Cell Capacity Test Procedure

Battery cell capacity is the fundamental measurement from which state of health, degradation rate, and pack-level energy calculations are derived. It is expressed in ampere-hours (Ah) and represents the total charge a cell can deliver from full to its lower cut-off voltage under defined conditions of temperature and discharge rate.

Capacity is not a fixed property. It varies with C-rate, temperature, and cell age, which is why repeatable measurements require strict control of each variable. A poorly conditioned or thermally unstable cell will produce results that cannot be compared across test cycles or between laboratories.

The procedure below covers the standard electrical sequence for a lithium-ion cell capacity verification test. For the broader standards context, chemistry-specific considerations, and advanced diagnostics, see Cell Capacity Testing: Standards, Diagnostics and Safety.

Test Equipment

Programmable charge/discharge cycler with four-wire Kelvin sensing at the cell terminals
Environmental chamber capable of holding test temperature to within ±1°C
Data logging at a minimum of 1 Hz for voltage, current, and temperature

Standard Electrical Test Sequence

Step 1: Pre-Conditioning

Run 1 to 3 baseline CC-CV cycles at a moderate C-rate (typically C/5 to C/2) before the measurement cycle. This stabilises the solid electrolyte interphase (SEI) layer and removes any residual capacity offset from prior storage or shipping. Cells received from a manufacturer at a partial state of charge should complete a full pre-conditioning sequence before any reference measurement is taken.

Step 2: Environmental Stabilisation

Place the cell in the environmental chamber and allow a full thermal soak at the target test temperature, typically 25°C per IEC 61960-3 or as defined by the test plan. The soak duration must be sufficient to bring the cell core, not just the surface, to a uniform temperature. For a standard 18650 or 21700 cylindrical cell, 2 to 4 hours is generally adequate at 25°C; larger prismatic or pouch cells require longer soaks. Do not proceed until temperature readings from multiple sensor positions have stabilised.

Step 3: Constant Current Charge

Apply a constant current (CC) charge at the rated charge C-rate, typically C/2 or 1C, until the upper voltage limit is reached. The upper voltage limit is cell-chemistry-specific and must be taken from the manufacturer’s datasheet — typical values are in the range of 4.1 to 4.25 V for NMC/NCA, 3.5 to 3.8 V for LFP, and around 2.85 V for LTO, but variation between cell designs is significant.

Step 4: Constant Voltage Charge

Hold at the upper voltage limit with a constant voltage (CV) phase until the charge current decays to the termination threshold, typically C/20 or C/50 depending on the test specification. This CV phase is critical: without it the cell is not fully saturated and the subsequent discharge will underreport true capacity.

Step 5: Relaxation Rest

Rest the cell at open circuit for 20 to 60 minutes. This allows concentration gradients and polarisation overpotentials to decay, returning the cell to electrochemical equilibrium before discharge begins. Skipping or shortening this rest introduces a systematic error in the discharge capacity measurement.

Step 6: Constant Current Discharge

Discharge at the rated C-rate to the lower cut-off voltage. Capacity is calculated by integrating current over time:

C (Ah) = I (A) x t (h)

For higher accuracy, use trapezoidal numerical integration rather than rectangular (left-hand Euler) summation, particularly if current is not perfectly flat during discharge. Record voltage, current, and temperature continuously throughout.

Step 7: Post-Test Rest

Rest the cell at open circuit for a minimum of 30 minutes after discharge. This stabilises the open-circuit voltage (OCV) for any subsequent measurements, such as DC internal resistance or OCV curve mapping.

Pass / Fail Criteria

Under IEC 61960-3, the measured discharge capacity must meet or exceed the manufacturer’s declared nominal capacity. Incoming quality control inspections typically apply a tighter acceptance band, commonly ±2% of nominal, to screen for capacity spread within a cell batch.

Common Sources of Measurement Error

Insufficient thermal soak — core temperature lag causes the discharge to begin at a different temperature than intended, shifting capacity by 1 to 3% per 5°C error for NMC chemistry
CV phase cut-off set too high — terminating the CV phase at C/10 rather than C/20 leaves 1 to 2% of capacity unfilled, producing a repeatable but systematic underread
No relaxation rest — residual polarisation depresses the initial discharge voltage and can cause premature cut-off at the lower voltage limit
Two-wire voltage sensing — lead resistance and contact resistance corrupt terminal voltage measurement; always use four-wire Kelvin connections at the cell terminals