Fire Safety of Li-ion Battery Packs for Aviation

By: Ulises Rojas-Alva¹, Matija Uršič², Grunde Jomaas¹

¹Department for Fire-Safe Sustainable Built Environment (FRISSBE), ZAG, Slovenia

²Department of Building Physics, ZAG, Slovenia

Abstract

Lithium-ion battery (LIB) technology is being widely used in many sectors that range from transportation to the built environment. In aviation, there have been developments for using LIB technology for the propulsion of commercial-leisure sailplanes. If not designed carefully, LIB solutions for such planes can pose a safety hazard as a cell can go into thermal runaway (due to a multitude of reasons), which could potentially endanger the life of the plane’s occupant(s). While the safety requirements for large commercial aircraft that use traditional fossil fuels are strictly regulated, the safety requirements are less strictly defined for lighter passenger aircraft with many boutique or custom solutions. The burden of safety regulation is borne by the European and US aviation agencies, EASA and FAA, which carry out case-by-case oversight for such products. One such product is the FES electric propulsion system from LZ Design in Slovenia. Following EASA requirements, several fire tests were conducted with a new product, namely a battery pack based on type 18650 Li-ion cells, that is used for the propulsion system in powered sailplanes. The thermal runaway of the cell was intentionally triggered, and the thermal behaviour of neighbouring cells was monitored. The successful tests, where the temperature of the neighbouring cells was below 100 °C at an EASA-defined test duration of 15 minutes after the thermal runaway of the target cell, are the basis for the certification of the new battery pack for use in powered sailplanes.

Keywords: lithium-ion batteries, aviation, fire safety, thermal runaway,

Introduction

The use of lithium-ion batteries (LIB) in aviation has been debated for a long time [1]. In both commercial and private jets, weight optimisation is very important, but there is also a push for reducing the use of fossil fuels. As conventional lead batteries have a significantly lower energy density compared to lithium batteries, the latter is the best candidate to be used [2]. The problem arises with safety, as most types of LIBs are potentially quite dangerous due to the large amount of stored energy and the potential for thermal runaway [3]. Although the possibility of a thermal runaway due to mechanical failure or internal short-circuiting is relatively low, the consequences can be serious, as energetic fires or even explosions may occur. The previous fire incidents with LIB applications on planes have resulted in increasingly stricter safety requirements by the European EASA (European Aviation Safety Agency) and the US FAA (US Federal Aviation Administration) [4].

The use of LIB in ultralight and powered sailplanes is nothing new. The use of lithium batteries to start the engine in light aircraft (e.g., Cessna) commenced shortly after they became commercially available. Stefan Gehrmann and his company flew the first ultra-light sailplane with an electric motor powered by LIB [5]. The LIB technology used was mostly based on commercial pouch lithium cells [6]. One of the advantages of this type of cell is its flat form and thus very efficient use of space, which is of great importance to the aviation industry.

The FAA and EASA approve the use of these battery packs on a case-by-case basis [7]. The problem with "pouch" cells is mainly their non-standardisation, since the specifications for the battery cells are completely dependent on the manufacturer, and over the years the accessibility and specifications of these products were subject to change. Battery pack providers have been slowly turning to more standard cells. The most prevalent design is the 18650 cylindrical cell (18 mm diameter and a height of 65 mm) [8], which has been used for battery packs for laptops, power tools, electric bikes, scooters and also large capacities such as the Tesla Model S [9]. The cylindrical design limits space utilization to a maximum of 90%. Thus, those LIB cells allow for the extremely wide availability of different specifications and the rapid application of improvements to commercial products.

LZ Design has been the manufacturer of the innovative electric propulsion system, Front Electric Sustainer (FES), for powered sailplanes since 2009. More than 350 sailplanes from various European manufacturers are already equipped with their LIB propulsion system. The core of the FES system is a powerful electric motor mounted in the nose of a sailplane with a folding propeller that fits the shape of the fuselage during soaring. Figure 1 shows a photo of a typical sailplane and of the battery pack location just behind the cockpit. The electric motor is powered by a LIB battery pack. This type of propulsion has become very popular over the years, as it offers many advantages in terms of classic pull-out drives with petrol two-stroke engines. In a bid to expand the supply, LZ Design developed a new battery pack based on the 18650-type of Li-ion battery cells. With the widespread use of lithium batteries over the past decade, safety conditions for auxiliary-powered sailplanes have also become more evident. The European EASA has taken over and slightly adapted the US standard RTCA DO-311A for sailplanes, which refers to Li-ion batteries built into passenger aeroplanes [10]. The main safety requirement for battery packs is resistance to the thermal runaway of one battery cell. This means that the catastrophic failure of one battery cell (thermal runaway) must not lead to a chain reaction involving other cells of the battery pack within a specified time frame. The purpose of this requirement is to enable the safe landing of the aircraft and to protect against loss of life. The EASA requirement for auxiliary power-driven sailplanes is a minimum of 15 minutes of resistance to individual cell failure. The standard test must be carried out in three places, with cells diagonally on two corners and with the centre cell of the battery pack. Testing shall be carried out under the conditions of a reasonable worst-case scenario.

The following will present the findings from three tests carried out with battery packs composed of 18650-type batteries. The goal is to show a transparent test process and share information and test data from these forced thermal runaway tests. It is encouraged that others do the same, so that battery-powered vehicles can be made safer through shared experiences and design details.

Figure 1 – Sailplanes in flight (left, photocredit: Luka Hojnik) and battery pack location behind cockpit (right).

Experimental method

The battery pack for a one-passenger sailplane consists of 280 (or 320) uni ts of 18650 type cells while the one for two-passengers sailplanes consists of 420 or 480 units. The version tested was the 16S 30P with 480 type 18650 cells arranged in two parallel layers of 240 cells each. There are two such packs in a two-passenger sailplane. Several features were introduced to improve the design of the battery pack to improve the heat dissipation of a LIB cell during thermal runaway (venting path and side venting, using insulating materials such as mica paper and silicone).

During the tests, the battery pack was placed in a testing enclosure with a top exhaust vent (to evacuate the fumes during thermal runaway), and a water valve to be used in case the neighbouring cells were compromised, as shown in Figure 2. Following the test protocol, the battery pack was heated to 55 °C in an industrial dryer and, just before the test, the battery pack was placed in a chamber that simulated the battery compartment in the aircraft (see Figure 3). In three separate experiments, the selected cells (charged to 100%) were disabled and subjected to overcharging abuse to induce the thermal runaway. This was done in three tests for cells positioned in various places, see Figure 2. The temperature of the target cell, the neighbouring cells and the battery pack was recorded during the tests. In addition, the voltage of the target cell was monitored, and videos were taken. The battery pack can be seen in Figure 4. After the cell failure, the temperatures of adjacent and distant cells were monitored using thermocouples.  

Results

The results for the temperature and voltage from the first test are depicted in Figure 5. The temperature of the target cell (TR) increased very abruptly, exceeding 150 °C. When the cell was subjected to overcharging abuse, its temperature increase resulted in decomposition of the polymer separator. In turn, the anode and cathode reacted in what is known as thermal runaway, where flaming combustion frequently occurs. In this experiment, flaming combustion was not observed, the neighbouring cells did not reach thermal runaway (as evidenced by the temperature data), and the temperature in the centre of the battery pack did not witness a significant increase in temperature.

Figure 5 – Temperature and voltage recordings of the target cell (TR), and temperature of the neighbouring cells during the first test.

Figure 6 shows the test data from the second test. In both, the second and third tests (not shown, as the test results were very similar to those of the first two tests), the recorded temperatures of the target cells were much higher than the first test. Likewise, the temperature of some of the neighbouring cells was also high. These high temperatures were due to the flaming combustion from the target cells, as post-test voltage measurements of these adjacent cells showed that they did not undergo thermal runaway. After 15 minutes (900 s) had passed from the forced thermal runaway, the temperature of all the adjacent cells dropped to below 75 °C with no further increases. Therefore, the battery pack successfully passed the safety test.

An overview of the results is listed in Table 1, where the temperatures reveal that flaming combustion took place for a short duration during thermal runaway, as evidenced by the photo shown in Figure 7, which is a frame extracted from the test video. As soon as the flames extinguished, the spiked temperature went down quickly (see Figures 5 and 6), and the battery packs eventually satisfied the test criterion at the 15-minutes mark after the forced thermal runaway.

Figure 6 – Temperature and voltage recordings of the target cell (TR), and temperature of the neighbouring cells during the second test.

Figure 7 – Flames appearing after thermal runaway during the second test at 14 min and 39 seconds. The duration of the flaming was very short and corresponded to the burning of the gas released from the forced thermal runaway. The flames were only in the gas phase and did not ignite any of the materials in the battery pack or the test setup. As soon as the flames were gone, the spiked temperature went down quickly (as seen in Figures 4 and 5), and the battery packs passed the test criterion with a solid margin.

Table 1 – Temperatures recorded for the target and neighbouring cells. Temperatures of the neighbouring cells after 15 minutes are also listed.

Concluding remarks

As for other light transportation vehicles, sailplanes are very suited for battery-operated propulsion. Still, management of thermal runaway situations is essential, and battery packs used for sailplanes are therefore also in need of testing and approval. The innovative design of a sailplane battery pack that allows the discharge of hot gases and the strategic use of insulating materials enabled the test battery pack to pass the safety test conducted in this study. With these successful tests, LZ Design provided the foundation for the successful further development of their FES system for propulsion of EASA-certified single-seater or two-seater powered sailplanes. The safety of LIB battery packs can be improved further if the appropriate passive design accounts for the correct venting of hot gases and fumes and the use of insulating materials. Other test facilities and companies carrying out tests are encouraged to share their data and experiences in order to facilitate that others can learn from their successes (or failures).

Acknowledgements

The FRISSBE project has received funding from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement No. 952395. We are grateful to Luka Žnidaršič from LZ Design (https://front-electric-sustainer.com/) for allowing open use of the test data and results.

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