CN110828935B - Safety protection method for lithium ion battery electric vehicle - Google Patents

Abstract

The invention is a safety protection method for a lithium-ion battery electric vehicle, which mainly includes a secondary response mode. The first-order response is the rapid cooling when the lithium-ion battery generates abnormal heat and the second-order response is the efficient fire extinguishing and cooling when the lithium-ion battery is thermally out of control. The primary and secondary responses use the same hardware design, mainly including pump sets, nozzles, refrigerants, temperature sensors, CO sensors, and control units. This method is based on the battery pack design of lithium-ion battery electric vehicles. It can aim at the different characteristics of the gestation and development of lithium-ion battery thermal runaway, through cooling before thermal runaway, fire extinguishing during thermal runaway, and cooling after fire extinguishing. All in one, multi-level and all-round protection for battery safety.

Description

A kind of lithium-ion battery electric vehicle safety protection method

technical field

The invention belongs to the technical field of safety, and relates to a safety protection method for an electric vehicle, in particular to a safety protection method capable of realizing a secondary response to the thermal runaway of a lithium-ion battery transferred to an electric vehicle.

Background technique

In recent years, with the emergence of environmental problems such as global warming and smog weather, the demand for new energy industries in various countries is increasing. The lithium-ion battery has become a leader in the new energy industry due to its high energy density and low self-discharge rate. At this stage, the annual constants of lithium-ion batteries and lithium-ion electric vehicles in my country are showing an upward trend year by year. However, due to the special properties of lithium-ion batteries, when the lithium-ion battery is in a state of abuse, it is very easy to form heat accumulation conditions, prompting an irreversible decomposition reaction inside the battery, releasing a large amount of heat and combustible gas in a short period of time. The gas is ignited, and the temperature of the battery shows an exponential increase, which means that the battery is considered to have thermal runaway. At this stage, electric vehicle fire accidents are showing an increasing trend year by year. Therefore, the safety protection of lithium-ion batteries assembled in electric vehicles is a technical problem that needs to be solved urgently. It is of great significance to configure efficient and intelligent security protection strategies on lithium-ion electric vehicles for the safe and efficient operation of electric vehicles.

At present, the safety protection strategies of lithium-ion battery electric vehicles are mostly focused on passive protection, while active protection strategies are less. The current lithium-ion battery safety protection strategies mainly have the following deficiencies: (1) Most of the existing lithium-ion battery safety protection strategies operate after the battery thermal runaway, which cannot effectively prevent or suppress the occurrence of thermal runaway. (2) Most of the existing safety protection methods aim at extinguishing open flames, and do not consider the characteristics that the battery surface is still at a relatively high temperature after the open flames are extinguished. That is, after the open flame of the battery is extinguished, there is still the possibility of thermal runaway transmission among the non-thermal runaway batteries of the same module. (3) Most of the fire extinguishing systems in the existing lithium-ion battery safety protection strategies only target a single battery module, and the open flame of the battery can easily spread to nearby normal working modules.

Aiming at the characteristics of thermal runaway of lithium-ion batteries, the present invention proposes a more intelligent, systematic and efficient safety protection method, that is, the safety protection method has a two-stage response, which can realize rapid external Cooling intervention; multi-position integrated safety protection design that quickly extinguishes the open flame when the battery is thermally out of control, and quickly cools down after the open flame is extinguished.

Contents of the invention

The object of the present invention is to provide a safety protection method for lithium-ion electric vehicles. During the incubation and development of lithium-ion battery thermal runaway, by implementing cooling before thermal runaway, fire extinguishing during thermal runaway, and cooling after fire extinguishing, etc. A variety of response strategies provide multi-level and all-round protection for battery safety. The problem solved by the present invention is to propose a safety protection method for lithium-ion battery electric vehicles, which intervenes in advance when abnormal heat production occurs in the early stage of battery thermal runaway, and takes away the heat of the battery through the refrigerant to prevent the generation of battery thermal runaway. Furthermore, when the thermal runaway of the battery occurs, the fire extinguishing medium is released to the ignition point through the nozzle, and the open flame of the battery is quickly extinguished. Finally, after the open flame of the battery is extinguished, the refrigerant is released again to take away the heat generated by the battery after the fire is extinguished, preventing thermal runaway from spreading between different batteries in the same module or even between different modules.

In order to solve the above problems and achieve the above objectives, the technical solution of the present invention is: a safety protection method for lithium-ion battery electric vehicles. This safety protection method mainly includes a two-level response mechanism, namely, the first-level response: rapid cooling when the lithium-ion battery produces abnormal heat; the second-level response: efficient fire extinguishing and rapid cooling when the lithium-ion battery is thermally out of control.

The first-order response is the rapid cooling of lithium-ion batteries when abnormal heat generation occurs. For the battery management system, the upper limit of its operating range is 60°C. Once the battery temperature exceeds the upper limit of the battery management system, the battery management system will reduce power or shut down. Therefore, the first-order response is mainly aimed at batteries that generate abnormal heat. The realization of the first-level response function mainly depends on hardware devices such as pumps, nozzles, refrigerants, temperature sensors, and control units.

Further, the selection of the pump should be determined based on the release pressure requirements of the refrigerant and the atomization requirements of the nozzle. The calculation of pump outlet pressure should be based on the following formula (1):

P＝H·ρ (1)

Among them, H is the lift of the pump, m; P is the outlet pressure, Pa; ρ is the density of the refrigerant, kg/m ³ .

Further, the nozzle should be an atomizing nozzle.

Further, in this method, both the refrigerant and the fire extinguishing medium are perfluorohexanone. Perfluorohexanone is a colorless and transparent liquid at room temperature, suitable for transportation and storage without risk of leakage. Perfluorohexanone has a large heat absorption and good insulation, which is very suitable for cooling batteries under normal working conditions. At the same time, the vapor pressure of perfluorohexanone is 12 times that of water, and it can quickly vaporize after release and spread to the fire extinguishing space, which is especially suitable for extinguishing fires in the cavity.

Furthermore, in order to make the temperature measurement more accurate, the temperature signal used in this part is jointly provided by the BMS system and the temperature sensor.

Further, in order to make the temperature measurement result more accurate, the temperature sensor is a contact temperature sensor. Its distribution point should be located at the bus bar of the battery.

Further, the control unit receives the signals from the temperature sensor and the BMS, and after judgment, sends a signal to the pump to control the release of the perfluorohexanone refrigerant. At the same time, a fault signal is sent to the BMS, requiring the driver to stop in time to evacuate passengers for inspection.

Further, the preset temperature for turning on is generally the temperature at which the SEI film decomposes. According to different material systems, the temperature at which the SEI film decomposes varies slightly. At this stage, it is recognized that the decomposition temperature of the SEI film is about 70-80°C. Therefore, in order to reduce irreversible damage to the battery and minimize the probability of false alarms, the opening temperature of the cooling system in this part is 80±10°C. The judgment conditions of this part are shown in the following formula (2):

[T _BMS ,T _sensor ] _max >80±10℃ (2)

Among them, T _BMS is the maximum battery temperature detected by the BMS system, ℃; T _sensor is the maximum battery temperature detected by the temperature sensor, ℃; [] _max operation is the maximum value operation;

Further, the amount of refrigerant used should be calculated according to the heat output of the battery. In this part, in order to save the cost of fire extinguishing agent, the calculation is based on the amount of fire extinguishing agent used in a single cell. As shown in the following formulas (3)-(4):

c·m·ΔT ₁ ＝Q _C6F12O (3)

Among them, c is the specific heat of the battery, generally 1.1kJ/kg °C; m is the mass of the battery, kg; ΔT ₁ is the temperature drop of the battery, here is 45 °C;

Q _C6F12O ＝Q ₁ +Q ₂ (4)

Wherein, Q _{C6F O} is the total heat absorbed by perfluorohexanone, _Q1 is the latent heat of perfluorohexanone, and _Q2 is the sensible heat of perfluorohexanone, and its calculation is calculated according to formulas (5)-(6) respectively:

Q ₁ =C·W·ΔT ₂ (5)

Q ₂ =γ·W (6)

Among them, C is the specific heat of perfluorohexanone, which is 1.013kJ/kg·℃; W is the amount of refrigerant used, kg; ΔT ₂ is the temperature rise of perfluorohexanone, which is 24.5℃ in this method. γ is the heat of vaporization of perfluorohexanone, which is 88.0 kJ/kg.

Furthermore, after the release of the refrigerant, the battery may be accompanied by a temperature recovery process. Therefore, this part is equipped with a feedback cooling mechanism. That is, after each cooling is completed, the controller will receive the temperature signal from the BMS. When the received temperature signal is greater than the preset value, the cooling system will start again until the temperature received by the controller is less than or equal to the preset value , the cooling system no longer works. The amount of refrigerant required in the feedback part will be introduced in the embodiment part. This feedback multiple cooling function can ensure the effectiveness of the cooling system and block thermal runaway in the incubation stage.

Furthermore, if for some battery systems, when the system temperature is higher than 60°C, the BMS system shuts down, at this time, a separate temperature monitoring device can be set up, and the power supply of the fire protection system can be used to replace the BMS system to realize the temperature monitoring function.

Furthermore, when the amount of perfluorohexanone released by the primary response exceeds 15% of the total design volume of the two-stage response, in order to prevent the insufficient amount of perfluorohexanone in the secondary response, the primary response does not perform feedback cooling. When the primary response fails, if the heat exchange conditions remain unchanged at this time, the battery temperature is likely to rise further, which will trigger a series of reactions inside the battery, and the battery temperature will continue to rise and produce a series of characteristic gases. The secondary response is triggered when the battery surface temperature and CO concentration reach the threshold of the secondary response.

The secondary response is efficient fire extinguishing and rapid cooling when the lithium-ion battery is thermally runaway. The secondary response mainly corresponds to the safety protection after the battery thermal runaway. The realization of the secondary response function mainly depends on hardware equipment such as pumps, nozzles, fire extinguishing media, BMS systems, temperature sensors, CO sensors, and control units.

Furthermore, the pumps, sprinklers, and fire extinguishing media used in the second-level response are the same as those in the first-level response. That is, in order to reduce the complexity of the system, the distinction of responses at all levels is implemented at the software level, and the responses at all levels use the same set of hardware control systems.

Further, the secondary response is triggered by temperature sensor, CO sensor and control unit.

Further, the temperature trigger signal is jointly provided by the temperature sensor and the BMS system. The temperature trigger signal mainly includes the absolute value of temperature and the rate of temperature rise.

Further, the absolute value of the trigger temperature of the secondary response is 150-200° C., and the temperature rise rate is 1.0° C./s. The absolute value of the thermal runaway temperature of batteries of different systems is different, and should be flexibly adjusted according to the actual situation. The triggering CO concentration of the secondary response depends on the experimental chamber.

Further, the amount of perfluorohexanone fire extinguishing agent in the module where the thermal runaway of the battery occurs should be determined according to the following method:

At the present stage, the extinguishing concentration of perfluorohexanone to extinguish the electrolyte fire of lithium-ion batteries is about 6.2% to 6.7%. According to the extinguishing concentration, the amount of fire extinguishing agent _W1 should be according to the regulations of NFPA2001 as follows (7):

W ₁ =(V/S)[E/(100-E)] (7)

Among them, E is the fire extinguishing concentration, %; V is the volume of the protected area, m ³ ; W is the amount of fire extinguishing agent, kg; S is the specific volume of the superheated steam of the fire extinguishing agent at 101kPa atmospheric pressure and the lowest ambient temperature in the protected area, m ³ /kg , which should be calculated according to the following formula (8):

S＝0.0664+0.0002741t (8)

Where, t is the minimum ambient temperature, °C.

Further, in this method, in order to leave a safety margin, the fire extinguishing concentration is designed to be 8%.

Further, after the secondary response is triggered, in order to prevent the spread of open battery flames among battery boxes with different temperatures, all battery boxes are sprayed with perfluorohexanone fire extinguishing agent at the same time, and the fire extinguishing concentration of each battery box is sprayed to 8%. According to the existing theory, when the concentration of perfluorohexanone reaches 8%, the cavity will not be able to support open flame combustion.

Further, it is assumed that after the fire extinguishing agent is released, the fire extinguishing concentration is reached, and the open flame disappears at this time. At this time, the fire extinguishing agent is released again in the module where the thermal runaway occurs, taking away the heat of the battery after fire extinguishing, speeding up the cooling of the battery after fire extinguishing, and preventing the battery from re-ignition and the spread of thermal runaway.

Further, each release line is equipped with a one-way valve to prevent the backflow of the fire extinguishing agent.

Furthermore, in order to prevent a large-scale fire from burning out the sub-controller, the one-way valve is a custom-made battery-controlled normally open one-way valve.

Further, the electromagnetic one-way valve is controlled by a sub-controller.

Among them, in order to reduce the electromagnetic interference generated during the working process of multiple solenoid valves, each battery valve is electromagnetically shielded and grounded.

Further, in the secondary response part, the amount of fire extinguishing agent can be calculated according to the following two methods.

Method 1: It can be calculated according to the balance between the heat production of the battery and the heat absorption of the fire extinguishing agent during the thermal runaway process. As shown in the following equation (9),

Q _t1 ＝Q _C6F12O ＝112.8185W ₂ (9)

Among them, Q _C6F12O is the heat absorbed by perfluorohexanone, kJ; Q _t1 is the heat released during the thermal runaway process of the battery, kJ; W ₂ is the amount of perfluorohexanone calculated according to the heat balance of thermal runaway heat production, kg.

Further, relative to the actual situation, since the reduction of heat production caused by the weakening of the internal reaction of the battery after the suppression of the thermal runaway of the battery by perfluorohexanone has not been considered, the _W calculated by this method is relative to the actual situation. big.

Method 2: It can be calculated based on the balance between the heat released when the peak temperature of the battery flame is suppressed and the temperature dropped to the desired temperature and the heat absorbed by the fire extinguishing agent. As shown in the following equation (10),

Q _t2 ＝Q _C6F12O ＝112.8185W ₃ (10)

Among them, Q _C6F12O is the heat absorbed by perfluorohexanone, kJ; Q _t2 is the heat released when the peak temperature that can be reached after the battery flame is suppressed drops to the desired temperature, kJ; W ₃ is calculated according to the heat balance of cooling and heat production The dosage of perfluorohexanone, kg.

Furthermore, in actual engineering applications, the amount of fire extinguishing agent used for cooling can be determined comprehensively based on the space reserved for the fire protection system, the cost of the fire protection system, and the battery system.

Further, the amount of fire extinguishing agent and refrigerant is determined by controlling the opening time of the pump. The pump opening time can be calculated by the following formula (11).

t=W/Q (11)

Among them, W is the amount of fire extinguishing agent, kg; Q is the mass flow rate of the pump, kg/s; t is time, s.

The advantages of the present invention are as follows: 1. The device effectively protects against thermal runaway in multiple stages such as the incubation and development of thermal runaway. The device can realize multi-level and all-round protection for the lithium-ion battery. 2. The device uses perfluorohexanone spray feedback to cool down before thermal runaway. The efficiency and success rate of cooling are greatly improved, and the abnormal temperature rise of the battery can be effectively controlled. In addition, the release of perfluorohexanone will not produce smoke and toxic gases, and will not cause damage to normal working batteries. While ensuring the normal operation of the system, it can prevent the panic of the passengers caused by the cooling system. 3. In the fire extinguishing stage, the device adopts the method of spraying all the nozzles in the battery box at the same time, which can effectively suppress the spread of thermal runaway among different battery boxes.

Description of drawings

Figure 1 is a flow chart of the safety protection strategy for the lithium-ion battery system of a passenger car;

Figure 2 is a schematic diagram of two-stage response control.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

When the temperature sensor in the battery box or the BMS system detects that the surface temperature of the battery exceeds 80±10°C, it is determined that the battery is abnormally heated, and the primary response is triggered. The BMS system will send an electrical signal to the main controller, which will then send the signal to each fractional controller and pumps via the CAN bus. Since all electromagnetic one-way valves are designed to be normally open, at this time, except for the sub-controller in the battery box where the temperature rises abnormally, the other sub-controllers will receive the signal from the main controller, and close the solenoid valve after action. , to prevent the fire extinguishing agent from being released to other battery boxes causing unnecessary losses and reducing cooling efficiency.

In the first-level response, in order to improve cooling efficiency, we designed a feedback cooling mechanism. That is, after each cooling is completed, the controller will receive the temperature signal from the BMS. When the received temperature signal is greater than the preset value, the cooling system will start again until the temperature received by the controller is less than or equal to the preset value , the cooling system no longer works. This feedback multiple cooling function can ensure the effectiveness of the cooling system and block thermal runaway in the incubation stage.

It is worth noting that in multiple feedback cooling, since the battery will return to a different temperature after each cooling, the amount of fire extinguishing agent required for each cooling is different. In conjunction with the above (3)-(6) formulas, the amount of fire extinguishing agent required for each cooling is shown in the following formula (12):

W _cool ＝c·m·(T _x -60)/(C·ΔT ₂ +γ) (12)

Among them, W _cool is the amount of fire extinguishing agent required for each cooling, kg; c is the specific heat capacity of the battery, kJ/kg °C; T _x is the temperature fed back by the BMS each time, and C is the specific heat capacity of the fire extinguishing agent, kJ/kg ·°C; ΔT ₂ is the temperature rise of perfluorohexanone, which is 24.5°C in this method. γ is the heat of vaporization of perfluorohexanone, which is 88.0 kJ/kg.

It is worth noting that, considering the amount of fire extinguishing agent required for feedback control, this part of the refrigerant is designed in excess. To simplify control, the dosing of extinguishing agent will depend on when the pump is on. The time at which the pump is turned on is as follows (13):

t＝∑W _cool /Q (13)

Among them, W _cool is the amount of cooling agent, kg; Q is the mass flow rate of the pump, kg/s. t is time, s.

When the first-level response cannot effectively suppress the rise in battery temperature, the battery temperature will rise further until the safety valve opens or thermal runaway occurs. At this time, a second-level response is required, that is, rapid fire extinguishing and cooling after the battery thermal runaway. If the primary response cannot effectively block the temperature rise of the battery and the battery temperature rises rapidly, the abnormal temperature rise may continue to develop into thermal runaway.

After the battery is thermally out of control, the battery releases a large amount of high-temperature smoke, and it is very likely to form a jet fire. At this time, the battery may release a large amount of CO gas, and the battery surface temperature shows an exponential rise, and the absolute value and temperature rise rate of the battery surface temperature are kept at a very high level. Therefore, we use this characteristic of battery thermal runaway to trigger a secondary response.

In this method, the CO concentration auxiliary battery temperature and temperature rise rate changes are used as the trigger signal of the secondary response. In order to prevent CO from burning out during thermal runaway, the absolute value of temperature and the rate of temperature rise are used as the main trigger signals in this method, even if the CO concentration does not reach the threshold, the secondary response will be triggered. The reason for using the CO concentration auxiliary temperature absolute value and the temperature rise rate as the trigger signal is to advance the warning time.

After the secondary response is triggered, the controller will issue a command to the pump and the pump will start. After _t2 , the controller sends a signal to all sub-controllers except the sub-controller of the abnormal battery pack, the solenoid valve receives the signal from the sub-controller, and the normally open battery valve closes. _t2 can be calculated according to the following formula (14):

t ₂ =(n*W ₁ )/Q (14)

Among them, n is the total number of battery packs; W ₁ is the amount of fire extinguishing agent in a single battery pack calculated according to the fire extinguishing concentration, kg; Q is the mass flow rate of the pump, kg/s. t is time, s.

This action has two main meanings. First, it prevents the high-temperature and high-pressure gas from the thermally runaway battery pack from rushing to other battery packs that are not out of control, causing the related batteries to go out of control. Second, it can prevent the unnecessary loss of perfluorohexanone .

This embodiment is only an exemplary description of the present invention, and does not limit its protection scope. Some improvements and modifications made by those skilled in the art without departing from the principle of the present invention should be regarded as the protection scope of the present invention.

Claims (2)

1. A safety protection method for an electric vehicle with a lithium ion battery is characterized by comprising the following steps: the safety protection method mainly comprises two-stage response, namely rapid cooling when the lithium ion battery abnormally generates heat and efficient fire extinguishing and rapid cooling when the lithium ion battery is out of control in heat;

the first-level response is rapid cooling when the lithium ion battery abnormally generates heat; the primary response has the main functions of quickly releasing a high-efficiency cooling refrigerant when the battery is abnormally heated, taking away the heat of the battery and preventing the battery from being abnormally heated and developing thermal runaway; the first-stage response hardware device mainly comprises a pump set, a spray head, a refrigerant, a temperature sensor and a control unit;

the secondary response is high-efficiency fire extinguishing and rapid cooling when the lithium ion battery is out of control due to heat; the secondary response mainly has the advantages that when the battery is out of control due to thermal runaway, the battery open fire is controlled in a short time, and the damage of battery jet fire to a battery system is reduced; on one hand, fire extinguishing medium is released in other battery modules which are not out of control, so that the propagation and spread of open fire in the battery compartment are prevented; on the other hand, after the open fire of the battery is extinguished, the high-efficiency cooling medium is released, the battery after the fire is extinguished is rapidly cooled, and the thermal runaway is prevented from spreading among different batteries and even among different modules in the module; the hardware device for the secondary response comprises a spray head, a storage tank, a conveying pipeline, a fire extinguishing medium, a temperature sensor, a CO sensor and a control unit;

the refrigerant and the fire extinguishing medium used in the method are all perfluorohexanone; the perfluorohexanone absorbs a large amount of heat after being released;

the amount of the refrigerant is calculated according to the heat production quantity of the battery and is calculated aiming at the amount of the fire extinguishing agent of the single battery cell, and the amount is shown as the following formula 3-4:

c·m·∆T ₁ = Q _C6F12O formula 3

Wherein c is the specific heat of the battery and is 1.1kJ/kg DEG C; m is the mass of the battery, and the unit is kg; Δ T ₁ The temperature of the battery is reduced, here to 45 ℃;

Q _C6F12O = Q ₁ + Q ₂ formula 4

Wherein Q is _C6F12O Total heat absorbed for perfluorohexanone, Q ₁ Is the latent heat of perfluorohexanone, Q ₂ The sensible heat of perfluorohexanone was calculated according to formulas 5 to 6, respectively:

Q ₁ = C·W·∆T ₂ formula 5

Q ₂ = gamma.W type 6

WhereinC is the specific heat of the perfluorohexanone and is 1.013kJ/kg DEG C; w is the amount of refrigerant in kg; Δ T ₂ Is the temperature rise of perfluorohexanone, and in the method, the temperature is 24.5 ℃, and gamma is the heat of vaporization of perfluorohexanone and is 88.0kJ/kg.

2. The safety protection method for the lithium ion battery electric vehicle according to claim 1, characterized in that: the amount of perfluorohexanone fire extinguishing agent in the module where thermal runaway of the battery occurs should be determined according to the following method:

the fire extinguishing concentration of the perfluorohexanone for extinguishing the lithium ion battery electrolyte fire at the present stage is 6.2% -6.7%, and according to the fire extinguishing concentration, the dosage W of the fire extinguishing agent ₁ The following formula 7 should be specified according to NFPA 2001:

W ₁ =(V/S)[E/(100-E)]formula 7

Wherein E is the extinguishing concentration; v is the volume of the protected zone in m ³ (ii) a W is the amount of the fire extinguishing agent in kg; s is the specific volume of the fire extinguishing agent superheated steam under the atmospheric pressure of 101kPa and the environmental temperature of the protection area, and the unit is m ³ Kg, which should be calculated according to the following formula 8:

s =0.0664+0.0002741t formula 8

Where t is the ambient temperature in degrees Celsius.

Images