The safety problem of power batteries is summed up as "thermal runaway", that is, after reaching a certain temperature, it becomes uncontrollable, the temperature rises linearly, and then it burns and explodes. Overheating, overcharging, internal short circuit, collision, etc. are several key factors that cause thermal runaway of power batteries.
(1) Overheating triggers thermal runaway
The reason for the overheating of the power battery comes from the unreasonable battery selection and thermal design, or the temperature rise of the battery caused by the external short circuit, the loosening of the cable connector, etc., which should be solved from two aspects of 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, so as to perform hierarchical alarms.
(2) Overcharge triggers thermal runaway
This year's pure electric bus fire incident was caused by "thermal runaway triggered by overcharge". Specifically, the battery management system itself lacked the safety function of the overcharge circuit, which caused the battery's BMS to be out of control but still being charged.
For this type of overcharge, the solution is to find the fault of the charger first, which can be solved by the full redundancy of the charger; secondly, it depends on whether the battery management is reasonable, for example, the voltage of each battery is not monitored.
It is worth noting that as the battery ages, the consistency between the batteries will get worse and worse, and overcharge is more likely to occur at this time. This requires balancing the entire battery pack to maintain battery pack consistency.
For example, a series-connected battery pack that adopts the most common battery pack combination method of "first in parallel and then in series", after solving the monomer consistency problem, the best case is to have the same capacity as the smallest capacity monomer. With this consistency, the capacity has rebounded, and at the same time, overcharging can be prevented.
In order to achieve consistency, there must be a way to estimate the capacity of each cell. Ouyang Minggao suggested that the state of the entire battery pack can be estimated based on the similarity of the charging curves.
In other words, as long as the charging curve of one of the single cells is known, the other curves should be similar to it. After the curve changes, they can approximately coincide, and these differences in the course of the curve change are easy to calculate. According to one monomer, other monomers can be calculated. With this method, the above-mentioned consistency balance can be carried out. Of course, this algorithm takes too long and needs to be simplified.
(3) Internal short circuit triggers thermal runaway
The Boeing 787 passenger plane caught fire due to a battery explosion. When searching for the cause of the accident, it was found that there were metal objects on the electrode and the diaphragm, which caused an internal short circuit. Although experts cannot 100% confirm that thermal runaway is triggered by an internal short circuit, it is the most probable cause because there is no other reason, and the internal short circuit cannot "emerge".
Battery manufacturing impurities, metal particles, expansion and contraction of charge and discharge, lithium evolution, etc. may all cause internal short circuits. This kind of internal short-circuit occurs slowly and takes a very long time, and it is not known when thermal runaway will occur. If it is tested, it cannot be repeated. At present, experts all over the world have not found a process that can repeat the internal short circuit caused by impurities, and they are all under study.
To solve the problem of internal short circuit, we must first find a battery manufacturer with good product quality, select the battery and battery cell capacity; secondly, make a safety prediction of the internal short circuit, and find the monomer with internal short circuit before thermal runaway occurs.
This means that the characteristic parameters of the monomer must be found, and the consistency can be started first. The battery is inconsistent, and the internal resistance is also inconsistent. As long as you find the monomer with variation in the middle, you can distinguish it.
Specifically, the equivalent circuit of a normal battery and the equivalent circuit of a micro-short circuit, the equation form is actually the same, except that the parameters of the normal cell and the micro-short cell have changed. You can study these parameters and see some of their characteristics in internal short-circuit changes.
One of the characteristics is the potential difference of the internal short-circuit monomer, comparing its internal resistance with other monomers. Ouyang Minggao proposed that R&D personnel should use models to identify monomers. After measuring the voltage and current of each cell, using these data and combining the model, the internal resistance of each cell can be estimated. After all the parameters of the monomer are estimated, according to the changes of the parameters, it can be judged whether the consistency has changed significantly.
(4) Mechanical trigger thermal runaway
Collision is a typical mechanical trigger for thermal runaway. Tesla's repeated fire accidents are the reason for this. Ouyang Minggao revealed that Tsinghua University and MIT have worked together to analyze Tesla's collision in the United States. If a collision simulation is performed in the laboratory, the closest thing is acupuncture.
The way to solve the thermal runaway triggered by collision is to do a good job of safety protection design of the battery. This requires R&D personnel to first understand the process of thermal runaway.
Generally speaking, after thermal runaway occurs, it will spread downward. For example, after the heat is out of control in the first section, heat will be transferred and spread, and then the whole group will follow one by one like firecrackers. For this kind of propagation, a model can be established, including the intermediate temperature rise rate, the heat generation of chemical energy and electric energy, and the heat transfer convection. The entire thermoelectric coupling model can be used for a related quantitative analysis with a calorimeter.
With the propagation model, R&D personnel can design how to block and suppress, which requires heat insulation. However, it is not simple to add a thermal insulation layer. On the one hand, the volume is thickened and large, and on the other hand, the thermal insulation layer and cooling are contradictory. These are all issues that need to be resolved.
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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