Analyzing the cause of the Incheon electric car

[Analyzing the cause of the Incheon electric car accident and suggesting countermeasures]

This accident is unfortunate, but I think it is simply a hasty judgment: "Electric cars are dangerous, so let's exclude Chinese battery electric cars," and "Let's move the electric car parking lot to the ground." We examined the root cause of why the accident occurred and why the work became so big that more than 100 vehicles were lost, and summarized what measures should be applied in the future.

(Additional: It is believed to be a foreseen accident. Problems such as low ride-high electric vehicles, high repair costs, gradient of entry to underground parking lots, NMC811 battery characteristics, and large apartment complexes where testing of fire fighting equipment is difficult are believed to overlap.)

1. Thermal runaway/explosion issues with NMC811 batteries
In the event of a thermal runaway, NMC811 with a high nickel content can generate a high temperature of 900 degrees Celsius, so the thermal runaway from one cell can cause an explosive fire in the entire pack. The results of the experiment with CATL's NMC811 and LFP batteries show that the thermal runaway transition rate of a single cell is five times faster than that of LFP. This experiment was conducted by placing cells in one row, but it is estimated that the actual pack will have a thermal runaway speed of more than 25 times faster because the cells are placed in two dimensions. In other words, once a thermal runaway occurs, the NMC811 cell can rapidly spread throughout the pack, which then generates a large amount of heat and gas as the vehicle explodes.

Battery makers and car makers are well aware of the problem and are taking measures. They typically wrap cells in a heat-resistant material, circulate cooling water, and design packs to quickly expel organic solvent gas. Tesla proposed and developed these technologies while developing lithium-ion battery electric car packs. What matters then is the capacity and proximity of the individual cells. Tesla's early electric vehicles used a 3-Ah cylindrical cell (18650), which was later converted to a 5-Ah cylindrical cell (21700), and recently increased it to 24 Ah (4680).

CATL's NMC811 cells start at a 50Ah capacity, and products over 100Ah are common. It is extremely difficult to prevent thermal runaway when these large-capacity brick cells are brought close together.

A cylindrical cell creates an empty space when it is closely attached to the cell, and its energy density decreases when the cooling water channel is taken into account. Other electric car makers boldly applied the design of sticking the brick-like square battery and pouch battery together. This design will increase energy efficiency, but it will aggravate the thermal runaway problem. Therefore, the thermal runaway response performance test should be strengthened for close-knit packs between batteries. NMC811 batteries with high nickel content, NMC9(1/2)(1/2), are more vulnerable to thermal runaway, so they need to be regulated more conservatively.

2. Vehicle design and inspection issues
When the +electrode plate (anode plate) and the -electrode plate (anode plate) contact each other and short-circuit, the battery generates a large amount of heat. The organic solvent inside the battery burns with that heat, which completely decomposes the battery, and when lithium comes into contact with oxygen, another fire occurs. When such a fire occurs very quickly, it is called thermal runaway.

The separator separating the electrodes fundamentally prevents short circuits, but the separator has become much thinner than the hair to increase energy capacity. The separator covers the electrode plate, and in order to maximize the battery capacity available, it is fashionable to make the separator really, very slightly larger than the electrode plate. So, if the battery is shocked and the inside is shaken, the electrode plate and the separator move slightly, which can lead to a minute short circuit. This minimum short does not cause a problem at first, but it can cause thermal runaway when the cell's temperature is gradually raised and the threshold is exceeded.

The Mercedes-Benz EQE vehicle, which caused the accident, had a long wheelbase and a low ground height, which made it more likely to hit the floor or be impacted while driving. Problems have already been reported several times, and drivers are reported to be stressed about the issue. However, because the battery pack is horizontally installed, and the cells are vertically erected, a vertical G shock is likely to cause a minimal short between the electrodes. Driving a car is difficult to completely prevent floor shocks, such as jumping over the chin or splashing stones, and Tesla also suffered a lot from the fire caused by the floor shock in the early stages. Electric car manufacturers should reconsider their designs, such as giving an alarm, wrapping it in cushioning, or changing the cell arrangement inside the pack when the floor shock occurs. Design changes reduce pack productivity, but safety is more important.

The routine inspection of electric vehicles also requires a procedure to check whether the floor shock has occurred and how shocked the pack is. When a minimum short circuit occurs, the voltage level of the cell drops and shows an abnormal pattern during charging and discharging, which is difficult to check with a vehicle BMS, but it can be identified by a precise check with an external device. Efforts should be made for electric vehicles, such as strengthening ride height standards, ensuring that vehicle shocks occur during regular inspections, and the government releasing data on the frequency of shocks when driving.

3. Underground parking lot sprinklers, dedicated parking areas, and floor water maintenance facilities
The Incheon fire caused more damage due to the malfunction of the sprinklers installed. Had the sprinklers been operated, the heat from the electric vehicle fire would have transferred to the surrounding cars, preventing a series of fires, and the haze would have been weakened by the water. Currently, only the pressure and alarms on the Hydrant line are checked when inspecting the fire fighting equipment. It does not always inspect the dual installed fire pump for pressure or whether the actual sprinklers burst and spray water. It is cumbersome, but in the case of underground parking lots, it is necessary to conduct actual sprinkler operation tests in consideration of emergency situations. As confirmed and proposed in recent demonstration tests, there should be measures to improve regulations in consideration of increased sprinkler capacity, switching to a sensitive response type, and malfunctioning fire pumps.

Unfortunately, water pipes and electrical wiring passed directly above the parking surface where the fire broke out, and the smoke exhaust system was not activated due to water supply/power failure