Since the commercial application of lithium-ion batteries, technology has been continuously advancing and has been widely used in electric vehicles, energy storage, electronic information and other fields, greatly promoting the development of human society. However, at the same time, different forms of explosions and fires have occurred, which has raised concerns about the safety of lithium-ion battery use. Previous researchers have conducted extensive research on the fire hazards of lithium-ion batteries.

Through conducting combustion spread tests on electric vehicle charging and swapping stations, the thermal stability of different types of lithium-ion batteries was compared and analyzed. It was found that the thermal stability of ternary lithium materials is lower than that of lithium iron phosphate and lithium manganese oxide, and their combustion temperature is more likely to reach its maximum value in a short period of time. In addition, studies have shown that the thermal explosion hazard of lithium-ion batteries is closely related to the state of charge (SOC) of the battery, and the hazard usually increases with the increase of SOC.

During the storage, transportation, and use of lithium batteries, accidental heating may expose them to a hot environment, posing a risk of thermal abuse. The experiment shows that the equivalent critical temperature for thermal runaway of lithium batteries is between 123.8 and 139.2 ℃. Under overheated conditions, the active substances inside lithium batteries are enhanced, which can easily trigger chemical exothermic reactions between battery materials, leading to rapid internal heating exceeding the critical value and causing thermal runaway and ignition of the battery. In order to simulate an overheated environment, external heating is a commonly used test method that can accelerate the simulation of the thermal abuse process of lithium batteries.

However, the literature mainly focuses on the thermal runaway characteristics and combustion characteristics of lithium-ion individual batteries under overheating conditions, and there is still a lack of research on the thermal abuse of modular battery packs. This article uses a heating furnace to simulate the thermal abuse process of adjacent batteries after thermal runaway of the ternary 18650 lithium battery pack, analyzes the combustion characteristics and fire behavior of the lithium battery pack under different heating positions and heating power conditions, and provides theoretical guidance and data support for the safe use of the ternary 18650 lithium ion battery pack and the development of efficient fire extinguishing technology.

1 Test preparation

1.1 Battery type

This article adopts a ternary lithium-ion battery pack as the research object, with a length of 240 mm, a width of 220 mm, a height of 85 mm, and a weight of 7 kg. The voltage of the battery pack during normal operation is 48 V, with a rated capacity of 30 Ah. It is composed of 156 (12 x 13) standard 18650 battery cells, each with a height of 65 mm and a maximum diameter of 18.4 mm. Before the test, remove the outer packaging of the battery pack, as it is made of flame-retardant material and cannot ignite the lithium battery through external heating. In addition, the internal circuits and battery management system remain unchanged, and the State of Charge (SOC) of the battery is 100%.

1.2 Test Layout

The entire test was conducted in a narrow confined space, with a length of 12 meters, a width of 2 meters, and a height of 2.4 meters, as shown in Figure 2. There are doors on both longitudinal sides of the confined space, which remain closed during the test.

A 1.2 m long and 0.6 m high observation window is installed in the middle of the side, and a smoke exhaust fan is installed on the upper side of the opposite wall to maintain ventilation during the test. A total of 11 water mist nozzles are installed on the top of the confined space, with an interval of 1 m between the nozzles, to conduct water mist fire extinguishing tests and serve as backup fire extinguishing and cooling measures to prevent uncontrolled combustion during fires.

Place the lithium battery pack horizontally on a bracket with the positive pole facing upwards. The bracket is a mesh structure that facilitates heating from the bottom of the battery. Two heating methods were used in the test:

(1) The heating furnace is located at the bottom of the lithium battery pack, and the heating surface of the electric furnace is 8 cm away from the bottom surface of the battery. It continuously heats the negative electrode on the bottom surface of the battery cell;

(2) The electric heating furnace is located on the side of the lithium battery, 8 cm apart, and continuously heats the side of the battery. The effective heating surface of the electric heating furnace is 12 cm long, 12 cm wide, and has an area of 144 cm ²， The heating power is adjustable from 0 to 2000 W.

The process of using an external heat source to heat the lithium battery pack and causing ignition is to open the electric furnace for continuous heating until the lithium battery catches fire, then turn off the power and stop heating.

In the test, four K-shaped armored thermal couple (T 1, T 2, T 3, T 4) were arranged inside the battery pack, with a diameter of 1 mm. The thermal couple were located in the middle of the battery cell, 30 mm away from the bottom, to collect temperature changes at different positions of the battery pack. However, for bottom heating and side heating, the placement of the thermal couple was not the same. In addition, a high-definition camera is installed on the front to record the combustion spread process of the battery pack.

1.3 Test conditions

A total of 5 test were conducted. The heating method for test 1-3 is bottom heating, while side heating is used for test 4 and 5, mainly to study the influence of different heating positions on the combustion propagation characteristics of lithium battery packs. On this basis, a set of fire extinguishing test, namely test 3, were conducted using water mist as the extinguishing method.  test 1 and 2 were repeated using bottom heating method. In addition, test 3 and 5 have a higher heating power of 2 kW compared to the other three groups, used to study the changes in internal temperature and combustion characteristics of lithium battery packs under the condition of increasing external heat source power.