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Der Artikel untersucht die Brandgefahren von Lithium-Ionen-Batterien (LiB), die in E-Rollern durch experimentelle Studien außer Kontrolle geraten. Es untersucht die Mechanismen des thermischen Ausreißens, einschließlich elektrischer, thermischer und mechanischer Beanspruchung, und quantifiziert die Wärmefreisetzungsrate (HRR) von LiB-Bränden. Die Experimente zeigen, dass LiB-Brände in E-Rollern zu schnellem Brandwachstum, großen Feuerbällen und der Freisetzung brennbarer Gase führen können, die in Wohnräumen erhebliche Gefahren darstellen. Die Studie bewertet auch die Auswirkungen von LiB-Bränden auf Wohngebäude und zeigt das Potenzial für eine rasche Brandausbreitung und Überhitzung auf. Die Ergebnisse unterstreichen die Notwendigkeit weiterer Forschungen zur Verringerung der Brandrisiken im Zusammenhang mit LiB-betriebenen Mikromobilitätsgeräten.
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Abstract
Over the last 20 or so years, lithium-ion battery (LiB) powered devices have infiltrated our day to day lives through the devices that rely on them for energy storage including phones, cordless tools, toys, wristwatches, and vehicles. With the meteoric rise in popularity of LiBs in society, failures due to misuse, mishandling, or a poor understanding of necessary safety features may be expected. This study quantifies the fire hazard of a seated E-scooter with a LiB in thermal runaway during overcharging. Heat release rate (HRR) was measured for LiB battery in isolation and subsequently installed in an E-scooter to determine the battery’s contribution compared to the complete device. Two scenarios were evaluated to assess the hazard an E-scooter may pose in a residential scale building: an overcharging E-scooter in a closed bedroom and an overcharging E-scooter in a common living-kitchen area. The results of this study clearly demonstrates the extreme hazard posed by an overcharging E-scooter to the occupants.
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1 Introduction
Modern society relies on lithium-ion battery (LiB) powered devices for communication, construction, entertainment, large-scale energy storage systems, wrist watches, and transportation among many other technologies in daily life. Globally, most of the developed world’s populations already use LiB powered devices routinely with little thought given to the serious consequence displayed by a LiB suffering thermal runaway.
When a technology or product is on a rapid development trajectory and is being widely adopted in society, failures due to misuse, mishandling, or a poor understanding of safety design requirements may be expected. Over the past decades, there have been numerous incidents where batteries in cellphones and laptops have overheated to the point of catching fire. The most noteworthy case was the Samsung Galaxy Note 7 where the battery was found to be defective and resulted in the recall of the battery [1]. Unfortunately, the replacement was also found to be defective and that led to the product being withdrawn from the market. In addition, the US Department of Transportation with the Federal Aviation Administration and the Pipeline and Hazardous Materials Safety Administration issued an emergency order to ban all Samsung Galaxy Note 7 smartphone devices from aircraft in the United States [2].
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Lithium-ion battery cells function like other chemistries of rechargeable and non-rechargeable cells (e.g., nickel–cadmium) in that they are composed of a cathode, an anode, a separator, and an electrolyte blend. However, the chemistry of a lithium-ion battery results in a higher energy density at a relatively lower cost. LiB cells can deliver up to 4 V, which is roughly three times higher than previously popular rechargeable battery chemistries. A drawback is that materials in a LiB’s separator, and electrolyte are combustible.
When a LiB fails and starts a fire, it is typically the result of the battery suffering a thermal runaway. Thermal runaway occurs when an exothermic reaction generates heat faster than the heat can dissipated leading to the reaction running out of control. Thermal runaway can be initiated by physical damage, overheating, and/or failure of the battery management system, which could result in overcharging or over discharging. When considering thermal runaway, the mechanisms typically categorized into three forms abuse: electrical, thermal, or mechanical [3]. Electrical abuse [4] occurs when a LiB is overcharged, over discharged, or short circuits either internally or externally. Thermal abuse [5] results from overheating the battery such as exposure to a fire [6]. Mechanical abuse [7] includes physical damage from a crushing, such as, crushing a cell in a garbage truck or penetration [8] such as driving a nail into the cell.
From a fire safety perspective, LiBs that experience thermal runaway, can result in overheating, flaming, and in some cases, an explosion [9]. During thermal runaway, the temperature of the battery increases which increases the number and rate of exothermic chemical reactions. This produces an exponentially accelerating feedback loop. This feedback loop continues until the battery cell vents combustible gases, via a pressure relief vent or in a potentially violent case rupture. When the case ruptures, vented gases may ignite immediately or accumulate outside the cell, which when ignited, can result in an explosion. Gases released from the cell include hydrogen, carbon monoxide, carbon dioxide, a mixture of hydrocarbons and other chemical species [10]. When battery cells are packed together, the thermal runaway process may spread from cell to cell until all the cells in the battery pack have been affected. A recent study on battery packs used in micro-mobility battery packs, has shown the benefit of improving the thermal insulation between cells in battery packs [11].
Tragic fatal fires involving a lithium-ion battery (LiB) are easily found in the media throughout the developed world [12‐14]. Anecdotally, New York City appears to suffer more micromobility fires than other major cities in the US. In 2023, there were 268 reported fires, 150 reported injuries and 18 fatalities from fires started from micromobility devices [15]. The large number of micromobility fires and associated high casualty rate is believed to be due to the more than 65,000 app-based food couriers [16] and the large number of intercity apartments. In March of 2023, New York City passed legislation requiring an independent laboratory to certify battery-powered mobility devices and their batteries.
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Following the “Lithium-Ion Batteries: Challenges for the Fire Service” symposium hosted by the Fire Department of New York (FDNY) in September 2022, it was clear that technical gaps remained with respect to the fire behavior of battery powered micromobility devices, particularly at their intersection with the residential environment. To increase the knowledge base in this area, the FSRI team partnered with researchers from UL Solutions to conduct a series of experiments to help better quantify the potential hazards of LiB entering thermal runaway. The experiments focused on characterizing the thermal runaway of the lithium-ion battery pack in a commercially available E-scooter. An E-scooter is classified as a micromobility device that according to the Department of Transportation [17], is any small, low-speed, electric-powered transportation device. This definition typically includes electric-assist bicycles (E-bikes—where user can provide pedal assistance when riding). In comparison, an electric scooters (E-scooters) the users is seated but does not have pedals to help propel the device. Small, lightweight, wheeled electric-powered transport such as hoverboards and E-skateboards are also considered micromobility devices. For this study, an E-scooter for a seated a passenger was examined due to their popularity in New York City. The E-scooter battery, which was installed under the seat, had an advertised capacity of 60 V (V), 20 amp-hour (Ah) 1.2 kilowatt-hours (kWh). With the battery installed, the E-scooters weighed between 145 to 165 pounds (66 to 74 kg), depending on the tires, wheels and trim model.
To quantify the hazard from LiB fires, four scenarios were evaluated experimentally:
1.
Standalone battery pack HRR,
2.
Battery pack installed in an E-scooter HRR,
3.
E-scooter in a purpose-built single-family residential home with the scooter in a closed bedroom,
4.
E-scooter in a purpose-built single-family residential home with the scooter positioned in the living room next to the front door.
2 Standalone Battery Pack HRR Experiments
The standalone battery scenario was chosen to isolate the HRR of the battery pack from the scooter to evaluate the threat and to estimate the contribution of the battery pack from the scooter in the full vehicle scenario. Several mechanisms can initiate thermal runaway of LiBs, in these experiments, two were examined: external overheating of the battery cells and overcharging the battery cells. The direct heating method reflects scenarios where it is believed there are defects in cell design or battery design that result in internal short circuits. The overcharging method reflects scenarios where the wrong charger is used, the battery management system on the battery or charger that does not have the appropriate specification, battery pack safety features failed, or the cells lacked the relevant cell-level protections.
In these experiments, the battery pack was placed under an exhaust hood equipped with oxygen consumption calorimetry (OCC). Oxygen, carbon monoxide, and carbon dioxide were recorded along with the mass flow rate in the duct.
2.1 Overheat HRR Experiments
For the overheating experiment, two thin film electrical resistance heaters were applied to outer (exposed) half of 5 cells. Figure 1 shows the matrix of 18,650 cells removed from the metal casing and the heaters installed over the cells. Only one of the two heaters was ramped up at a rate of 6 °C per minute. The second heater was installed for redundancy, in case the first heater failed to operate. Electrical interconnections and charging circuitry were not modified for this experiment. Thermal runaway was identified approximately 45 min after the start of heat application. There was complete propagation of thermal runaway of the battery pack with a peak estimated vertical flame extension of 3–4 feet over a 2 min duration.
Fig. 1
Photograph of the battery pack removed from metal case showing the matrix of 18,650 lithium-ion cells. A Shows the matrix of violet colored 18,650 cells. B Shows the cells covered with an electrical resistance heater used to force the thermal runaway
Figure 2 shows the HRR history for the two isolated battery and two scooter experiments. To aid in comparing the HRR, the time scale for each experiment was shifted such that time 0 s was the point when the HRR first exceeded 20 kW. The 20 kW is considered to be the point where the HRR becomes large enough to be measurable on this scale of OCC and is deemed to be the point where thermal runaway may be observed. The blue line shows the HRR for the overheat scenario, which peaked at 410 kW just 114 s after the thermal runaway was evident. Figure 3 shows the state of the battery pack following the experiment. The case was deformed on three sides and near the power plug, where the gas vented and flaming combustion was observed. A large number of small burning brands and other debris were ejected (ejecta) during the experiment, as seen in Fig. 3 scattered around the battery pack. This demonstrates the large number of potential ignition sources created and thrown some distance from the battery pack that can ignite other combustibles in the space.
Fig. 2
HRR comparison for isolated battery and E-scooter experiments
For the overcharge scenarios, the batteries were supplied with 157 V directly to the charging cable of the battery pack with the current limited to 10 Amps (A). The battery management system for the battery pack was bypassed. Electrical connections between the individual cells were not modified. Thermal runaway was identified approximately 65 min after the start of the overcharge. Similar to the external heating experiment, there was complete propagation of thermal runaway of the battery pack. In this experiment, the event duration was approximately 1 min, resulting in flaming combustion 6–7 ft (2 m) above the battery pack. In Fig. 2, the red line shows the HRR for the isolated intentional overcharge experiment. In this scenario, the peak HRR exceeded 1600 kW 14 s after the HRR exceeded 20 kW. Both experiments’ intense burning period (HRR > 50 kW) lasted less than 100 s.
Figure 3 shows the state of the battery pack following the experiment. The lid remained attached to the enclosure, but the outer enclosure bowed out on three sides (more severely than in the overheating experiment). The power plug burned out, and small debris was ejected near the battery pack. The damage pattern and large quantity of ejecta material were nearly identical to the overheat experiments shown in Fig. 3 so is not included here.
Based on the results from these experiments and the scenarios, the two methods most closely resemble, it was determined that the remaining experiments would utilize intentional overcharging which was determined to be the more severe of the two failure mechanisms.
3 Battery Failure in Vehicle Experiments
For the free-burn experiments, the scooter with the battery installed was placed under an OCC, as shown in Fig. 4. The battery was connected to a power supply (like the standalone overcharge experiment) to apply 157 V with the current limited to 10 A.
Fig. 4
E-scooter in place under oxygen consumption calorimetry hood
The HRR experiments included a video of the battery transitioning into thermal runaway approximately 1 h and 50 min after the start of the overcharge. The first outward sign that the scooter was in thermal runaway was the large quantity of visible smoke emanating from under the seat. Figure 5 is a sequence of video images, with the first image being 10 s after the first visible smoke. Ignition occurred 13 s after the first visible smoke, thus providing limited visible warning and little time to activate a smoke alarm before conditions in the room deteriorate. At ignition, a large fireball engulfed the scooter, followed immediately by a jet flame approximately 2 m (6–7 ft) high from the battery compartment below the seat. The seat was blown off the scooter by the initial fireball and landed immediately adjacent to the scooter. In Fig. 2, the green line shows the HRR for the experiment shown in Fig. 5, and the yellow is from a replicate experiment. In this experiment, the peak HRR reached 1570 kW just 10 s after the HRR exceeded 20 kW. It should be noted that OCC cannot accurately respond to rapid changes in the heat release, so the fireball from the ignition of the battery gas is unlikely to be adequaely measured. From the video observations, the fireball lasted only 2–3 s. The HRR from the battery pack lasted approximately 60 s, at which point the flaming combustion was fueled by plastic components and scooter tires. It should be noted that for the first scooter, there was a problem with the data acquisition system and the data was lost after 250 s. For the replicate scooter experiment (orange line in Fig. 2) the fire lasted approximately 20 min.
Fig. 5
Video sequence for isolated scooter under the OCC, sequence starts 10 s after first visible smoke
To put the fire hazard posed by an E-scooter battery into perspective, a typical HRR curve from a single seat upholstered chair was chosen from the Fire Safety Research Institute Material and Product database [18] (MPD). The HRR curve for upholstered chair can be seen as the gray line in Fig. 2. The chair was ignited with a simulated match and exhibits a nearly ultrafast t2 fire growth rate as might be expected from a residential overstuffed polyurethane chair. The HRR can also be compared with a gasoline powered 50 cc scooter found in the literature [19]. The HRR for the 50 cc scooter is shown in Fig. 2 as the black dashed line which was digitized from the original published graphical results. In this experiment, the gas tank was empty, and the battery had been removed. The HRR reached a peak of nearly 1500 kW and mimics a fast t2 growth rate. The total heat release rate (THRR) from the experiments was determined by numerically integrating the area under the HRR curves. The THRR calculated from the HRR results in Fig. 2 are shown in Table 1. The THRR shows the three different fuel packages had comparable amounts of stored energy, but the near instantaneous fire growth rate from the LiB makes the E-scooter a more severe hazard.
Table 1
Comparison of the Total Heat Release Rate (THRR) for the experimental HRR results shown in Fig. 2
*The HRR results for Scooter 1 were lost after 250 s when the data acquisition failed, thus, the THRR was not calculated for this experiment
4 Building Fire Experiments
To quantify the hazard in a residential building, fire experiments were conducted in a purpose-built single-story residential structure constructed on the grounds of the Delaware County Emergency Services Training Center in Sharon Hill, PA. The building footprint is approximately 150 m2 (1600 ft2) with interior experimental area of approximately 135 m2 (1450 ft2) and featured four bedrooms, two bathrooms, and an open plan kitchen/living room. Figure 6 shows representative photographs of the four sides of the structure, with side A as the front. Room windows that are visible in the photographs have been labeled.
Fig. 6
Exterior photographs of the four sides of the experimental structure
The structure was constructed following standard practice for light wood frame construction and was supported on large concrete blocks to allow easy underfloor access to install instrumentation. The exterior wood stud walls were protected with 6 mm (1/4″) thick fiber cement board siding, a layer of olefin home wrap, and 11 mm oriented strand board (OSB). The exterior walls were filled with R-13 fiberglass insulation and lined on the interior with 16 mm gypsum board and finished with two coats of latex paint. A dimensioned floor plan of the structure is shown in Fig. 7.
The structure had a single fiberglass exterior door (0.91 m by 2.03 m). Internal doors to the bedrooms and closets (0.76 m by 2.03 m) were hollow-core wood frame. The bedroom windows were double-hung, dual pane windows each measuring 0.91 m wide by 1.22 m high with a center mullion for a total size of 1.83 m by 1.22 m. Living room windows were similar to the bedroom windows, with two double-hung, dual pane windows with a center mullion but slightly taller with an overall size of 1.83 m wide and 1.52 m high. The bathroom windows were dual-pane, non-operable windows 0.91 wide by 0.61 high. The kitchen window was double-hung, dual-pane, measuring 0.91 m wide by 0.91 m high. Figure 7 also includes the location of the exterior vent and instrumentation layout. Additional details about the building construction can be found in ref [20].
Fig. 7
Dimensioned floor plan of the structure used in the compartment experiments
A residential heating, ventilation, and air conditioning (HVAC) system was also installed in the structure. A closed system (i.e., no fresh air intake on the return) was installed that recirculated the air within the structure. The HVAC system used rigid metal ductwork for the main trunk lines, supply lines, and to connect the returns once they reached the attic. Within the living space of the structure, each return was created by the volume between stud bays and the enclosing walls. Each bedroom (×4), bathroom (×2), the living room (×2) and the kitchen (×1) had supplies with surface-mounted registers in the ceiling for a total of nine supplies. Each bedroom (×4), the hallway (×2), and the living room (×1) had returns with surface-mounted registers along interior walls, 200 mm (8 in.) above the floor, for a total of eight returns. The system included an 18 kW heater with a 0.37 kW (1/2 horsepower) five-speed motor, which resulted in an airflow of approximately 570 l/s (1200 scfm). R410A refrigerant was used as the cooling fluid that conditions the air in a single-stage air handler. The condensing unit for the HVAC system was located along the back side of the structure below the mechanical room. The system was operating during the experiments. Additional details about the HVAC system can be found in reference [20].
For each fire scenario, the fire was started by an overcharge battery with a 100% state of charge. In the first experiment, the scooter was placed in bedroom 1 near the bedroom door. The bedroom was decorated with typical furnishings, including a queen-sized bed, mattress topper, duvet, pillows, a side table, a lamp, curtains, and a deep pile carpet with underlay. Figure 8 shows the bedroom layout with the E-scooter in place and the visible instrumentation annotated in the photo. The bedroom door was closed as might be expected in a shared living arrangement.
Fig. 8
Photograph of the bedroom fire scenario showing the fuel load and scooter in situ, just prior to the experiment
The second scenario placed a scooter charging in the living room near the main entry door to the residence. The living room furnishing included 2 three-seater sofas, ottoman, coffee table, side table, lamp, curtains and deep pile carpet with underlay. Figure 9 shows the living room fire scenario layout with the scooter in the center of the photo, just visible behind the sofa. The front door of the house is open on the left side (door was closed for the experiment) and the open plan kitchen is visible in the right side of the photo. The instrumentation on the target wall that is visible in the photograph has been annotated.
Fig. 9
Photograph of the living room with the fuel load in place, the scooter is just visible behind the sofa for the living room scenario
The house was instrumented to measure gas temperatures, velocities, pressures, total heat fluxes, and species concentrations throughout the structure. Figure 7 shows partial layout of the instrumentation used during these experiments as well as the scooter placements for both the bedroom 1 and living room scenarios. The target walls, as labeled in Fig. 7 were the same for the two scenarios, except that the gas species measurements were more concentrated around the scooter.
Gas temperatures were measured with 1.25 mm (0.05 in.) bare-bead, Chromel–Alumel (type K) thermocouples and 1.6 mm (0.0625 in.) inconel-sheathed thermocouples. Small-diameter thermocouples were used during these experiments to limit the impact of radiative heating and cooling. The total expanded uncertainty associated with the temperature measurements from these experiments was estimated to be ± 15%, as reported by researchers at NIST [21, 22]. Bare-bead thermocouple arrays were installed throughout the structures in specific spatial locations, which can be found on the floorplan in Fig. 7. Each thermocouple array consisted of eight thermocouples with the top thermocouple in each array located 25 mm (1 in.) below the ceiling and the remaining seven thermocouples spaced at 0.305 m (1 ft) intervals. Single inconel sheathed thermocouples were also installed throughout the HVAC duct network at each of the supplies, returns, and in the main trunk.
Pressure rise inside the compartment was measured using high-frequency piezoelectric pressure transducers. A flush-mount PCB Piezotronics high-frequency ICP® 113B28 piezoelectric pressure transducers connected to a 481A PCB signal conditioner with a 003C10 low-noise coaxial cable was used for measuring time-resolved pressure. The sensor utilized a quartz piezoelectric element, with a measurement range of 344.7 kPa, sensitivity ± 14.5 mV kPa, rise time ≤ 1.0 μs and a resolution of 0.007 kPa [23]. The data was captured continuously 10 kHz and stored for a rolling 20 s period. The data was then written to a file when the pressure exceeded a threshold value.
Total heat flux measurements were made with water-cooled Schmidt-Boelter gauges. Each of the 12 heat flux gauges were oriented horizontally in bedroom 1, mid hall, start of hall, and living room target wall, at 0.91 m (3 ft) above the floor on the beds in the bedrooms, and at 0.30 m (1 ft) and 0.91 (3 ft) above the floor at the bedroom 2 and 3 windows. Results from an international study on total heat flux gauge calibration and response demonstrated that the uncertainty of a Schmidt-Boelter gauge is typically ± 8% [24].
Sixteen gas concentration sampling ports were installed in the structure. The sampling ports were installed at 1 ft and 3 ft above the floor in the hallway and living room, at 1 ft above the floor in the bathrooms and kitchen, at 3 ft above the floor in the bedrooms, and at 1 ft and 3 ft above the floor at the bedroom 2 and 3 windows. Gas samples were analyzed using oxygen (paramagnetic alternating pressure) and a combination carbon monoxide/carbon dioxide (non-dispersive infrared) analyzers. The gas sampling instruments used throughout the series of tests discussed in this research have demonstrated a relative expanded uncertainty of ± 1% when compared to span gas volume fractions [25]. Given the non-uniformities and movement of the fire gas environment and the limited set of sampling points in these experiments, an estimated uncertainty of ± 12% was applied [26].
To minimize transport time through the system, samples were pulled from the structure using a vacuum/pressure diaphragm pump rated at 0.75 CFM. The sampling ports consisted of 9 mm (3/8 in.) OD stainless steel tubing within the structure. Once outside the structure, the sample was drawn through a condensing trap to remove moisture and filtered through a 5 micron polyester filter and a 3 micron polyester filter. At the exit of these filters, the sample line transitioned from stainless steel to polyethylene tubing until the sample reached the analyzer/pump rack. At the inlet to the rack, the gas flowed through a 0.3 micron HEPA filter before reaching the sample pump. Downstream of the pump, but upstream of the analyzer, the sample flowed through a drierite filter to remove any remaining moisture and finally a 0.01 micron filter. Before every experiment, the transport time of a known calibration gas from each sample port to each respective analyzer was measured. This time lag was accounted for in post-processing to ensure the gas data was in sync with the other measurements.
4.2 Closed Bedroom
For the closed bedroom experiment, the time from first signs of smoke from the thermal runaway of the scooter battery until the battery exploded and bedroom windows failed was approximately 20 s. Flashover of the space occurred within 30 s after visible smoke. In this experiment, the closed door and natural swing into the bedroom, limited the failure to only the exterior bedroom windows. Despite the closed door, the bedroom window failure established a flow path that was connected to the exterior and provided a sufficient supply of air to support the transition to flashover within the space. Figure 10 shows a sequence of 9 images captured from a floor level video camera at 1 s intervals. The first 3 images show the buildup of thermal runaway gases, prior to ignition. The remaining 6 images show the explosion and rapid fire spread within the bedroom. Within 40 s of visible smoke from the scooter, flames were visible out of the failed bedroom 1 window. Had suppression been delayed, the bedroom door may have failed. Failure of the door would have likely resulted in additional flame spread throughout the structure. First water on the fire was through the open window of bedroom 1.
Fig. 10
Video sequence for the closed bedroom scenario, the sequence starts 17 s after first visible smoke
Figure 11 shows the history of the average temperature calculated from the top three thermocouples on each TC array in the center of each bedroom, hallway, kitchen and living room. In bedroom 1 there was near instantaneous temperature rise, going from ambient to over 800 °C in less than 10 s. It is not until 140 s after ignition before there is a measurable temperature rise is observed outside bedroom 1 at the end of hall TC array outside the bedroom 1 door. This is after the suppression crew opened the bedroom door to complete suppression. This demonstrates that even a closed hollow core door can provide some level of protection to the occupants outside the room of origin. This is supported by the successful “Close Before You Dose” [27] public safety campaign promoting sleeping with one’s bedroom doors closed. The other bedrooms, kitchen, and living room do not indicate a significant temperature rise for the first 180 s after observing smoke from the scooter.
Fig. 11
Average temperature for the three highest thermocouples on the center TC trees in each room along the escape path from each bedroom to the final exit door from the living room
Figure 12 shows the CO concentrations history for the room of origin (bedroom 1), hallway and living room. Note: the hallway and living room represent the primary means of egress for all four bedrooms. Bedroom 1 target and window wall locations are highlighted in Fig. 7. There were multiple sampling heights at each location that are provided in the legend of Fig. 12. The CO concentrations in the room of origin increased rapidly and exceeded the calibration limit of the CO analyzer of 5%. Although exceeding the 5% calibration limit is undesirable experimentally, this is not considered significant since the gases are considered lethal at CO concentrations well below this level. According to the National Institute for Occupational Safety and Health (NIOSH) website [28], the Immediately Dangerous to Life and Health (IDLH) level for CO is 0.12% (1200 ppm) and the Emergency Exposure Guidance Levels (EEGL) for 10 min is 0.15% (1500 ppm). Figure 12 also shows that outside the room of origin, the CO concentrations do not increase until more than 60 s after suppression activities have started. This confirms that the closed door to bedroom 1 provides some level of protection to the occupants outside the room of origin.
Fig. 12
CO concentrations for the closed-door scenario as measured along the escape path from the bedrooms to the final exit door from the living room
When assessing the impact of the toxic smoke on the occupants, the Fractional Effective Dose (FED) is often used to evaluate the cumulative effect of the toxic products in the smoke and the occupants increased respiration rate caused by heightened CO2 concentrations. The procedure for calculating the FED was taken from ISO 13571 Life-threatening components of fire—Guidelines for the estimation of time to compromised tenability in fires [29]. An occupant’s actual FED depends upon the escape route, travel speed, and local species concentration. The FED shown in Fig. 13 was calculated for each room along the escape route including bedroom 1, hallway, and living room. In bedrooms 2, 3, and 4, the FED does not rise significantly until more than 60 s after suppression activities begin, corresponding to the increase in CO concentration in these spaces. When assessing the impact of the FED on the building occupants, a Log-normal distribution, with a standard deviation equal to 1, is often used to characterize the human population as described in the appendix of ISO 13571 [29]. An FED = 1 is considered to be fatal to 50% of the population. An FED = 2 would be considered fatal to 76% of the population, an FED = 0.3 (a common FED used in fire safety design) would be considered fatal to 11% and FED = 0.1 would be fatal to 1% of the population. Thus an FED ≥ 1 is considered to be fatal to the occupants in the room. Looking at Fig. 13, for bedroom 1 at the target wall, FED > 1 just 35 s after smoke from the thermal runaway becomes visible, thus making egress for any alert occupant difficult and making any sleeping occupant highly unlikely.
Fig. 13
Fractional effect dose for the closed-door scenario calculated from the CO, CO2 and O2 measurements in each room
In these experiments there was a measurable overpressure of the fire compartment when the battery gases ignited into the fireball seen in Fig. 10. It should be noted that this building is an experimental structure built for repeated fire exposure experiments and is typically repaired and relined with sheetrock after every each experiment, at least in the room of fire origin. In most cases, the repairs also require the replacement of the windows that are standard residential PVC framed, double hung, double glazed windows. Although the building is constructed following standard building practice, it is not intended for habitation thus the window are fixed to the wall structure with at least 4 screws, they are not fixed as robustly as required by the building code. The consequence of reduced fixing of the window is that entire window frame may separate from the structure or deform enough to dislodge the individual double glazed window element from the frame. Therefore, the overpressure relief in these experiments were expected to be less than what would be experienced in a modern residential building. In the video, the window can be seen being dislodged from the frame and fixing out of plane before breaking and throwing shards of glass up to 10 m from the structure. Regardless of the failure mechanism for the window, the explosive overpressure dislodged the window frame and shattered the glass of the Bedroom 1 and caused significant damage to the bedroom door. Figure 14 shows the pressure history for the bedroom explosion (blue line). The time scale for the graph is a relative time based on the 20 s that the pressure data was captured. The data was smoothed using 21-point center moving-average. The maximum pressure was 1.54 kPa which is well below the level 24–34 kPa where a human ear drum may rupture from a blast wave [30] or the 50 kPa [31].
Fig. 14
High-speed pressure history for the battery gas explosions, captured at 10 kHz
For the living room experiment, the scooter was in a larger volume due to the open floor plan and open bedroom 2 door. The larger volume diluted gas species and dispersed expanding gases from the explosion, thus reducing pressure rise. Yet even with the larger volume, within 10 s of sustained visible battery gas from the E-scooter, the transition to flaming combustion resulted in a pressure increase that caused the failure of the window frame dislodging the individual window elements in the living room, kitchen and bedroom 2. The bedroom 1 windows and kitchen access doors also failed as a result of the overpressure caused by the ignition of the battery gases. It is important to note that while bedroom 1 door was closed, it was the same door that was damaged in the first bedroom 1 experiment and had a diminished structural capacity. The windows in bedrooms 3 and 4 remained intact during the experiment. The measured overpressure for the E-scooter in the living room is shown in Fig. 14 as the yellow line. The raw data showed the maximum pressure was 1.23 kPa, slightly lower than the bedroom experiment. The pressure profile is also more complicated than the bedroom experiment as a result of the multiple vented rooms.
Each of these window and/or door failures caused by the explosion, created flows paths with lower-pressure exhaust openings connected to the higher-pressure fire compartment (living room). Bi-directional flows were established within these flowpaths, and additional air was supplied to the fire. As a result, flames spread down the hallway toward the bedrooms and the kitchen. Within 40 s of visible smoke from the scooter, flames were visible out of the failed front (side A) and side living room windows (side D).
Figure 15 shows a sequence of 9 images captured from a floor level video camera at 1 s intervals. The first 4 images show the buildup of thermal runaway gases, prior to ignition. The remaining 5 images show the explosion and rapid fire spread within the living room. In the last 4 images in Fig. 15 (post ignition) several spot fires can be seen in the carpet around the E-scooter. These burning brands are made up of the combustible components from the scooter that have been thrown from the device in the initial explosion or hot metal or other material ejected from the battery itself following the initial explosion. The ejected material was seen in all of the experiments and resulted in ignition of nearby combustible contents in the room fire experiments. The burning brands were not visible in the bedroom 1 experiment due to the large volume of flames in the smaller room filled the entire view area of the camera with orange flames as seen in Fig. 10.
Fig. 15
Video sequence for the living room scenario, sequence starts 6 s after first visible smoke
Figure 16 shows the history for the average temperature calculated from the top three thermocouples on each TC array in the center of each bedroom, hallway, kitchen and living room. The larger volume resulted in a slower overall temperature rise compared to the closed bedroom scenario, even though the temperature rise is considered extremely rapid compared to a typical residential fire growth rate. It is important to note that the hollow core doors to bedrooms 3 & 4 stayed intact after the explosion and prevented significant temperature rise in these rooms until after suppression activities began. However, an alternative escape route would be required for self-evacuation from these bedrooms. In this case, the only alternative for self-evacuation would be the bedroom window. Bedrooms 1 and 2 did see substantial temperature rise after the explosion as a result of bedroom 2 door being left open and bedroom 1 door being damaged in the two explosions.
Fig. 16
Average temperature for the three highest thermocouples on the center TC trees in each room along the escape path from each bedroom to the final exit door from the living room
Figure 17 shows the CO concentration history for the scooter in the living room for all of the bedrooms and the rooms that make up the primary escape route for the bedrooms. This includes the hallway, kitchen, and living room. The target and window wall locations in the living room are highlighted in Fig. 7. There were multiple sampling heights at each location that are provided in the legend of Fig. 17. The CO concentrations on the living room target wall increased more rapidly based on the proximity of the sampling location to the scooter. In 3 locations, the concentration exceeded the calibration limit of the CO analyzer of 5%. It can also be seen in Fig. 17 that behind the closed doors of bedrooms 2 and 3, the CO concentrations do not increase significantly until more than 60 s after suppression activities have started.
Fig. 17
CO concentrations for the living room E-scooter scenario as measured along the escape path from the bedrooms to the final exit door from the living room
Figure 18 shows the (FED) calculated for each of the bedrooms and the rooms along the escape route including hallway, kitchen, and living room. In bedroom 2 and 3 the FED does not rise until more than 60 s after suppression activities begin, corresponding to the increase in CO concentration in these spaces. Looking at Fig. 18 for the living room target wall, the FED > 1 just 34 s after smoke from the thermal runaway becomes visible, thus making egress for any alert occupant difficult and making it highly unlikely for any sleeping occupant.
Fig. 18
Fractional effect dose for the living room E-scooter scenario calculated from the CO, CO2 and O2 measurements in each room
The thermal runaway of LiBs used in micromobility devices, are large and portable enough to create an extremely rapid, hazardous fire in a residential scale building. The initial explosive ignition of the battery gases results in a large fireball that is sufficient to shatter windows and damage hollow core doors. The fireball is large enough (~ 2 m in diameter) to threaten occupants and nearby combustibles. Following the initial fireball, the battery fire spreads to the combustible fuels on the scooter, i.e. seat, deck, fairings, fenders, tires, etc. As the battery burns, hot molten droplets and particles of metal and other materials are discharged from the battery as ignition sources that can quickly spread the fire to nearby combustibles, i.e., upholstered furniture, bedding, carpet, etc. Once the battery fire starts to diminish in size, the fire on the scooter is sufficient (> 500 kW) to drive the fire spread to the other combustible contents, bringing the room to flashover if the battery alone has not already taken the room to flashover. With flashover times less than a minute, the LiBs found in micromobility devices pose a significantly greater hazard than typical residential upholstered furniture.
6 Future Research
With LiB devices ubiquitous in our society, the fire hazards associated with LiB devices require further investigation, including:
Evaluate the efficacy of automatic fire sprinklers on E-scooter fires,
Identify and quantify the air and water contamination caused by LiB fires,
Investigate the contamination of personal protective equipment (PPE) form LiB fires,
Determine the best practice for cleaning PPE contaminated in LiB fires.
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