Lithium-ion batteries, such as the ones used in EV’s or electronic devices, may catch fire or overheat if they have manufacturing errors, have been damaged, or the controlling and/or monitoring of the battery is not functioning correctly. EV batteries run at high voltage, typically between 400v and 800v, where 800v will probably become the norm in the future.
An EV battery catching fire, due to the above defined issues, or any other causes, looks like a spontaneous combustion, e.g., like in fuel consumption vehicles. However, the internal burning process within the battery is called a “thermal runaway” and is different from a normal burning process.
Unlike a fire in a fuel consuming vehicle where just one single reaction occurs, a fire in an EV battery comprises multiple steps. Essentially, an uncontrolled, cascading loop of violent chemical reactions releases a tremendous amount of energy and heat, and as the individual battery cells warm up, energy and heat drives through the rest of the battery in a kind of domino effect.
What makes this worse is the fact that, as the thermal runaway accelerates, the burning battery’s stored energy essentially creates its own fuel (oxides). It’s a chemical fire which doesn’t necessarily need oxygen, hereby making it far harder to extinguish than a petrol fire. With the oxides present in the battery cells, temperatures from approx. 180°C causes oxygen to be released in the battery, which reacts with cell components, in particular the electrolyte, which again leads to an exothermic reaction that cannot be stopped in practice, during which the battery burns.
The examined method of cooling batteries in thermal runaway via Brine fluid submersion seem to show a very sharp cooldown curve for the tested battery pack samples. Any ongoing thermal runaway should be halted and spread of the runaway effect to adjacent batteries avoided.
Depending on the size of the battery, the threshold for thermal runaway is passed in a time between 38 seconds for the smallest powerpack and 73 seconds for the largest one. After this time, any ongoing thermal processes in cells which have already experienced runaway may continue a controlled burn until they burn through their fuel, but adjacent healthy cells should not be affected, and would therefore not contribute either fuel or thermal energy to the process. In all the tested cases, the settling temperature is below 0°C, meaning that the margin for error is large.
In the material, ongoing thermal processes have not been accounted for. A full, simultaneous burnout has instead been assumed.
Thermal runaway is a chain reaction within a battery cell that can be very difficult to stop once it has started. It occurs when the temperature inside a battery reaches the point that causes a chemical reaction to occur inside the battery.
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