1. Introduction

1. When the battery is working normally, the pressure inside the battery box is discharged through the breathable membrane of the explosion-proof valve to balance the pressure inside and outside the box and prevent water, liquid and dust from entering,

2. Abnormal pressure occurs inside the box and increases sharply in a short period of time. When the pressure reaches the critical value set by the explosion-proof valve, the pressure spring of the explosion-proof valve instantly compresses backwards to connect the inside and outside of the battery box directly and discharge quickly. After the pressure is released, the spring automatically recovers and is completely sealed to reduce pressure damage to the box and internal electronic components.

2. Selection principles

1. Selection of opening pressure:

Generally speaking, the selection of opening pressure It depends on the voltage withstand capability of the battery pack and the gas production rate and temperature rise of the cell thermal runaway.

If the pressure failure pressure of the battery box is 20kpa, the opening pressure of the explosion-proof valve should take a safety factor, such as 80%, on the maximum pressure failure pressure of the box. Select the opening pressure 16kpa.

2. During normal operation, the selection of air permeability:

According to “8.2.8 Temperature Shock” in the national standard GB38031-2020 ” requirements, the battery pack needs to be placed in an alternating temperature environment of -40-60°C, and the conversion time between the two extreme temperatures is within 30 minutes. It is assumed here that the net air volume in the battery pack is 50L. According to the national standard requirements, in Within 30 minutes, when the battery pack heats up from -40°C to 60°C, the expansion of the gas volume is the ventilation required by the explosion-proof valve.

According to Guy-Lussac’s law of ideal gases, the pressure When constant, the volume of a certain mass of gas is proportional to the thermodynamic temperature.

That is:

V1/T1=V2/T2 =C(constant)(1)

V3=V2-V1(2)

Q=V3/t×1.5(3)</p >

In the formula: V1 is the volume of gas in the box (50L) at the initial temperature (-40℃); T1 is the initial temperature (-40+273)K; V2 is

60 The volume of gas in the box at ℃; T2 is the maximum temperature (60+273) K; V3 is the volume of gas generated after the temperature rises inside the box; Q is the air permeability; t is the temperature rise time, that is, the required exhaust time is 30min; 1.5 is the safety factor.

According to Equation (1)-Equation (3), the air permeability Q can be calculated to be equal to 1.07L/min｡

< strong>3. Flow selection after turning on:

Thermal runaway and thermal diffusion of lithium batteries are a complex failure phenomenon composed of multiple factors. In-depth study of the mechanism of thermal runaway and The phenomenon of battery core explosion is a very cumbersome project and requires a lot of test analysis. In order to facilitate the quantitative calculation here, the following assumptions are based on:

(1), cylindrical 18650 battery core (volume is 0.0165L) is selected;

(2), The explosion heat of a single cell spreads to the surrounding 6 cells and fails;

(3). The gas production rate of 7 cylindrical cells failing simultaneously is 25L/s;

(4), the exhaust rate of the through hole with a diameter of 10mm (area 78.5mm2) is 6L/s (under 24kPa pressure);

(5), battery core explosion The high temperature generated after thermal runaway causes air expansion 10 times the volume of the battery core.

Based on the above assumptions, the volume of 7 battery cells can be obtained as 0.0165×7≈0.12L. The explosion of the battery core causes The gas expansion amount is 0.12×10=1.2L, and the gas generated by the thermal runaway of the battery core is 25+1.2=26.2L/s. Therefore, the pressure relief amount of the battery pack explosion is 26.2L/s. According to assumption (4), it can be It is concluded that the required through hole area is 26.2/6×78.5=342.8mm2, and the through hole diameter can be calculated to be 20.9mm.

Based on the two parameters of air permeability and through hole diameter obtained above, select the specifications of the explosion-proof valve suitable for the actual project. It should be noted that the phenomenon of thermal runaway of lithium batteries is complicated. Variable, the selection of explosion-proof valve also needs to consider the material of the box itself, the working conditions of the battery pack, etc., and also needs to cooperate with simulation analysis and test data.

3. Installation position

The following requirements need to be paid attention to when installing the explosion-proof valve:

(1), because the explosion-proof valve will explode when the battery core explodes. It has the function of directional pressure relief and exhaust, so the discharged flammable and toxic gases should be kept away from the people in the vehicle, and there should be sufficient space to ensure gas circulation;

(2), in order to avoid water accumulation and In case of ash falling, etc., the explosion-proof valve should be arranged on the side of the battery box so that the explosion-proof valve is in a vertical or tilted state; Dust floating particles carried in the air will impact the breathable membrane of the explosion-proof valve. In order to prevent the breathable membrane and internal parts from being damaged by impact, the explosion-proof valve should be designed to be installed on the leeward side of the car’s driving direction and avoid installation into the wind.