Cell Measurement Parameters

An A to Z list of measurement parameters.

ACIR

As the name suggests, ACIR means Alternating Current Internal Resistance. An alternating current of 100mA 1000 Hz is applied to the cell via ACIR measurement equipment. The working principle of this equipment is; applying the I_ac and then measuring V_ac. Then

Impedance Z = V_ac / I_ac

When measuring the impedance, there will be a phase shift. So, only the Real part is considered to measure the internal resistance ignoring the reactance part. ACIR gives R_int only and normally defined as a single value at 1kHz.

ACIR = Re (V_ac / I_ac)

Appearance

A visual inspection of the cell, any surface deformation, discolouration or deposits. This should be noted on receipt of the cell as well as before and after every test.

Calendar Ageing

Here the cells are held at a fixed temperature and state of charge (SoC). Then every month the cells are removed from the fixed temperature storage and moved to a standard 25°C facility where the capacity and internal resistance are tested. It should be noted that calendar ageing increases with temperature and SoC, hence 90% SoC and 50°C is going to age faster than a cell held at 30% SoC and 15°C. This means that you are going to bias the testing towards the higher temperatures and SoC values. See: Developing a Lifecycle Test Procedure.

Capacity

The amount of energy available from a battery. Battery capacity is expressed in ampere-hours.

The battery is soaked at a nominal temperature and then charged at a defined rate until it reaches the maximum cell voltage. The cell is then discharged at a defined C-rate until it reaches a lower defined cell voltage. The current versus time (coulomb counting) is then used to establish the Ah capacity of the cell or pack.

Charge Rate

Charge rate is expressed as a C-rate, where 1C charges a cell in one hour. A 2C charge rate halves that time; a C/2 rate doubles it. Characterising the cell across a range of charge rates is essential for determining the upper current limits used in BMS charge tables and for assessing fast-charge capability.

Charge rate testing is typically conducted at a reference temperature (25°C) and repeated at low and high temperature extremes to capture the full operating envelope. At low temperatures, lithium plating risk on the anode limits the acceptable charge rate significantly, making temperature-dependent charge rate data a safety-critical input.

Typical range tested: C/10 (reference), C/5, C/2, 1C, 2C, 3C, 5C (cell-dependent)
Output: accepted charge (Ah), charge time, temperature rise, voltage profile during CC and CV phases
See also: Fast Charge; DCIR

Coulombic Efficiency

Coulombic Efficiency (CE) is the ratio of charge delivered on discharge to the charge consumed on the preceding charge cycle:

CE = Q₀ᵣᵈ / Qᶜʰᵃʳᵍᵉ

For a cell at room temperature, CE should be very close to 1.0 (typically 99.9%+ for a healthy NMC or LFP cell). Small deviations signal parasitic side reactions consuming lithium, and are therefore an early indicator of ageing rate and cycle life.

High Precision Coulometry (HPC) and Ultra High Precision Coulometry (UHPC) can resolve CE to the fourth decimal place, enabling detection of lithium plating, electrolyte decomposition, and SEI growth well before capacity fade becomes visible in standard discharge tests. CE data from the first 50 cycles is increasingly used as a predictive incoming quality screen.

Typical incoming QA threshold: CE > 99.95% by cycle 5 at 25°C (chemistry-dependent)
Low CE early in life strongly predicts short cycle life
See also: Calendar Ageing; Self Discharge Rate

Cut-off Voltages

The upper and lower cut-off voltages define the safe operating voltage range for a cell. Exceeding either limit risks irreversible damage or a safety event.

Upper Cut-off Voltage (V_max)

The maximum terminal voltage at which charging is terminated. Exceeding Vₘₐˣ risks cathode structural breakdown, electrolyte oxidation, and lithium plating at the anode. Typical values: NMC 4.2 V to 4.35 V; LFP 3.65 V; LCO 4.2 V.

Lower Cut-off Voltage (V_min)

The minimum terminal voltage at which discharge is halted. Deep discharge below Vₘᵢⁿ risks copper dissolution from the anode current collector, which can redeposit as metallic dendrites and cause internal short circuits on subsequent charging. Typical values: NMC 2.5 V–3.0 V; LFP 2.5 V.

Both cut-off voltages are temperature-dependent and are mapped as part of the BMS safe operating area definition. They are directly embedded in the BMS charge and discharge control logic.

See also: Nominal Voltage; Battery Management System

Cycle Ageing

Cycle ageing tests quantify how cell capacity and internal resistance degrade as a function of cumulative charge and discharge cycles. Cells are cycled between defined voltage limits at a reference C-rate and temperature, with periodic Reference Performance Tests (RPTs) inserted to measure capacity, resistance, and OCV without interrupting the ageing programme.

The test matrix covers combinations of temperature, Depth of Discharge (DoD), charge rate, and discharge rate to build a multi-stress degradation model. End-of-life is typically defined at 80% of initial rated capacity, though application-specific thresholds apply (e.g. 70% for some stationary storage applications).

See: Developing a Lifecycle Test Procedure

DCIR

Hybrid Pulse Power Characterization (HPPC)

Battery test used to determine the dynamic performance characteristics of a battery, in particular the DC Internal Resistance of the cell. The battery is pulse discharged typically at 1C for 10s. The voltage and current profile is then used to determine the internal resistance of the cell. These measurements are repeated for charge and over different temperatures, States of Charge, different pulse lengths and at different rates. In this way a complete map of the internal resistance of the cell can be created and using this a power map versus SoC and temperature.

Dimensions

Dimensional characterisation of a cell covers the physical size measurements required for module and pack design, tolerance stacking analysis, and incoming quality inspection. Measurements are taken on the cell at a defined state of charge (typically 50% SoC) and temperature (25°C), as cell dimensions vary with both SoC and temperature due to electrode expansion and contraction.

Cylindrical Cells

Diameter: measured at mid-height; tolerance typically ±0.1 mm on nominal (e.g. 21.0 mm for a 21700)
Height (length): measured under a defined axial load to account for cap spring compliance
Terminal recess / protrusion depth: affects contact pressure in spring-loaded holders

Prismatic Cells (Hard Case)

Height, width, and thickness at defined SoC
Terminal position and pitch: critical for busbar design
Terminal thread size and torque specification

Pouch Cells

Length, width, and nominal thickness at defined SoC and under defined stack pressure
Tab dimensions, position, and material (Cu anode / Al cathode)
Thickness growth vs. SoC and cycle number: required for module gap analysis (see Swelling / Thickness)

See also: Cell Formats; Module Design; Swelling / Thickness

Discharge Rate

Discharge rate is expressed as a C-rate and defines how quickly a cell is discharged relative to its nominal capacity. At 1C the cell delivers its nominal capacity in one hour; at 2C in 30 minutes; at C/5 in five hours.

Rate capability testing characterises how capacity and efficiency change across the full discharge rate range the cell may experience in service. At high discharge rates, internal resistance losses increase, the usable capacity reduces, and heat generation rises. This data feeds directly into the power map and thermal model used in pack design.

Standard reference discharge: C/5 or C/10 (full capacity baseline)
Pulse discharge: used in HPPC for DCIR measurement at each SoC point
Continuous high-rate discharge: 2C, 5C, 10C as applicable; run to Vₘᵢⁿ or until temperature limit reached
Output: capacity (Ah) and energy (Wh) at each rate; voltage profile; temperature rise; efficiency (Wh out / Wh in)

EIS (Electrochemical Impedance Spectroscopy)

Electrochemical Impedance Spectroscopy applies a small sinusoidal AC perturbation (typically 5–10 mV amplitude) across a wide frequency range (typically 10 mHz to 100 kHz) and measures the resulting current response. The ratio of voltage to current at each frequency gives the complex impedance Z(ω).

The results are typically displayed as a Nyquist plot (imaginary vs. real part of impedance) or as Bode plots (magnitude and phase vs. frequency). From the Nyquist plot, an Equivalent Circuit Model (ECM) can be fitted to extract individual contributions:

R₀ (ohmic / series resistance): x-axis intercept at high frequency; dominated by electrolyte and contact resistance
Rᴬᴸ (SEI resistance): first semicircle; reflects the Solid Electrolyte Interphase on the anode
Rᶜᵅ (charge-transfer resistance): second semicircle; reflects electrode kinetics at the electrode/electrolyte interface
W (Warburg element): low-frequency tail; reflects solid-state lithium-ion diffusion within the electrode particles

EIS is non-destructive and can be run at different temperatures and SoC levels to build a full parametric model. It is the primary technique for parameterising dynamic cell models used in BMS SoC and SoP estimation, and it provides diagnostic sensitivity to ageing mechanisms that are invisible to DC measurements.

See: Electrochemical Impedance Spectroscopy. See also: DCIR / HPPC; ACIR

Entropic Heat Coefficient (dOCV/dT)

The entropic heat coefficient, also written as dU/dT or ∂OCV/∂T, describes how the equilibrium cell voltage varies with temperature at a fixed state of charge. It is a thermodynamic property arising from the entropy change of the lithium intercalation reaction.

The coefficient can be positive or negative depending on the electrode material and SoC, meaning the reversible heat contribution can be either endothermic (absorbs heat) or exothermic (releases heat) during charge or discharge. In LFP cells particularly, reversible entropic heat can be of comparable magnitude to Joule heating at moderate C-rates, making it a significant term in the cell heat generation equation:

Qᵈᴫᶜ = I²R (irreversible) + T × I × dOCV/dT (reversible)

Measuring this coefficient requires OCV to be measured at a fixed SoC across a range of temperatures (e.g. 5°C to 45°C in 5°C steps) after full thermal equilibration. The data is an essential input to accurate cell-level thermal models and calorimetry interpretation.

Fast Charge

At pack level this is typically defined as the time (minutes) to charge from 10% to 80% State of Charge (SOC). In cells it is often easier to show SOC vs time for different charge rates.

Formation Capacity

Formation is the first controlled charge applied to a freshly assembled cell. During formation, the electrolyte reacts with the anode surface to form the Solid Electrolyte Interphase (SEI), an electronically insulating but ionically conductive layer that is essential for long-term cell stability.

Formation capacity is the capacity measured during and after the formation protocol, typically a slow C/10 or C/20 first charge followed by a C/5 reference discharge. This measurement serves two purposes:

Quality confirmation: the discharge capacity should match the cell specification within tolerance; significant deviation indicates a formation or assembly defect
Incoming QA baseline: the capacity at formation is the reference from which all subsequent ageing-related fade is measured

The irreversible capacity loss on the first cycle (First Cycle Loss, FCL) is the capacity consumed in forming the SEI and is not recovered. FCL typically ranges from 5–15% of initial charge capacity depending on chemistry, anode material (especially silicon-containing anodes, where FCL is higher), and formation protocol.

See also: Coulombic Efficiency, Cycle Ageing, Cell Capacity Testing

Leakage Current

Leakage current (also termed self-discharge current) is the small, continuous current that flows from a fully charged cell at open circuit due to parasitic electrochemical reactions. It is the electrical equivalent of the self-discharge rate, but expressed in current units (microamperes) rather than capacity loss per month.

Leakage current is measured by high-precision current sensing or derived from OCV decay converted through the OCV-SoC curve. A sudden increase in leakage current during storage or ageing can indicate the onset of an internal short circuit and is one of the parameters monitored in safety screening protocols.

Typical values: 1–10 µA for consumer cells; higher for larger-format automotive cells proportional to electrode area
Safety relevance: a leakage current significantly above baseline is a trigger for cell rejection or investigation

See also: Self Discharge Rate, Relaxation delta Voltage, Safety Diagnostics

Nominal Capacity

The nominal capacity of a cell is the capacity (Ah) measured at a standard temperature and charge / discharge rate. The discharge capacity test is preceded by a standard charge cycle and ends at a given terminal voltage. The charge capacity is preceded by a standard discharge cycle and ends in Constant Voltage (CV) mode when the current drops to that defined by the standard charge cycle.

Nominal Energy

The nominal energy of a cell is the measured charge / discharge electrical energy at a given c-rate and temperature. The cycles are specified as the standard charge discharge cycles and hence end at a given terminal voltage during discharge and at a end charge current during the constant voltage charge cycle.

Nominal Voltage

The nominal voltage of a cell is specified at a given charge / discharge rate and temperature. It is calculated from the measured Nominal Capacity and Nominal Energy. Nominal Voltage equals Nominal Energy divided by Nominal Capacity.

Pressure

Internal gas pressure and external mechanical pressure are both important measurement parameters at cell level, for different reasons.

Internal Gas Pressure

Gas is generated inside a cell during abuse conditions (overcharge, over-temperature) and to a lesser extent during normal cycling through electrolyte decomposition. In pouch cells this gas causes visible swelling; in cylindrical and prismatic hard-case cells it raises internal pressure until the vent mechanism activates.

Measurement: pressure sensor integrated into a sealed test fixture; cell pierced under controlled conditions
Application: vent activation pressure characterisation; thermal runaway gas generation quantification

External Stack Pressure

Pouch and prismatic cells require controlled compressive pressure to maintain electrode-electrolyte contact and prevent delamination. The cell expands and contracts with SoC and temperature, and irreversibly expands over its lifetime due to lithium plating, gas generation, and electrode particle cracking.

See: Mapping Pouch Cell Pressure Distribution.

See also: Swelling / Thickness, Module Design

Open Circuit Voltage (OCV)

The Open Circuit Voltage (OCV) is a fundamental parameter of the cell. The OCV of a battery cell is the potential difference between the positive and negative terminals when no current flows and the cell is at rest.

Measurement of OCV

The measurement of Open Circuit Voltage can be time consuming as it needs to be established versus the State of Charge (SoC) of the cell. Doing this in fine steps from 0% to 100%, with the measurement being made “at rest”, means we have to charge/discharge and then let it rest before making the voltage measurement.

Power

The power a cell can charge and discharge is measured versus time, SOC, temperature and State of Health (SOH).

Relaxation delta Voltage

A safety diagnostic that can be used to find cell internal short circuits.

Self Discharge Rate

The capacity loss per month given a percentage of the nominal capacity. Typically this is declared at room temperature.

Short Circuit Current

The peak current produced when a cell is externally short-circuited through a very low resistance (typically ≤5 mΩ per test standards). The short circuit current is primarily limited by the cell internal resistance at the moment of the event, so it is directly related to DCIR. For a high-power cell with very low internal resistance this peak current can reach several hundred amperes.

Short circuit current measurement is conducted as part of abuse testing and legislative qualification. It characterises the thermal and electrical load placed on the cell during a fault condition, informing fuse and contactor sizing as well as cell-level safety response.

Standard: UN 38.3 T.5, IEC 62133, UL 2054
Outputs: peak current (A); temperature rise; cell voltage recovery after event; pass/fail on fire/explosion criteria

SOH (State of Health) Reference Measurement

State of Health (SOH) quantifies the degree to which a cell has aged relative to its as-new condition. At cell level, SOH is not a single measured quantity but a definition that must be agreed before testing begins, because different applications weight capacity fade and resistance growth differently.

The two primary SOH definitions used in cell testing are:

Capacity-based SOH: SOHᶜ = Qₘᵉₐˢᵘʳʳʳ / Qⁿᵒᵐᵉⁿᵉᴸ × 100%. Most common definition; end-of-life at SOH = 80% for most automotive applications.
Resistance-based SOH: SOHᴬ = Rⁿᵒᵐᵉⁿᵉᴸ / Rₘᵉₐˢᵘʳʳʳ × 100%. Used where power fade (rather than energy fade) is the critical end-of-life criterion.

The reference capacity and resistance measured at the start of life (or at the beginning of ageing testing) are therefore critical baseline measurements that all subsequent SOH calculations depend upon. These must be measured under tightly controlled, standardised conditions to ensure they are reproducible.

Specific Heat Capacity

You can initially use a generic specific heat capacity for a cell to do some initial thermal modelling. However, the heat capacity of a cell varies enough with chemistry and design that it is worth measuring it for the specific cell you are using. This will allow you to develop more accurate thermal models of the cell.

Measuring Specific Heat Capacity

Sample Container: At the heart of the chamber is a sample container where the material under investigation is placed.
Temperature Sensors: The chamber is equipped with fast response thermocouples to monitor the temperature of the sample continuously.
Heating System: A resistive heating system surrounds the chamber, capable of precisely controlling the temperature of the sample according to the experimental protocol.
Insulation: The entire assembly is heavily insulated to ensure adiabatic conditions, meaning that once the external heating is stopped, no heat should enter or leave the system.

Swelling / Thickness Change

All lithium-ion cells expand and contract as lithium ions intercalate into and de-intercalate from the electrode materials during charge and discharge. This volume change is reversible on a cycle-by-cycle basis but an additional irreversible thickness growth accumulates over the cell lifetime due to SEI growth, gas generation, and mechanical degradation of the electrode particles.

Swelling measurement is particularly important for pouch and prismatic cells, where it directly determines:

Module gap allowance: the module design must accommodate the maximum lifetime thickness including both reversible SoC-dependent swelling and irreversible ageing-related growth
Contact pressure maintenance: a module compression system designed for a new cell may apply insufficient or excessive pressure at end-of-life if swelling is not correctly characterised
Early fault detection: accelerated or non-uniform swelling can indicate lithium plating, gas generation, or separator damage

Measurement Methods

Dilatometry: a linear displacement sensor (LVDT or laser) monitors cell surface displacement under a defined constant load throughout cycling
Optical / laser profilometry: full surface scan of a pouch cell to capture thickness distribution, not just centre-point thickness
In-operando pressure mapping: captures the combined effect of swelling and load distribution (see Pressure above)

See also: Mapping Pouch Cell Pressure Distribution; Module Design; Pressure [above]

Thermal Conductivity

Thermal conductivity (k, W·m⁻¹·K⁻¹) describes how readily heat flows through the cell body. Because the internal structure of a lithium-ion cell is a highly anisotropic laminate of thin metallic foils, active material coatings, and separator, the thermal conductivity differs significantly in the two principal directions:

In-plane conductivity (parallel to electrode layers): 20–40 W·m⁻¹·K⁻¹ — dominated by the high-conductivity copper and aluminium current collector foils
Through-plane conductivity (perpendicular to electrode layers, i.e. the direction heat must travel to reach the cell surface): 0.3–1.0 W·m⁻¹·K⁻¹ — dominated by the low-conductivity active material coatings, electrolyte-saturated separator, and polymer binder

The low through-plane conductivity is a fundamental constraint in cell thermal management: heat generated at the core of a large-format cell must travel through a poor thermal conductor to reach the cooling surface. This drives the preference for surface cooling of pouch and prismatic cells, and for tab or end-cap cooling in large cylindrical formats.

Measurement is typically by the laser flash method for through-plane conductivity and the hot-disk or steady-state heat flow method for in-plane conductivity. Both measurements should be made at multiple temperatures to capture the temperature dependence.

See also: Specific Heat Capacity, Heat Generation in a Cell, Battery Cooling Options, Thermal Conductivity of the Active Layers

Vent Activation Pressure

Cylindrical and prismatic hard-case cells incorporate a pressure relief vent that opens at a defined internal pressure to prevent catastrophic case rupture during abuse events. The vent activation pressure is a safety-critical parameter that defines the threshold between controlled venting and uncontrolled case failure.

Vent activation is characterised by pressurising a cell (typically through gas injection at a controlled, safe state of charge) and measuring the pressure at which the vent opens. The pressure at which the vent re-seals (if it does) is also of interest for scenarios where the source of gas generation is temporary.

Typical values: 0.5–2.0 MPa for 18650/21700 cylindrical cells; varies significantly by manufacturer and cell format
Design implication: the vent activation pressure defines the maximum internal pressure the cell case is designed to withstand; module and pack venting pathways must be sized to manage the gas flow rate at vent activation

See also: Thermal Runaway, Cell Formats, Pressure

Voltage Relaxation Profile

After a current pulse or the end of a charge or discharge, the cell terminal voltage does not immediately settle to its equilibrium OCV. Instead it relaxes exponentially (or as a sum of exponentials) as concentration gradients within the electrode particles and electrolyte dissipate. This transient behaviour is the voltage relaxation profile.

The shape and time constants of the relaxation profile are diagnostic of cell impedance and are used to:

Fit Equivalent Circuit Model (ECM) parameters: RC pair time constants (τ = RC) extracted from relaxation data match EIS-derived values and feed the dynamic voltage model used by the BMS
Determine adequate rest time for OCV measurement: the cell must be rested long enough for the relaxation to complete before the terminal voltage can be taken as a true OCV
Distinguish between fast (charge-transfer) and slow (diffusion) processes: the fast component settles in seconds to minutes; the slow diffusion tail can take hours in large-format cells

Voltage relaxation is distinct from the Relaxation Delta Voltage (RdV) safety diagnostic, which specifically monitors the long-term OCV drift caused by internal micro-short circuits. See: Relaxation delta Voltage.

See also: OCV Measurement, DCIR / HPPC, EIS

Weight

Cell mass is measured on a calibrated balance at a defined state of charge and temperature; unlike dimensions, weight does not vary meaningfully with SoC, so it is typically a one-off measurement rather than a repeated one across the test matrix. It is the denominator for gravimetric energy density (Wh/kg) and gravimetric power density (W/kg), and is checked on receipt as a basic incoming QA screen — a cell outside its specified mass tolerance can point to an electrolyte fill or assembly defect.

See also: Key Cell Metrics