Capacity testing tells you how much energy a battery holds, internal resistance tells you its health and power capability, and EIS maps the internal chemistry in detail. For most users, capacity + internal resistance is enough; EIS is the lab-grade tool for diagnosing exactly what is aging.
The Short Answer
Battery testing answers two questions: how much energy is left (capacity) and how healthy is the battery (internal resistance and impedance). Capacity testing discharges the battery at a defined rate and measures delivered energy — the gold standard for state of health. Internal resistance (IR) measurement applies a brief load and measures the voltage drop — a quick, powerful health signal, since IR rises as batteries age. EIS sweeps a range of frequencies to separate the different internal processes — the diagnostic microscope of battery testing. Each method has its place: field checks, warranty verification, and R&D.
Capacity Testing
Capacity is measured in ampere-hours (Ah) delivered at a defined discharge rate and temperature. The standard test: fully charge the battery, discharge at a rated current (e.g., 0.2C or 1C) to the cutoff voltage, and measure the energy delivered. Compare the result to the rated capacity: a battery delivering 85% of rated capacity after years of use is typically considered end-of-life for many applications. This ratio is formally defined as state of health in battery management. Ampere-hour capacity depends on rate (higher rate = less delivered Ah, the Peukert effect) and temperature (cold reduces capacity). Test at a consistent, documented protocol — otherwise results are not comparable.
Internal Resistance (IR)
Internal resistance is the battery’s own opposition to current flow — a combination of ionic and electronic resistance inside the cell. IR is measured by the voltage drop method: apply a known current pulse, measure the instantaneous voltage change, and divide (R = ΔV/ΔI). Fresh cells have low IR (milliohms for large cells); IR rises as the battery ages, the electrolyte degrades, and connections corrode. Rising IR means more heat and voltage sag under load — the reason an old battery “feels weak” even at the same SOC. IR is the quickest, most repeatable health metric for field testing and is the basis of many handheld battery testers.
Electrochemical Impedance Spectroscopy (EIS)
EIS applies a small alternating current over a range of frequencies (e.g., 1 mHz to 10 kHz) and measures the impedance response. The result is a Nyquist plot that separates the internal processes: high-frequency behavior reflects ohmic resistance (electrolyte, contacts), mid-frequency arcs reflect charge-transfer resistance at the electrode interfaces, and low-frequency behavior reflects diffusion processes. EIS is the most informative test — it can identify electrolyte loss, SEI layer growth, lithium plating, and connection issues — but it requires lab equipment and skilled interpretation. It is the tool of choice for battery R&D, failure analysis, and quality audits, not for daily field checks.
Test Methods Compared
| Method | What It Measures | Speed | Equipment | Best For |
|---|---|---|---|---|
| Capacity discharge | Ah / energy delivered | Hours | Load + meter | SOH verification, warranty |
| Internal resistance | Voltage drop under pulse | Seconds | Handheld tester | Field health screening |
| EIS | Impedance spectrum | Minutes | Lab impedance analyzer | Failure analysis, R&D |
| Open-circuit voltage | SOC proxy | Instant | Multimeter | Rough SOC check |
How to Interpret Results
- Capacity drop but normal IR: likely calendar aging or undercharge — check usage and charging.
- IR rise with normal capacity: power capability degrading — poor for high-load use, fine for low-load.
- Both degraded: normal aging — plan replacement.
- Sudden IR jump: connection problem, dry-out, or internal damage — inspect and retire.
Field Testing Basics
- Multimeter SOC check: measure open-circuit voltage after rest (no load for 1-2 hours) and compare to the chemistry’s voltage-SOC curve.
- Handheld IR tester: for quick health screening — compare cells in a pack to spot a weak cell.
- Load test: apply a known load and watch voltage sag — a battery that sags hard under load has high IR.
- Capacity verification: only reliable way to confirm capacity claims — do it before accepting a used or warranty battery.
For pack-level decisions, test each cell or module: LiFePO4 packs with a BMS can report per-cell data, making weak-cell identification much easier.
FAQ
How do you test a battery’s capacity?
Fully charge, then discharge at a defined rate to the cutoff voltage and measure the delivered Ah. Compare to the rated capacity; consistent protocol (rate and temperature) is essential for comparable results.
What is internal resistance and why does it matter?
Internal resistance is the battery’s opposition to current flow. It rises as the battery ages, causing voltage sag and heat under load. Measuring it is the quickest health check available.
What is EIS in battery testing?
Electrochemical impedance spectroscopy measures impedance across a frequency range, separating internal processes like electrolyte resistance, charge transfer, and diffusion. It is a lab-grade diagnostic tool.
At what capacity should a battery be replaced?
Generally below 80% of rated capacity for most applications, or when the battery can no longer deliver the required power without excessive voltage sag. Application-specific limits vary.
Can I test battery health with a multimeter?
You can check open-circuit voltage (rough SOC) and do a basic load test, but accurate health assessment needs a capacity test or internal resistance measurement with proper equipment.
Tested & Certified Batteries from Jetray
Sources: Wikipedia – Electrochemical Impedance Spectroscopy; Wikipedia – Ampere Hour.
