With the rapid development of new energy vehicle industry, the power battery of electric vehicles is prone to Thermal runaway due to its own characteristics and use environment. Thermal runaway will cause thermal damage to the power battery, leading to auto ignition and even explosion. Therefore, it is of great significance to analyze the causes of Thermal runaway of electric vehicle power batteries and propose preventive measures. At present, the main reasons for Thermal runaway of electric vehicle power battery are as follows:

1) overcharge, overdischarge and overload;

2) External short circuit, internal short circuit;

3) Reduced insulation;

4) Thermal runaway of electric core.

Among them, internal short circuit and Thermal runaway of battery cell are the main reasons for Thermal runaway of electric vehicle power battery. In order to reduce the Thermal runaway of electric vehicle power battery, this paper analyzes from four aspects: overcharge, over discharge, overload, external short circuit and Thermal runaway of battery cell, and proposes corresponding solutions for these four aspects.

1 Overcharge, Discharge, Overload

The power battery will generate a lot of heat during charging, discharging and parking, and the heat accumulation will lead to Thermal runaway. Therefore, it is necessary to do a good job in the management of the power battery to avoid overcharging, over discharging and overload.

Overcharging: During the use of the electric vehicle power battery, if the charging current or voltage is too high for a long time, the internal pressure of the battery will be too large. When the pressure reaches a certain level, the internal temperature of the battery will rise, and eventually the heat will be out of control. So it is necessary to strictly control the charging current and voltage to avoid exceeding the safe range.

Therefore, it is necessary to strictly control the usage time of electric vehicle power batteries to avoid excessive discharge for a long time.

Overload: if the power battery of electric vehicle is overloaded for a long time during use, it will also cause Thermal runaway.

2 External short circuit, Internal short circuit

There are generally several types of short circuit faults in the power battery of electric vehicles:

The short circuit between the positive and negative poles of a battery is usually caused by overcharging and discharging. During the overcharging or discharging process of the battery, if it cannot be charged in a timely manner, the internal resistance of the battery will increase, which will cause the internal temperature of the battery to rise.

When the temperature rises to a certain degree, the electrolyte will decompose into gas, causing gas expansion and explosion. When the power battery is short circuited, a large amount of heat will be generated, which will lead to the rapid rise of the internal temperature of the battery, thus causing Thermal runaway.

The short circuit fault of a single battery in the battery pack did not cause Thermal runaway of the whole battery pack. In practical work, sometimes the overall Thermal runaway of the battery pack may be caused by local short circuit. When there is an external short circuit in a single battery, it may have an impact on other single batteries inside the battery; When an internal short circuit occurs in the battery pack, it may affect the safety of the entire battery pack; When an external short circuit occurs in the battery pack, it may cause the entire battery pack to ignite and burn.

3 Insulation Degradation

Insulation performance is one of the important indicators to evaluate the safety of electric vehicle power batteries. The decline of insulation performance refers to the loss of Galvanic isolation ability of insulating materials from surrounding media due to mechanical damage. The decline of insulation performance leads to a short circuit in the power battery of the electric vehicle, which generates heat. When the heat reaches a certain level, the heat will spread along the line to the outside, eventually leading to Thermal runaway of the power battery.

After Thermal runaway of the battery, if the battery insulation drops to a certain extent, the electric spark generated by the external short circuit may break through the internal insulation material, causing the power battery to explode. Therefore, in order to prevent the power battery from explosion and combustion accidents after Thermal runaway, it is necessary to insulate the power battery to prevent internal short circuit. The commonly used method at present is to insert a conductive rubber pad between the positive and negative poles of the power battery for insulation treatment.

4 Cell’s Thermal Runaway

The Thermal runaway of the battery cell refers to the short circuit of the battery cell, which causes the battery cell to heat and ignite. The Thermal runaway of the electric core can be divided into two categories, one is the Thermal runaway of the electric core caused by external short circuit, and the other is the Thermal runaway of the electric core caused by internal short circuit of the internal electric core.

At present, there are many products on the market that are used for power battery cooling systems. As shown in Figure 8, these products generally use external cooling systems. For electric vehicles, these products cannot effectively cool the power battery, so it is difficult for these products to effectively cool the power battery in case of Thermal runaway of the power battery of electric vehicles.

5 Conclusion

(1) The Thermal runaway of the electric vehicle power battery is due to the fact that the heat generated inside the battery cannot be dissipated in time, which makes the internal temperature rise beyond its tolerance temperature, thus causing Thermal runaway. The Thermal runaway problem of the power battery should be considered in the design phase of the electric vehicle power battery.

(2) According to the causes of Thermal runaway of electric vehicle power battery, measures can be taken from the design stage and the production stage to prevent it, that is, according to the working characteristics and service conditions of the power battery, the safety protection system is designed on the power battery of electric vehicle during design, and standards are formulated for management in the production stage, so as to effectively prevent Thermal runaway of the power battery.

(3) The methods described in this paper can effectively prevent traffic accidents and fire accidents caused by the Thermal runaway of electric vehicle power batteries, thus protecting the safety of people’s lives and property.

With the rapid development of electric vehicles, the safety accidents caused by Thermal runaway of electric vehicle power batteries are gradually increasing. The Thermal runaway of power batteries will not only threaten the safety of electric vehicle drivers, passengers and property, but also threaten the life safety of people around electric vehicles. Therefore, in case of Thermal runaway of power battery, in order to prevent safety accidents caused by Thermal runaway of power battery and protect people’s life and property safety, measures should be taken as soon as possible to prevent Thermal runaway of power battery. In addition, users of new energy vehicles should try their best to avoid parking in a closed space for a long time, so as to avoid Thermal runaway caused by excessive temperature inside the vehicle due to long-term parking in a closed space.