Analysis of the causes of thermal runaway of four types of power CR2032 battery: R&D personnel have a long way to go

The safety problem of power batteries can be summarized as "thermal runaway", which means that after reaching a certain temperature, it is out of control, the temperature rises sharply, and then it will burn and explode. Overheating, overcharging, internal short circuit, collision, etc. are several key factors that cause thermal runaway of power batteries.

(1) Overheating triggers thermal runaway

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

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

(2) Overcharging triggers thermal runaway

The cause of a pure electric bus fire this year was "thermal runaway triggered by overcharging". Specifically, the battery management system itself lacks circuit safety functions for overcharging, resulting in the battery BMS being out of control but still charging.

For this type of overcharging, the solution is to first find the fault of the charger, which can be solved by full redundancy of the charger; the second is to see whether the battery management is reasonable, such as not monitoring the voltage of each battery.

It is worth noting that as the battery ages, the consistency between the batteries will become worse and worse, and overcharging is more likely to occur. This requires balancing the entire battery pack to maintain the consistency of the battery pack.

For example, the series battery pack adopts the most common battery pack combination method of "parallel first and then series", and after solving the problem of single cell consistency, the best case is to have the same capacity as the smallest single cell. With this consistency, the capacity has recovered and overcharging can also be prevented.

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

That is to say, as long as the charging curve of one single cell is known, the other curves should be similar to it. After the curve changes, they can overlap approximately, and these differences in the process of curve change are easy to calculate. Based on one single cell, other cells can be inferred. With this method, the consistency balance mentioned above can be performed. Of course, this algorithm takes too long and needs to be simplified.

(3) Internal short circuit triggers thermal runaway

A Boeing 787 passenger plane once caught fire due to a battery explosion. When looking for the cause of the accident, it was found that there were metal objects on the electrodes and diaphragms, which caused 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 because no other causes can be found and the internal short circuit cannot "emerge".

Battery manufacturing impurities, metal particles, charge and discharge expansion and contraction, lithium precipitation, etc. may all cause internal short circuits. This type of internal short circuit occurs slowly, takes a very long time, and it is unknown when it will cause thermal runaway. If an experiment is conducted, it cannot be verified repeatedly. At present, experts around the world have not found a process that can repeat the internal short circuit caused by impurities, and they are all under research.

To solve the internal short circuit problem, first find a battery manufacturer with good product quality and select the battery and battery cell capacity; secondly, make a safety prediction for the internal short circuit and find a cell with an internal short circuit before thermal runaway occurs.

This means that it is necessary to find the characteristic parameters of the cell, and we can start with consistency. Batteries are inconsistent, and internal resistance is also inconsistent. As long as we find a cell with variation in the middle, we can identify it.

Specifically, the equivalent circuit of a normal battery and the equivalent circuit of a micro-short circuit are actually the same in the form of the equation, except that the parameters of normal cells and micro-short circuit cells have changed. We can study these parameters to see some characteristics of their changes in internal short circuits.

One of the characteristics is the potential difference of the internal short circuit cell, comparing its internal resistance with other cells. Ouyang Minggao proposed that R&D personnel should use models to identify cells. After measuring the voltage and current of each cell, these data combined with the model can be used to estimate the internal resistance of each cell. After all the parameters of the cells are estimated, based on the changes in the parameters, it can be determined whether their consistency has changed significantly.

(4) Mechanically triggered thermal runaway

Collision is a typical way of mechanically triggered thermal runaway. This is the reason why Tesla has repeatedly caught fire. Ouyang Minggao revealed that Tsinghua University and MIT have worked together to analyze Tesla's collision accidents in the United States. If a collision simulation is performed in the laboratory, the closest thing is a needle puncture.

The solution to collision-triggered thermal runaway is to do a good job of battery safety protection design. And this requires R&D personnel to first understand the occurrence process of thermal runaway.

Generally speaking, after thermal runaway occurs, it will spread downward. For example, after the first section of thermal runaway, there will be heat transfer, which will begin to spread, and then the whole group will follow one by one like setting off firecrackers. For this kind of propagation, a model can be established, including the intermediate temperature rise rate, heat generation of chemical energy and electrical energy, heat transfer convection, etc. The entire thermoelectric coupling model can be used to make a relevant quantitative analysis.

With the propagation model, R&D personnel can design how to block and suppress it, which requires adding a thermal insulation layer. However, adding a thermal insulation layer is not simple. On the one hand, the thickening makes the volume larger, and on the other hand, the thermal insulation layer is contradictory to cooling. These are all problems that need to be solved.

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

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