The validated solution to a two-term heat transfer model of a bomb calorimeter allows direct calculation of the heat released in an arbitrary process from the recorded temperature history without the need to correct for non-adiabatic behavior. The heat transfer coefficients and thermal capacities of the bomb calorimeter used in the heat calculation are determined parametrically from the temperature response to a known heat impulse (i.e., benzoic acid combustion). This methodology allows accurate measurement of heat released intermittently or during an extended period of time in a bomb calorimeter, as occurs during electrical resistance heating and subsequent thermal runaway of lithium ion batteries.

The energy released by failure of rechargeable 18-mm diameter by 65-mm long cylindrical (18650) lithium-ion cells/batteries was measured in a bomb calorimeter for four different commercial cathode chemistries over the full range of charge using a method developed for this purpose. Thermal runaway was induced by electrical resistance (Joule) heating of the cell in the nitrogen-filled pressure vessel (bomb) to preclude combustion. The total energy released by cell failure, ΔHf, was assumed to be comprised of the stored electrical energy E (cell potential x charge) and the energies of mixing, chemical reaction, and thermal decomposition of the cell components, ΔUrxn. The contribution of E and ΔUrxn to ΔHf was determined, and the mass of volatile, combustible thermal decomposition products was measured in an effort to characterize the fire safety hazard of rechargeable lithium-ion cells.

The high energy density of lithium-ion batteries (LIB) makes safe shipment as cargo on commercial aircraft a concern because of the potential for initiating or accelerating a fire. LIB failure caused by overheating, mechanical damage, or manufacturing defects results in rapid thermal energy release (thermal runaway), ejection of the cell contents, and the possibility of conflagration, burning, or explosion of the volatile organic electrolytes. Full-scale cargo fire tests at the Federal Aviation Administration have shown that these risks can be mitigated when LIBs are shipped at reduced electrical capacity (state-of-charge [SOC]). To quantify the safety benefit of shipping at reduced SOC, experiments were conducted using a bomb calorimeter to determine the relationship between the SOC of the LIB; its cell potential (volts) and electrical capacity (Coulombs); and the release of stored chemical energy during failure. Commercial LIBs in the form of single cylinders 18 mm in diameter and 65 mm long (18650 cells) were forced into failure in the bomb calorimeter using electrical resistance heating in a nitrogen atmosphere to preclude oxidation of the cell components. Data were collected for the release of stored energy as a function of electrical capacity and cell potential, and the composition of the combustible gases released during failure was determined by infrared spectroscopy. These studies showed that the stored electrochemical energy, which is the product of the actual charge and cell potential, is a better predictor of the energy release at failure than the fraction of the maximum rated charge capacity (SOC).

The fire hazard of lithium batteries carried on airplanes in passenger electronics and shipped as cargo includes the energy generated by individual cell failure, subsequent self-heating (thermal runaway), and the energy released by burning or explosion of the volatile cell components. In this study, a variety of non-rechargeable lithium metal (primary) cells and rechargeable lithium-ion (secondary) cells were heated to failure using radiant energy in a fire (cone) calorimeter and electrical resistance heating in a thermal capacitance (slug) calorimeter. In the fire calorimeter, hazard parameters were measured for several different cell chemistries over a range of electrical charge (states of charge [SOC]) and radiant heat flux, including the loss of cell mass at failure, the maximum rate of heat released in flaming combustion, the total heat released by combustion of the volatiles, and the specific heat of combustion of the cell contents. In the thermal capacitance calorimeter, the chemical energy released as heat and the cell temperature were measured as a function of the SOC and the heating rate. This report documents the results of these tests; namely, combustion energy measured for several lithium primary and secondary cells and the chemical energy released as heat during failure of a lithium-ion secondary cell. The results can be used to quantify the thermal hazard and the combustion hazard of individual electrochemical cells and cell assemblies (batteries).

One goal of this analysis is to characterize the stratification and localization of Halon 1211 in aircraft compartments. A second goal is to provide a methodology to determine stratification and localization multiplication factors that can be applied to the safe-use halocarbon concentrations in Advisory Circular (AC) 20-42D to allow the safe use of higher concentrations than currently recommended. The current safe-use concentrations are based on pharmacokinetic-based assessments of gaseous halocarbon concentration decay histories in a ventilated compartment with perfect mixing and instantaneous agent discharge. The AC 20-42D refers to “an upcoming report” (this report) to provide guidance for setting safe halocarbon limits with consideration of stratification and localization. Separate analyses and guidance is provided for a B-737 aircraft and an unpressurized general aviation aircraft. General guidance is provided for application to non-test aircraft.