Overheating, overcharging, internal short circuits, and collisions are some of the key factors that can cause thermal runaway in power batteries.
(1) Overheating triggers thermal runaway
The causes of overheating in power batteries can be due to improper battery selection and thermal design, external short circuits leading to increased battery temperature, loose cable connections, etc. These issues should be addressed from both battery design and battery management perspectives.
From the perspective of battery material design, materials can be developed to prevent thermal runaway and block the thermal runaway reaction; from the perspective of battery management, different temperature ranges can be predicted to define different safety levels, thereby enabling graded alarms.
(2) Overcharging triggers thermal runaway
The cause of a fire involving a pure electric bus this year was "thermal runaway triggered by overcharging." Specifically, the battery management system itself lacked the circuit safety function for overcharging, causing the battery's BMS to malfunction while it was still charging.
To address the causes of this type of overcharging, the first step is to identify the charger malfunction, which can be resolved by implementing full redundancy in the charger. The second step is to check whether the battery management is appropriate, such as whether the voltage of each battery cell is being monitored.
It's worth noting that as batteries age, the consistency between individual cells deteriorates, making overcharging more likely. This necessitates balancing the entire battery pack to maintain consistency.
For example, in a series-connected battery pack using the most common battery pack combination method of "parallel first, then series," the best-case scenario after solving the problem of individual cell consistency is that it has the same capacity as the smallest individual cell. With this consistency, the capacity recovers, and overcharging can also be prevented.
To achieve consistency, a method is needed to estimate the capacity of each individual cell. Ouyang Minggao suggests that the state of the entire battery pack can be estimated based on the similarity of its charging curves.
In other words, once the charging curve of one individual battery cell is known, the curves of the others should be similar. Through curve variations, they can approximately overlap, and the differences in the curve variations are easy to calculate. The other cells can be deduced from one individual cell. With this method, the consistency balancing mentioned above can be performed. However, this algorithm is too time-consuming and needs simplification.
(3) Internal short circuit triggers thermal runaway
A Boeing 787 passenger plane previously caught fire due to a battery explosion. During the investigation, metallic objects were found on the electrodes and diaphragm, indicating an internal short circuit. While experts cannot definitively confirm that the thermal runaway was triggered by this internal short circuit, it is the most probable cause because no other cause could be found, and the internal short circuit itself cannot be readily apparent.
Impurities from battery manufacturing, metal particles, expansion and contraction during charging and discharging, and lithium plating can all cause internal short circuits. These internal short circuits occur slowly over a very long period, and it's impossible to predict when thermal runaway will occur. Experiments cannot replicate the process. Currently, experts worldwide have not yet found a way to reproduce the internal short circuit process caused by impurities, and research is ongoing.
To solve the internal short circuit problem, the first step is to find a battery manufacturer with high-quality products and select the appropriate battery and cell capacity. The second step is to conduct a safety prediction of the internal short circuit and identify the cells with internal short circuits before thermal runaway occurs.
This means it's necessary to find the characteristic parameters of individual cells, and we can start with consistency. Batteries are not consistent, and their internal resistance is also inconsistent. As long as we find the individual cells with variations, we can identify them.
Specifically, the equivalent circuit equations for a normal battery and the equivalent circuit equations for a battery with a micro-short circuit are actually the same; the only difference is that the parameters of the normal cell and the micro-short-circuited cell change. These parameters can be studied to observe their characteristics during internal short-circuit changes.
One key characteristic is the potential difference of the internally short-circuited cell, which is compared to the difference in internal resistance between the two cells. Ouyang Minggao proposed that researchers should use models to identify individual cells. After measuring the voltage and current of each cell, these data, combined with the model, can be used to predict the internal resistance of each cell. After all the parameters of the cells are predicted, changes in these parameters can be used to determine whether there has been a significant change in their consistency.
(4) Mechanically triggered thermal runaway
A collision is a typical way to mechanically trigger thermal runaway. This is the reason why Tesla vehicles have repeatedly caught fire. Ouyang Minggao revealed that Tsinghua University and MIT have collaborated to analyze Tesla's collision accidents in the United States. If a collision simulation were conducted in a laboratory, the closest approximation would be a needle prick.
The solution to collision-triggered thermal runaway is to implement sound battery safety protection designs. This requires researchers to first understand the process by which thermal runaway occurs.
Generally, after thermal runaway occurs, it propagates downwards. For example, after the first thermal runaway, heat transfer begins, and the propagation starts, then the entire system continues like firecrackers, one after another. A model can be built to address this propagation, incorporating intermediate temperature rise rates, heat generation from chemical and electrical energy, heat transfer, and convection. The entire thermoelectric coupling model can be quantitatively analyzed using a calorimeter.
With a propagation model in place, researchers can design methods to block and suppress it, which requires adding a heat insulation layer. However, adding a heat insulation layer is not simple; on the one hand, it increases the volume, and on the other hand, the heat insulation layer and cooling are contradictory. These are all problems that need to be solved.
In summary, researchers need to focus on both safety protection design and battery management when it comes to thermal runaway propagation and suppression.
