Safety is often said to be the lifeblood of lithium-ion batteries. Recently, I've been analyzing the causes of battery system fires from the inside out. The key is to consider each cause layer by layer, then combine past and future accidents for comparison, and finally, based on the actual design of each vehicle model, predict the PPM (parts per million) of future accidents. The names of all manufacturers have been removed below; this discussion is not directed at any particular company and no judgment is made.
This is an experimental approach that starts by assuming a single problem and then expands to a global perspective, as can be seen from the diagram:
The entire analysis was only to match the extreme event of the battery system catching fire. We separated the content caused by mechanical abuse. The design basis of the battery system is to place it in a relatively safe position in the vehicle to prevent problems during vehicle use. The mechanical design is indeed the subject of a lot of safety tests such as nail penetration and crushing. However, the problems caused by mechanical abuse are actually the ones that are easier to solve.
Take the three TSLA accidents as examples:
1. A fire broke out in Smyrna, Tennessee, after the electric car crashed into a trailer hitch that had fallen off the road, causing the chassis to collide and catch fire.
2. The driver hit and went through a concrete wall while turning, eventually crashing into a tree and coming to a stop, where it caught fire.
3. In Seattle, the driver claimed he hit metal debris in the road, causing him to leave the highway. After the car malfunctioned, he smelled burning, at which point the vehicle caught fire.
This mechanical design is also simple; by considering more protection on the outer perimeter of the structure and the bottom cover, the effect can be achieved immediately.
If we remove the manufacturers from the events of this year, we can reconsider whether car fires are caused by the battery or something else. A large part of the problem is related to loads outside the battery, with overheating cables causing peripheral components to ignite.
Here we can divide the fire into three basic layers, the fundamental cause of the fire:
1. Unintended heat release within the battery + internal and external combustion products
2. Release of combustible gases within the battery + ignition point
3. Release of flammable liquids from the battery + ignition point: This mainly includes two parts: electrolyte leakage and coolant leakage.
We can consider the heat release of the battery system:
1. Overcharging of the battery pack or individual battery cells
Overcharging is indeed a common cause of heat release. Battery pack-level thermal runaway events can be further subdivided into multi-cell (module, single cell overcharging), battery overcharging and electrolyte evaporation overheating, overcharging caused by incorrect calculation of battery remaining capacity (SOC), overcharging caused by failure to perform energy recovery according to protection under high SOC state, and overcharging caused by charging control program jamming.
2. Heat release during short circuit overcurrent
Battery pack/high-voltage circuit failures can cause short circuits, leading to heat dissipation. This is primarily caused by internal and external short circuits within the battery pack, resulting in overheating of conductors and connectors, and ultimately, overheating of individual cells, triggering subsequent thermal events. Further subdivisions can include module-level short circuits causing component overheating. Examples include module-level short circuits, battery pack-level short circuits, and short circuits caused by the ingress of corrosive/conductive liquids from the surrounding environment.
3. Heat generation due to high connection impedance
A fault in the battery pack/high-voltage circuit can create a high-resistance point in the charging/discharging circuit. The temperature rises at this high-resistance point, potentially causing ignition of adjacent materials and subsequent heat transfer. Poor contact or corrosion at main circuit connection points can also cause overheating.
4. Increased internal resistance and internal overheating of the battery
Combustible gases are released during single-cell exhaust, and subsequent heat sources (electric arc, single-cell thermal runaway) lead to excess heat in the battery system. Single-cell single-point fault thermal runaway determination experiments can be considered to extend from individual cells to the overall system. Under given conditions, this will achieve the filing and disclosure of each battery pack, which has some reference value. However, the experimental conditions and the occurrence of the fault are unlikely to perfectly match.
Case Review
In reality, fire accidents are all intertwined:
Leakage and short circuit
Case 1: Car A catches fire
Phenomenon 1: More than two battery cells inside the battery box are leaking electrolyte.
Phenomenon 2: Insulation between individual battery cells and the aluminum casing of the battery box is damaged.
After the vibration occurred during operation, intermittent short circuits occurred in some batteries inside the battery box. The insulation failure point formed a high-voltage short circuit loop through the grounding of the battery box and bracket, releasing heat energy. The heating of the electrolyte and the battery body caused the accident.
The fire spread and ignited the interior.
Case 2: Car B catches fire
Phenomenon 1: After a violent collision, the battery structure is damaged, and the cooling pipe ruptures, causing coolant leakage.
Phenomenon 2: Slow chemical reaction occurs after coolant immerses the battery management unit (BMU) circuitry.
After the sampling point was submerged, the lack of a completely waterproof design on the plate level caused slow heating, triggering the coolant's ignition point, and the fire started after being left in the parking lot for several weeks.
summary:
1. A fire is an extreme event, but it receives a lot of media attention. Everyone's first reaction is that it's a problem with the battery system. Based on various analyses, if a fire starts in the battery system, there must be a malfunction, and heat must have accumulated and ignited it.
2. If more publicly available information is gathered together, broken down and compared, and the system analysis process is improved, some consensus can still be reached to prevent many future fire accidents.
