Adiabatic Heat-Wait-Search Testing

Adiabatic Heat-Wait-Search (HWS) testing is a screening method used in Accelerating Rate Calorimetry (ARC) to test the thermal stability of batteries. HWS finds the temperature at which the cell starts to heat itself and then follows that self-heating under adiabatic conditions. The calorimeter “heats” to raise the temperature in small steps, “waits” for thermal equilibrium, and “searches” for a tiny self-heating rate. As soon as the measured self-heating crosses a sensitivity threshold, the instrument switches to adiabatic mode so the cell’s own exotherm drives the temperature rise. This is the standard way to pinpoint thermal-runaway onset temperature, measure how fast the event accelerates, and measure how the voltage and pressure change.

Battery Charging

The cell is charged using the recommended protocol, typically at constant-current/constant-voltage (CC-CV) or constant-power (CP). In CC-CV, current is held until the voltage limit is reached, then voltage is held while current tapers to a cutoff. In CP, current is adjusted so electrical power stays constant. During charging, voltage, current, and power are recorded and plotted versus time.

Open vs Closed Test

Two basic setups are used. An open test vents to a hood or blast enclosure and is chosen when temperature/self-heating behavior is the main interest. A closed test seals the sample in an armored chamber with a pressure transducer. Closed tests record absolute pressure and pressure-rise rate and allow gas collection for later GC-MS or FTIR gas analysis.

Measurements & Analysis

• Start temperature: the temperature where the search begins (often 40–50 °C).
• Heat-step size: how much the setpoint jumps each loop before checking for self-heating (often 5 °C per step). Wait time: the pause after a step so the system settles before measuring (often 20–40 minutes).
• Seek/search time: the short window used to detect a real self-heating slope during each step (often 5–10 minutes).
• Slope-sensitivity threshold: the smallest self-heating rate used to switch into adiabatic follow (commonly 0.02 or 0.03 °C/min).
• Onset temperature: the temperature at which the sample first shows that self-heating rate (the HWS “found it” point).
• Maximum self-heat rate and temperature: the peak dT/dt observed and where it happened.
• Measured vs. adiabatic temperature rise: the actual temperature increase during the event and the rise corrected for thermal inertia to estimate the true adiabatic rise.
• Phi (Φ) or thermal inertia: a correction factor that accounts for extra heat capacity from the test hardware; used to convert temperature rise into released energy.
• The cell Cp together with the thermal-inertia (phi) factor turns the measured temperature rise ΔT run into total released energy.
• Cp can be measured directly with the ARC specific-heat option for accuracy or assumed from literature.
• Pressure trace and rise rate: absolute pressure and how fast it increased, if a transducer was fitted.
• Voltage logging, multiple thermocouples, and gas sampling, which line up the electrical, thermal, and gas data in time.
• RDCS (Rapid Data Collection System) log data at a much higher rate (10kHz vs. 1kHz standard) is also available (optional) to accurately capture the transient effects of harsher, rapid pressure and temperature spikes.

Deliverables

Deliverables include in addition to the PDF report, an Excel file with charts of T vs t, P vs t, V vs t, V vs T, dT/dt vs T, dP/dt vs T, P vs T, and a summary of the test setup and results. Pre- and post-test photographs of the cell and fixtures, notes on visible deposits or case changes are also included.

Instruments

A differential scanning calorimeter (DSC) and thermogravimetric analyzer (TGA) are excellent instruments to quantify impurities. They also measure the heat release and thermal degradation of battery raw and electrode materials.

In an Accelerating Rate Calorimeter (ARC^®), the normal cycling of battery discharging and charging can be simulated and exothermic runaway reactions can be quantified. In an ARC^®, the temperature and pressure response to heat is recorded and the following properties can be measured:

We Can Help

The professionals at ioKinetic will work with you to determine what is required for your particular needs. Request your free quote today or call us at 1-844-ioKinetic.