The rapid global adoption of electric vehicles (EVs) represents a pivotal shift in transportation technology, aligning with strategic energy goals and the advancement of high-tech manufacturing. While EVs offer significant benefits such as reduced noise, lower emissions, and higher energy efficiency, their safety, particularly concerning lithium-ion power batteries, has become a paramount concern. Incidents involving thermal runaway, leading to fires or even explosions, pose severe risks to occupants, emergency responders, and property. Statistical data indicates a notable increase in fire incidents associated with EV battery packs, underscoring the urgent need for reliable, integrated safety solutions. This paper delves into the design and simulation of an automatic fire suppression system specifically tailored for the EV battery pack. The proposed system prioritizes safety, cost-effectiveness, and ease of secondary installation, offering a multi-trigger control strategy to mitigate fire risks proactively and prevent secondary injuries, especially in collision scenarios where occupants may be incapacitated.
1. Thermal Runaway Mechanism in the EV Battery Pack
The fundamental safety challenge for the EV battery pack stems from the phenomenon of thermal runaway. A lithium-ion cell operates safely under a state of thermal equilibrium, where the heat generated internally is effectively dissipated to the environment. Thermal runaway initiates when this balance is disrupted, leading to an uncontrolled increase in temperature. This process is often exothermic and self-accelerating.
The core energy balance for a cell can be expressed as:
$$ m c_p \frac{dT}{dt} = Q_{\text{gen}} – Q_{\text{diss}} $$
where:
- $m$ is the mass of the cell,
- $c_p$ is the specific heat capacity,
- $\frac{dT}{dt}$ is the rate of temperature change,
- $Q_{\text{gen}}$ is the total heat generation rate, and
- $Q_{\text{diss}}$ is the heat dissipation rate to the surroundings.
When $Q_{\text{gen}} > Q_{\text{diss}}$, temperature rises. The heat generation term $Q_{\text{gen}}$ itself is a strong function of temperature due to various exothermic decomposition reactions. A critical model for this self-heating is the Arrhenius relationship. The rate of a key exothermic side reaction (e.g., SEI layer decomposition) can be modeled as:
$$ k = A \exp\left(-\frac{E_a}{RT}\right) $$
where $k$ is the reaction rate constant, $A$ is the pre-exponential factor, $E_a$ is the activation energy, $R$ is the universal gas constant, and $T$ is the absolute temperature. As $T$ increases, $k$ increases exponentially, leading to a rapid spike in $Q_{\text{gen}}$, which further increases $T$—a classic positive feedback loop.
Primary causes that trigger this imbalance in the EV battery pack include:
- Internal Short Circuit: Caused by mechanical deformation from collisions, lithium dendrite growth from overcharging, or manufacturing defects. This creates a low-resistance path, generating intense localized heat ($Q_{\text{gen}} \propto I^2 R_{short}$).
- Electrical Abuse: Overcharging or over-discharging can lead to destabilization of electrode materials and electrolyte decomposition.
- Thermal Abuse: External heating or failure of thermal management systems.
- Component Failure: Poor electrical connections, faulty wiring, or BMS failures leading to localized heating.
Once initiated, thermal runaway in one cell can propagate to neighboring cells within the EV battery pack through heat transfer, leading to a cascading failure. This process also produces large volumes of flammable and toxic gases (smoke), which can ignite, causing a fire or explosion. Therefore, early detection and rapid suppression are critical for the safety of the EV battery pack.
2. Design Scheme for the Automatic Fire Extinguishing Device
The proposed design for safeguarding the EV battery pack emphasizes independence, reliability, and non-invasiveness for aftermarket installation. The core philosophy is to create a dedicated safety system that does not rely on the vehicle’s main Battery Management System (BMS) or airbag control unit, ensuring functionality even if primary systems are compromised.
2.1 System Components and Architecture
The system comprises several key modules, as outlined in the table below:
| Module | Component | Function & Specification |
|---|---|---|
| Sensing Module | Temperature Sensors | Multiple, strategically placed within the EV battery pack enclosure. Trigger thresholds: Warning (~90°C), Fire (~130°C). |
| Smoke Sensors | Detects electrolyte vapor and combustion aerosols generated during thermal runaway in the EV battery pack. | |
| Collision Sensors (G-sensors) | Multiple, installed on the vehicle body/frame near the EV battery pack to detect impact forces. | |
| Control Module | Fire Extinguishing ECU | Dedicated microcontroller unit. Processes all sensor inputs, implements the control logic, and actuates outputs. |
| Manual Override Button | Located inside the cabin for driver-initiated suppression. Requires a long-press (e.g., 3s) for activation to prevent accidental discharge. | |
| Actuation & Alarm Module | Extinguishing Agent Storage Tank | Contains a clean agent suitable for lithium-ion fires (e.g., FK-5-1-12, aerosol). |
| Electrically Operated Valve (E-Valve) | Controls the release of the extinguishing agent into the EV battery pack. | |
| Interior Alarm (Visual/Audible) | Alerts the driver to a potential thermal event in the EV battery pack. | |
| External High-decibel Alarm/Speaker | Broadcasts a pre-recorded distress message (“Occupants inside, please call for help!”) in collision scenarios. | |
| Delivery System | Flame-retardant Pipes & Nozzles | Network of pipes and spray nozzles to distribute the extinguishing agent evenly within the EV battery pack enclosure. |
2.2 Installation Strategy for the EV Battery Pack
A significant design consideration is minimizing modification to the existing EV battery pack to ensure safety (water ingress protection) and reduce cost. The proposed method involves:
- Internal Mounting: Temperature sensors, smoke sensors, and spray nozzles are mounted inside the EV battery pack enclosure using adhesive clips or brackets, requiring no drilling on the pack casing itself for these components.
- Single Penetration Point: Only one or two carefully sealed openings are made in the EV battery pack casing. A flame-retardant conduit bundle, containing both the sensor wiring harness and the main extinguishing agent delivery pipe, passes through this opening.
- External Routing: The conduit is routed externally to the Fire Extinguishing ECU, the agent tank, and the alarm modules.
This approach maintains the integrity of the EV battery pack’s primary seal while enabling the integration of the safety system.
3. Control Strategy and Operational Logic
The intelligence of the system lies in its multi-layered, condition-based control strategy. The logic is designed to provide graduated warnings, allow for manual intervention, and enable automatic response in critical or occupant-incapacitating situations.
3.1 Formal Logic Representation
Let us define the following binary state variables (1 = Active/True, 0 = Inactive/False):
- $S_{\text{smoke}}$: Smoke sensor detection (1 if smoke concentration > threshold).
- $T_{\text{warn}}$: Temperature > Warning Threshold (e.g., 90°C).
- $T_{\text{fire}}$: Temperature > Fire Threshold (e.g., 130°C).
- $C$: Collision signal active.
- $M_{\text{long}}$: Manual button pressed and held for >3 seconds.
- $Alarm_{\text{int}}$: Command to activate interior alarm.
- $Alarm_{\text{ext}}$: Command to activate external alarm/speaker.
- $Valve$: Command to open the extinguishing agent E-Valve.
A time-delay confirmation function $Delay(x, \tau)$ is used, which outputs 1 only if input $x$ has been continuously 1 for a duration exceeding $\tau$ seconds (e.g., $\tau = 3$). This prevents false triggering from transient signals.
3.2 Detailed Control Modes
The control logic for the EV battery pack safety system can be summarized by the following state table and subsequent formulas:
| Mode | Trigger Condition (Input) | Action (Output) | Purpose |
|---|---|---|---|
| Warning Mode | $(S_{\text{smoke}} \ OR \ T_{\text{warn}}) \ AND \ Delay(\cdot, 3s)$ | $Alarm_{\text{int}} = 1$ | Early alert to the driver for investigation and potential manual action. |
| Manual Suppression Mode | $Alarm_{\text{int}}=1 \ AND \ M_{\text{long}}$ | $Valve = 1$ | Driver-confirmed fire in the EV battery pack, initiating controlled suppression. |
| Automatic Fire Suppression Mode | $T_{\text{fire}} \ AND \ Delay(\cdot, 3s)$ | $Valve = 1$ | Automatic response to a confirmed, severe thermal event within the EV battery pack, regardless of driver state. |
| Collision & Rescue Mode | $C \ AND \ [(S_{\text{smoke}} \ OR \ T_{\text{warn}}) \ AND \ Delay(\cdot, 3s)]$ | $Valve = 1$, $Alarm_{\text{ext}} = 1$ | Prioritizes immediate suppression and calls for external help if a collision coincides with signs of EV battery pack failure, assuming occupant incapacity. |
The corresponding Boolean logic equations implemented in the Fire Extinguishing ECU are:
$$ Alarm_{\text{int}} = Delay(S_{\text{smoke}} \ OR \ T_{\text{warn}}, \ 3) $$
$$ Valve_{\text{manual}} = Alarm_{\text{int}} \ AND \ M_{\text{long}} $$
$$ Valve_{\text{auto}} = Delay(T_{\text{fire}}, \ 3) $$
$$ Valve_{\text{collision}} = C \ AND \ Delay(S_{\text{smoke}} \ OR \ T_{\text{warn}}, \ 3) $$
$$ Alarm_{\text{ext}} = Valve_{\text{collision}} $$
The final command to open the extinguishing valve is the OR combination of all suppression triggers:
$$ Valve = Valve_{\text{manual}} \ OR \ Valve_{\text{auto}} \ OR \ Valve_{\text{collision}} $$
This strategy ensures the EV battery pack is protected through multiple, redundant logical pathways.
4. System Simulation and Validation
The control strategy was modeled and simulated using the MATLAB/Simulink software platform to verify its dynamic response and timing. The model included signal source blocks for sensor inputs, relay blocks with time-delay characteristics to implement the $Delay$ function, and logical operator blocks (AND, OR).
4.1 Simulation Scenario 1: Graduated Warning to Manual Suppression
This test validates the warning and manual intervention logic for the EV battery pack protection system.
- Inputs: A smoke sensor signal $S_{\text{smoke}}$ transitions from 0 to 1 at t=0.8s and remains high. The manual button signal $M$ is applied starting at t=3.8s.
- Observed Outputs:
- $Alarm_{\text{int}}$ becomes 1 at t≈3.38s (0.8s + 3s delay confirmation), alerting the driver.
- Assuming the driver holds the manual button, $M_{\text{long}}$ becomes 1 after the 3s hold period at t≈6.8s.
- The $Valve$ output ($Valve_{\text{manual}}$) subsequently activates at t≈6.8s, initiating suppression.
This simulation confirms that the system provides a warning period before requiring explicit driver confirmation for extinguishing agent release, preventing unnecessary discharge for minor anomalies in the EV battery pack.
4.2 Simulation Scenario 2: Collision with Thermal Event
This test validates the automatic collision response logic crucial for occupant rescue when the EV battery pack is compromised.
- Inputs: A temperature signal exceeds the $T_{\text{warn}}$ threshold (90°C) at t=7.8s. A collision signal $C$ is activated at t=13.6s.
- Observed Outputs:
- Upon collision detection at t=13.6s, the condition $C \ AND \ Delay(T_{\text{warn}}, 3)$ is evaluated. Since $T_{\text{warn}}$ has been active since t=7.8s, the delay condition is already satisfied.
- Consequently, both $Valve$ ($Valve_{\text{collision}}$) and $Alarm_{\text{ext}}$ are set to 1 immediately at t=13.6s, without any additional waiting period related to the temperature signal.
This result demonstrates the system’s priority during a collision: it bypasses the standard warning sequence and immediately initiates both fire suppression for the EV battery pack and a call for external assistance, addressing the critical risk of occupant incapacitation.
5. Further Considerations for EV Battery Pack Safety
While the proposed design addresses active fire suppression, a comprehensive safety approach for the EV battery pack involves several additional layers.
5.1 Extinguishing Agent Selection
The choice of agent is critical. An ideal agent for an EV battery pack fire should:
- Have high thermal conductivity to cool the cells.
- Be electrically non-conductive.
- Be clean (leave no residue) and environmentally acceptable.
- Inhibit chemical chain reactions.
Common agents include clean gaseous agents (e.g., FK-5-1-12, Novec 1230), aerosol generators, and fine water mist systems. The required mass $m_{agent}$ can be estimated based on the enthalpy of the fire and the agent’s heat capacity:
$$ m_{agent} \approx \frac{Q_{\text{fire}}}{\Delta h_{\text{agent}}} $$
where $Q_{\text{fire}}$ is the estimated total heat release from the involved EV battery pack modules and $\Delta h_{\text{agent}}$ is the effective heat absorption per unit mass of the agent.
5.2 System Integration and Testing
For widespread adoption, the system must be rigorously tested:
- Environmental Testing: Vibration, thermal cycling, and water resistance tests to ensure reliability.
- Fire Suppression Efficacy Testing: Full-scale tests on real EV battery pack modules under various failure initiators (nail penetration, overcharge, external flame).
- False Positive Rejection Testing: Ensuring the system does not activate during normal vehicle operation, fast charging, or minor impacts.
5.3 Cost-Benefit and Standardization
The addition of such a system increases vehicle cost. A formal analysis weighs this against the reduced risk of total vehicle loss, potential liability, and most importantly, enhanced occupant safety. Regulatory bodies may eventually mandate integrated automatic fire suppression systems for high-energy EV battery packs, driving standardization of performance requirements, installation protocols, and agent types.
6. Conclusion
This work presents a comprehensive design and analysis of an automatic fire extinguishing device specifically for the EV battery pack. The system is designed with a focus on independent operation, cost-effective secondary installation, and a robust, multi-condition control strategy. By utilizing dedicated temperature, smoke, and collision sensors, the system can differentiate between a developing thermal anomaly, a confirmed fire, and a high-risk post-collision scenario. The MATLAB/Simulink simulations validate the logical flow and timing of the control strategy, demonstrating its ability to provide early warnings, allow for manual override, and execute automatic suppression and rescue calls when necessary. The proposed device represents a significant proactive safety measure that can mitigate the severe consequences of thermal runaway in the EV battery pack, thereby enhancing the overall safety and public confidence in electric vehicles. Future work will involve prototyping, extensive physical testing with various EV battery pack configurations, and optimization of the extinguishing agent delivery dynamics within the complex geometry of a real EV battery pack enclosure.