What is the reason for thermal runaway of power batteries ?

The safety issue of power batteries can be summarized as "thermal runaway", which means that after reaching a certain temperature, it becomes uncontrollable, the temperature rises sharply, and then it will burn and explode. Overheating, overcharging, internal short circuits, collisions, etc. are several key factors that can cause thermal runaway of power batteries.

(1) Overheating leads to overheating and loss of control

The reasons for overheating of power batteries come from unreasonable battery selection and thermal design, or from external short circuits causing the temperature of the battery to rise, loose cable joints, etc. It should be solved from two aspects: battery design and battery management.

From the perspective of battery material design, materials can be developed to prevent thermal runaway and block the reaction of thermal runaway; From the perspective of battery management, different temperature ranges can be predicted to define different safety levels for graded alarms.

(2) Overcharging causes overheating and loss of control

The cause of this year's pure electric bus fire incident was "thermal runaway triggered by overcharging", specifically due to the lack of circuit safety function for overcharging in the battery management system itself, resulting in the BMS of the battery being out of control but still charging.

The first solution to this type of overcharging is to identify the fault in the charger, which can be resolved through complete redundancy of the charger; Secondly, it depends on whether the battery management is reasonable, such as not monitoring the voltage of each battery cell.

It is worth noting that as the battery ages, the consistency between different batteries will deteriorate, making overcharging more likely to occur. This requires balancing the entire battery pack to maintain consistency.

For example, a series connected battery pack using the most common battery pack combination method of "parallel first, then series", after solving the problem of cell consistency, the best situation is to have the same capacity as the smallest capacity cell. With this consistency, the capacity has increased while also preventing overcharging.

In order to achieve consistency, there must be a method to estimate the capacity of each individual cell. Ouyang Minggao suggested that the state of the entire battery pack can be estimated based on the similarity of charging curves.

That is to say, as long as the charging curve of one individual battery is known, the other curves should be similar to it. After the curve changes, they can approximately coincide, and the differences in the process of curve changes can be easily calculated. Based on one monomer, other monomers can be inferred. With this method, the consistency balance mentioned earlier can be achieved, but of course, this algorithm takes too long and needs to be simplified.

(3) Internal short circuit leads to overheating and loss of control

The Boeing 787 aircraft once caught fire due to a battery explosion. When investigating the cause of the accident, it was found that there were metal objects on the electrode and diaphragm, resulting in an internal short circuit. Although experts cannot 100% confirm that thermal runaway was triggered by an internal short circuit, it is the most likely cause as no other reasons can be found and the internal short circuit cannot 'emerge'.

Impurities in battery manufacturing, metal particles, shrinkage due to charging and discharging expansion, lithium deposition, etc. can all cause internal short circuits. This type of internal short circuit occurs slowly, for a very long time, and it is unknown when it will cause thermal runaway. If the experiment is conducted, it cannot be repeated for verification. At present, experts around the world have not found a process that can replicate internal short circuits caused by impurities, and are all studying it.

To solve the problem of internal short circuit, the first step is to find a battery manufacturer with good product quality, select the battery and battery cell capacity; Secondly, safety prediction should be made for internal short circuits, and before thermal runaway occurs, the individual units with internal short circuits should be identified.

This means that it is necessary to find the characteristic parameters of the monomer, and we can start with consistency. Batteries are inconsistent, and their internal resistance is also inconsistent. As long as the individual cells with variations in the middle are found, they can be identified.

Specifically, the equivalent circuit of a normal battery and the equivalent circuit of a micro short circuit have the same equation form, except that the parameters of the normal cell and the micro short circuit cell have changed. Research can be conducted on these parameters to observe some characteristics of their internal short circuit variations.

One of the features is the potential difference of the internal short-circuit unit, comparing its internal resistance with the differences of other units. Ouyang Minggao proposed that R&D personnel should use models to identify individual units. After measuring the voltage and current of each individual cell, these data can be combined with a model to estimate the internal resistance of each cell. After estimating all the parameters of the monomer, based on the changes in the parameters, it can be determined whether there has been a significant change in its consistency.

(4) Mechanical thermal runaway

Collision is a typical way of mechanical contact causing thermal runaway. Tesla's repeated fire accidents are due to this reason. Ouyang Minggao revealed that Tsinghua University and MIT have collaborated to analyze Tesla's collision accidents in the United States. If a collision simulation is conducted in the laboratory, the closest approach is needle puncture.

The solution to solve the problem of thermal runaway caused by collision is to design a safety protection system for the battery. And this requires R&D personnel to first understand the process of losing control of fever.

Generally speaking, after thermal runaway occurs, it will spread downwards. For example, after the first thermal runaway, there will be heat transfer that begins to spread, and then the whole group will follow one by one like setting off firecrackers. A model can be established to address this type of propagation, including the rate of intermediate temperature rise, heat generation from chemical and electrical energy, and convective heat transfer. The entire thermoelectric coupling model can be quantitatively analyzed using a calorimeter.

With the propagation model, researchers can design how to block and suppress it, which requires adding insulation layers. However, adding an insulation layer is not simple. On the one hand, it thickens the volume, and on the other hand, the insulation layer is contradictory to cooling. These are all issues that need to be addressed.

In short, in terms of thermal runaway expansion and suppression, R&D personnel need to start from two aspects: safety protection design and battery management.