emobility thermal runaway

Experiments Completed for Intentional Thermal Runaway on Lithium-Ion Batteries

November 10, 2022

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 mobility 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 posed if the lithium-ion batteries enter thermal runaway.

The experiments focused on characterizing the thermal runaway of the lithium-ion battery pack in a commercially available e-scooter as the result of an intentional overcharge. The single-passenger scooter featured a 1.2 kilowatt-hours (kWh) advertised capacity battery that was installed under the seat. The scooters, with their battery installed, weighed between 145 to 165 pounds (lbs), depending on the tires, wheels and trim model.

The series of experiments spanned three scales:

  1. Isolated battery pack in a laboratory
  2. Stand-alone e-scooter in a laboratory
  3. E-scooter in a purpose-built single-family residential home test prop

Isolated Battery Failure Experiments

There are several mechanisms that can initiate thermal runaway of lithium-ion batteries. These experiments examined two: external overheating of the battery cells and overcharging the battery cells. The direct heating method is reflective of scenarios where it is believed there are defects in cell design or battery design that result in internal short circuit. The overcharging method is reflective of scenarios where the wrong charger is used, controls on the battery or charger are not to the appropriate specification, battery pack safety features fail, or if the cells lack the relevant cell-level protections.

The battery pack in the e-scooters examined for this study were labeled as 60 Volts (V), 20 Amp Hours (Ah) and featured a 1.2 Kilowatt Hour (kWh) advertised capacity. Inside the pack, there were 136 individual 9 watt-hours (Wh) 18650 cells. Battery packs were subjected to both external heating and intentional overcharging. In both experiments, the batteries were initially at 100% state of charge from the manufacturer supplied charger.

External Heating

For the external heating experiment, one heater was applied to outer (exposed) half of 5 cells. The heater was ramped at a rate of 6 degrees Celsius per minute. Electrical interconnections and charging circuitry were not modified for this experiment. Thermal runaway was identified approximately 45 minutes after the start of heat application. There was complete propagation of thermal runaway of the battery pack with peak estimated vertical flame extension of 3-4 feet over a 2 minute duration. Figure 1 shows the state of the battery pack following the experiment. There was case deformation on three sides and near the power plug that were the sites of gas venting and flaming combustion. Small debris was ejected near the battery pack.

Post experiment battery packs

Fig. 1: Post-experiment images of battery pack following overheating initiated thermal runaway.
 

Intentional Overcharge

To intentionally overcharge the batteries, 157 V was applied 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 minutes 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 minute and resulted in flaming combustion 6-7 feet above the battery pack. Figure 2 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 severe compared to the overheating experiment). The power plug burned out and there was small debris ejected near the battery pack.

Post experiment battery packs

Fig. 2:  Post-experiment images of battery pack following overcharging initiated thermal runaway.
 

Based on the results from these experiments and the scenarios that the two methods most closely resemble, it was determined that the remaining set of experiments would utilize intentional overcharging.

Battery Failure in Vehicle Experiments

For the free-burn experiments, the bike with battery installed, was placed under an oxygen consumption calorimeter (see Figure 3). The battery was connected to a power supply in the same manner as the standalone overcharge experiment to apply 157 V with current limited to 10 A.

e-Scooter experiment

Fig. 3:  E-scooter in place under oxygen consumption calorimetry hood.
 

For the specific experiment included in the video, the battery went into thermal runaway approximately 1 hour and 50 minutes after the start of the overcharge. The heat release rate (HRR) from the thermal runaway event peaked at approximately 1.1 megawatts (MW) and lasted for about 1 minute at which point the flaming combustion transitioned to plastic contents of the scooter. The contents fire eventually grew to a peak of 1.2 MW before decaying. This phase lasted for several minutes.

Of particular interest is the peak HRR and the time from first measurable increase to peak. For reference, the 1.1 MW peak HRR is similar to an over-stuffed upholstered chair. In this free-burn setting where there was no radiative feedback from the environment, the scooter reached a peak HRR in 13 seconds. The upholstered chair in this same setup, is typically on the order of 2-3 minutes to reach its peak HRR.

Compartment Fires

The full-scale structure experiments were conducted in a 4-bedroom, 2-bathroom, single-story, single-family residential structure that was purpose built to be a fire test prop. Two experiments were conducted: one in a closed bedroom (bedroom 1 in Figure 4) and one in an open concept living room (living room in Figure 4). In both experiments, the thermal runaway was initiated via an intentional overcharge of the battery pack conducted identically to the isolated battery and free-burn tests. Additionally, all exterior doors and windows were closed prior to the start of the overcharge event.

Test structure layout

Fig. 4: Layout of test structure and approximate test volumes in transparent shading with location of scooter in solid shading.
 

In both experiments, the thermal runaway resulting in a sharp increase in pressure in the structure that caused a failure of exterior windows. Within 60 seconds of window failures, there was a call for suppression to limit additional fire spread throughout the structure.

Closed Bedroom

For the closed bedroom experiment, from the time of first signs of smoke from the scooter until the bedroom windows failed was about 12 seconds. Flashover of the space occurred within 30 seconds of the failure. In this experiment, the closed door, and the fact that its natural swing was into the bedroom, limited window failures to only the bedroom. However, despite the closed door, the bedroom window failure established a flow path that was connected to exterior and thus, there was a sufficient supply of air to support a transition to flashover within the space. Within 40 seconds 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.

Living Room

For the living room experiment, there was larger portion of the structure that was part of the experimental volume due to the open-concept floor plan and open bedroom 2 door. As a result, within 10 seconds of sustained visible smoke from the scooter, the transition to flaming combustion resulted in a pressure increase that resulted in failure of both living room windows, the bedroom 2 windows, the bedroom 1 windows (it is important to note that while this door was closed, it was the same door that was damaged in the first experiment and did not retain its original structural integrity), and kitchen doors. Each of these new vents created flows paths with lower-pressure exhaust openings that 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, there was flame spread both down the hallway toward the bedrooms and toward the kitchen. Within 40 seconds of visible smoke from the scooter, flames were visible out of the failed front and side living room windows.

Next Steps

A technical report is in development that will leverage the data collected from the instrumentation along with video data to provide an assessment of the fire dynamics associated with thermal runaway of lithium-ion batteries in these e-scooters.

Examining the Fire Safety Hazards of Lithium-ion Battery Powered e-Mobility Devices in Homes