The safety hazards of pure electric cars include high-voltage electric shock, power battery leakage and combustion, and other risks that may exist under special circumstances of the car.
(1) High voltage electric shock
The voltage and current levels of pure electric cars are relatively high. The voltage of the power battery is generally 300~600V, and the current can reach several hundred amperes during normal operation.
The level of the safe voltage that the human body can withstand depends on the current that the human body allows and the resistance of the human body. Human body electrogram is mainly composed of internal resistance, body surface resistance and body surface capacitance. The human body resistance varies in a wide range with different conditions, however, the human body resistance is generally not less than 1kΩ. The safety voltage in my country’s civil power grid is mostly 36V, which is roughly equivalent to the human body’s allowable current of 30mA (calculated when the human body resistance is 1200Ω).
In pure electric cars, the common form of electric shock to the human body is shown in Figure 1.
For high-voltage components in the system, if the interior is damaged or wet, an electromotive force may be transmitted to the housing. If there are two parts of such enclosures with different electromotive forces, a dangerous voltage will develop between the two enclosures, and there is a danger of electric shock to the human body if the hands touch these two parts.
The current threshold at which the human body does not feel anything is 2mA, which requires that if a person or other object constitutes an external circuit between the power battery system (or “high voltage” circuit) and ground, the leakage current cannot exceed 2mA in the worst case, i.e. When a person directly touches any point of the electrical system, the current flowing through the human body should be less than 2mA, so that the car insulation is considered qualified.
(2) Hidden danger of power battery safety
Taking the lithium battery widely used in electric cars as an example, the lithium battery will not have safety problems during normal use, but the improper use of the battery will lead to aggravation of the thermal effect of the battery, which is the fuse of the safety problem of the lithium battery. It is manifested as “thermal runaway” of the battery, resulting in a safety accident. Thermal runaway has the following situations.
①Overcharge and overdischarge. During car charging, especially at the end of battery charging, the concentration of ions inside the battery increases, the diffusion performance decreases, the concentration polarization increases, the battery acceptability decreases, and the battery will be overcharged when recharging. If the battery has good heat dissipation during overcharging, or the overcharge current is small, the temperature of the battery is low. After overcharging, only electro-hydraulic decomposition occurs, and the battery is still safe; if the battery has poor heat dissipation at this time, due to the high rate Charging leads to a high temperature of the battery and triggers a chemical reaction, which often poses a safety hazard.
The ability of the battery to provide a large current at the end of discharge decreases. When the remaining battery power is insufficient and a large current is required to discharge, the battery will be overdischarged. When overdischarge occurs, due to the reduction of lithium ions in the negative electrode of the battery, the desorption capacity decreases, and the polarization voltage increases, so it is easy to cause the active material of the negative electrode of the battery to fall off, and it is easy to cause an internal short circuit in the battery. The direct manifestation of a short circuit inside the battery is the rapid generation of heat that can cause a fire.
②Overcurrent. Lithium battery overcurrent mainly has the following situations.
a. Charge/discharge in low temperature environment. In a low temperature environment, due to the decrease in the conductivity and diffusivity of the battery, especially the lithium ion activity of the negative electrode of the battery, the ability of the battery to accept current decreases, which easily leads to overcurrent in the battery.
b. After the battery is aging and the performance of the battery is degraded (including capacity reduction, internal resistance increase, rate characteristic decrease, etc.), charging at the original current will easily lead to excessive relative current.
c. Batteries are grouped in parallel. In the process of parallel charging, due to the difference in battery consistency, the internal resistance of the single cells is different, and the charging current allocated to each single cell is different, which may cause the current allocated to some single cells to be much larger than the charging current.
d. Internal and external short circuits of the battery. A short circuit of the battery will generate a large current in an instant, and the internal temperature of the battery will rise sharply, resulting in safety accidents such as leakage and fire of the battery.
③The battery is overheated. In addition to the above-mentioned overcharge, overdischarge, and overcurrent that will cause the battery to overheat, the following conditions will also cause the battery to overheat.
a. The thermal management system of the battery fails. The main reason is that the battery temperature sensor in the power battery pack assembly is damaged, or the detection control circuit fails or the cooling fan is damaged.
b. Battery temperature sampling points are limited. There are a large number of batteries on the car, and it is difficult to detect the temperature of each single battery.
c. The temperature sampling location is limited. Due to the structure of the battery itself, the temperature sampling points of the battery management module of the electric car are generally on the positive and negative terminals of the battery, or the temperature of the battery shell is collected through the patch, which cannot reflect the actual internal temperature of the battery.
d. The working environment temperature is high. If the battery is near heat-generating components such as the drive motor or air compressor, it can cause the battery to overheat.
The hidden dangers caused by the increase of battery temperature include the gradual decline of the performance of the battery itself, which further aggravates the short circuit inside the battery. In addition, the high temperature of the battery itself will lead to thermal deformation of the battery, resulting in accidents such as leakage.
(3) Potential safety hazards under operating conditions
Due to the high voltage of pure electric cars, when an accident occurs during driving, if there is no good safety design, it is easy to have safety hazards. These safety hazards are mainly reflected in the following aspects.
①The high-voltage system is short-circuited. When the high-voltage circuit of the power system is short-circuited, it will cause the power battery to discharge a large current instantaneously. At this time, the temperature of the power battery and the high-voltage wiring harness will rise rapidly, which will lead to the burning of the power battery and the high-voltage wiring harness, and may also cause the battery to explode in severe cases. .
If the high-voltage bus of the power battery is short-circuited with the body, the occupant may touch the high-voltage power of the power battery, resulting in electric shock injury.
② A collision or rollover occurs. When an electric car collides or rolls over, the high voltage of the power system may be short, and the power system will instantly generate a large amount of heat, which may cause combustion or even explosion. In addition, high-voltage components may fall off, causing electric shock to the occupants. If the power battery is collided or the temperature is too high due to combustion, it may cause leakage of the battery electrolyte, causing injury to the occupants, and mechanical damage to the occupants in the event of a collision or overturning.
③Wade in water or encounter heavy rain. When the electric car is wading in water or encountering rainstorms and other working conditions, due to water vapor erosion, the insulation resistance between the positive electrode and the negative electrode of the high voltage may become smaller or even short-circuited, which may cause the battery to burn, leak or even explode. Through the car body, the occupants may be exposed to the risk of electric shock.
④ Unintentional movement of the car during charging. When the car is charging, if the car moves, the charging cable may be broken, putting the occupants and people around the car at risk of electric shock. risk.