIR During Cycling
DCIR measurement is typically embedded directly in the cycling procedure:
- After the rest step, insert a short pulse discharge step (e.g., 10-second pulse at 1C or 2C).
- Set the End Type to Step Time = 10 seconds.
- Follow the pulse with a rest step to allow voltage recovery before continuing the cycle.
- The DCIR value is computed from ΔV ÷ ΔI and stored in the data file.
- In MIMS Client, add DCIR or Pulse Resistance to a cycle-based chart to track the trend over life.
- Optionally, run a full EIS sweep at each Reference Performance Test interval for deeper diagnostics.
Where Internal Resistance Testing Adds Value
Resistance is a more sensitive aging indicator than capacity for many failure modes, especially in power-focused applications.
| Environment | Primary Goal |
|---|---|
| R&D Laboratories | Separate ohmic and kinetic resistance contributions to guide material design |
| Validation Programs | Verify power capability meets pulse discharge requirements at end-of-life |
| Production & QA | Detect high-resistance cells that indicate formation or welding defects |
| Field & Reliability | Track resistance growth as a predictor of remaining power capability |
Benefits for Engineers and Technical Buyers
Internal resistance measurement adds diagnostic depth to any cycling or characterization program.
- Earlier degradation signal: Resistance growth often precedes capacity fade, giving more lead time for intervention.
- Power-focused validation: For EV and power-tool applications, DCIR at end-of-life determines whether pulse power requirements are still met.
- EIS for root-cause analysis: Nyquist plots separate ohmic resistance, SEI resistance, and charge-transfer resistance — enabling more targeted design improvements.
- No extra channels required: DCIR is collected inline with the cycling procedure using standard pulse steps.
Frequently Asked Questions
How long should a DCIR pulse be?
Typical pulse durations range from 2 to 30 seconds. Shorter pulses (2–5 s) capture primarily ohmic resistance; longer pulses include polarization contributions. The pulse duration should be standardized across all measurements in a study for consistent comparison.
At what SoC should DCIR be measured?
DCIR is SoC-dependent — it is typically highest at very low and very high SoC, and lowest in the mid-SoC range. Most programs measure DCIR at 50% SoC as the standard reference point, with additional measurements at other SoC levels for characterization.
What does a rising Nyquist semicircle indicate?
Growth of the high-frequency semicircle typically indicates increasing SEI or cathode interface resistance. Growth of the lower-frequency feature suggests worsening charge-transfer kinetics at the electrode surface — often associated with active material degradation.