Analysis of several factors causing thermal runaway of power battery

The safety problem of power cells is summed up as "heat out of control", that is, after reaching a certain temperature, it is uncontrollable, the temperature rises in a straight line, and then it will burn and explode. Overheat, overcharge, internal short circuit, and collision are the key factors that cause thermal runaway of power cells.

(1) Overheat triggers fever out of control

The reason for the overheating of power cells comes from the unreasonable selection of batteries and thermal design, or the increase in the temperature of batteries due to external short circuits, the loosening of cables, etc.. It should be solved from two aspects of battery design and battery management.

From the point of view of battery material design, materials can be developed to prevent thermal runaway and block thermal runaway reactions; From the point of view of battery management, different temperature ranges can be predicted to define different safety levels, and hierarchical alarms can be performed.

(2) Overcharge touching heat out of control

A pure electric bus fire this year was caused by "overcharge-triggered thermal runaway". Specifically, the battery management system itself lacks the safety of overcharged circuits, causing the battery's BMS to be out of control but still charging.

For the reasons of overcharging, the first solution is to find the failure of the charging machine, which can be solved by the full redundancy of the charging machine; The second is to see that battery management is unreasonable, such as not monitoring the voltage of each cell.

It is worth noting that as the battery ages, the consistency between the various batteries will get worse and worse, and overcharging is more likely to occur. This requires the entire battery pack to be balanced to maintain battery consistency.

For example, a series battery with the most common battery combination method of "first and last", after solving the problem of monomer consistency, the best case is to have the same capacity as the smallest capacity monomer. With this consistency, capacity rises, but also prevents overloading.

In order to achieve consistency, there must be a method to estimate the capacity of each monomer. Ou yangming suggested that the state of all batteries can be estimated based on the similarity of the charging curve.

That is, as long as you know the charging curve of one of the monomer batteries, the other curves should be similar to it. After the curve changes, they can approximately coincide, and these differences in the process of curve changes are easy to calculate. According to one monomer, other monomers can be calculated. With such an approach, the consistency equilibrium mentioned above can be achieved, although this algorithm is too long and needs to be simplified.

(3) Internal short circuit trigger fever out of control

The Boeing 787 was caught in a battery explosion. In the search for the cause of the accident, metal was found on the electrodes and diaphragm, resulting in an internal short circuit. Although experts can not confirm 100 % that thermal runaway is triggered by an internal short circuit, it is the most likely reason because no other reason can be found and the internal short circuit can not be "emerged."

Battery manufacturing impurities, metal particles, shrinkage of charging and discharging expansion, lithium analysis, etc. may cause internal short circuits. This kind of internal short circuit occurs slowly, for a very long time, and it is not known when it will develop heat out of control. If the test is conducted, it can not be repeated. At present, experts around the world have not yet found a process that can repeat the internal short circuit caused by impurities. They are all under study.

To solve the problem of internal short circuit, we must first find a battery manufacturer with good product quality and choose battery and battery single capacity; Secondly, safety prediction of internal short circuit is carried out, and the monomer with internal short circuit must be found before heat runaway occurs.

This means that the characteristic parameters of the monomer must be found, starting with consistency. The battery is inconsistent and the internal resistance is inconsistent. As long as a mutated monomer is found in the middle, it can be identified.

Specifically, the equivalent circuit of a normal battery and the equivalent circuit with a micro-short circuit occur. The form of the equation is actually the same, except that the parameters of the normal monomer and the micro-short-circuit monomer have changed. These parameters can be studied to see some of their characteristics in the internal short-circuit changes.

One of the characteristics is the potential difference of the short-circuit monomer, and the difference between its internal resistance and other monomers is compared. Ou yangming proposed that R&D personnel use models to identify monomers. After measuring the voltage and current of each monomer, using these data and combining the model, the internal resistance of each monomer can be estimated. After all the parameters of the monomer are estimated, according to the changes in the parameters, it can be judged whether the consistency of the monomer has undergone significant changes.

(4) Mechanical contact heat out of control

Collision is a typical way for a mechanical trigger to get out of control. Tesla's repeated fire accidents are the reason. Ou yangming revealed that Tsinghua University and MIT worked together to analyze Tesla's collision in the United States. If a simulation of a collision takes place in the laboratory, the closest is the acupuncture.

The way to solve the runaway collision heat is to design the safety protection of the battery. This requires developers to understand the process of thermal runaway.

In general, after heat loss occurs, it will spread downwards. For example, in the first quarter, heat transfer will occur after the heat is out of control, and it will begin to spread, and then the whole group will follow one by one like a firecracker. For this kind of propagation, a model can be established, including intermediate temperature increase rate, heat generation of chemical energy, heat transfer and convection. The whole thermoelectric coupling model can be used to do a related quantitative analysis with a heat meter.

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

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

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