This report is a compilation of test data and results from projects conducted by the Fire Safety Branch designed to determine the hazard from and possible hazard mitigation for the bulk shipment of lithium batteries/cells as cargo on transport airplanes. Though the main focus of this report is transport on passenger aircraft (effectiveness of Halon and effect on Class C cargo compartment), general hazard data is also applicable to freighters.

Initial testing found that current packaging did little to impede thermal runaway propagation from cell to cell and package to package. Lowering the state of charge (SOC) was shown to reduce the hazard. A 30% SOC in some cell chemistries stopped cell-to-cell propagation but did not eliminate the hazard from an external fire.

Full-scale tests in a 727 aircraft demonstrated that, even in the presence of Halon 1301, cell-to-cell propagation occurred. The tests also demonstrated that the pressure and off-gassing from the cells could reduce the Halon concentration at an accelerated rate, thereby causing the loss of fire protection earlier then designed.

Recent testing showed that lithium-ion cells in thermal runaway emitted H2 gas that can be explosive even in a 5% Halon atmosphere. The ignition of the gases vented by a small number of cells in thermal runaway (the number depends on many factors, such as chemistry, design, and SOC of the cells, and size, design, and load factor of the compartment) can cause enough damage to a Class C cargo compartment as to render the Halon suppression system ineffective.

Materials for aircraft cabin interiors must meet the flammability requirements of Title 14 Code of Federal Regulations (CFR) Part 25.853. The 14 CFR 25.853 requirement includes a test for large-area materials that measures the heat release rate (HRR) during burning using a fire calorimeter originally developed at Ohio State University (OSU). In the standard 14 CFR 25 procedure, a sample is inserted into the combustion chamber of the OSU apparatus and subjected to a calibrated radiant heat flux of 35 kW/m2 and an impinging pilot flame. Room temperature air is forced through the combustion chamber and exits through the exhaust duct at the top of the apparatus where a thermopile senses the temperature of the exhaust gases. The HRR during the test is deduced from the sensible enthalpy rise of the air flowing through the combustion chamber using the temperature difference between the exhaust gases and the ambient incoming air to calculate the amount of heat released by burning after suitable calibration using a metered methane diffusion flame. Limits of 65 kW/m2 and 65 kW/m2-min for the maximum (peak HRR) and the total heat release (HR) up to 2 minutes into the test (2-min HR), respectively, are placed on large-area materials used in passenger cabins of transport category airplanes carrying more than 19 passengers.

Results from multi-laboratory studies of the same materials tested for HR and HRR according to 14 CFR 25.853 indicated that the laboratory-to-laboratory variation of the test results was relatively high. There are many factors that can contribute to poor agreement between OSU fire calorimeter results obtained in different laboratories (i.e., reproducibility), including the accuracy of the heat flux calibration, contaminated temperature sensors, thermal inertia of the apparatus and its components, and changes in the convective environment in the combustion and bypass chambers caused by airflow and airflow distribution. This study focused on the effect of the airflow through the 14 CFR 25.853 fire calorimeter on the repeatability and reproducibility of the HR and HRR by the sensible enthalpy (temperature rise) of the standard method, compared to HR and HRR measured simultaneously by the oxygen consumption method.

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).