The relentless global pursuit of environmental sustainability has propelled electric vehicles (EVs) to the forefront of green transportation. However, the performance of the vehicle’s power battery system, the very heart of an EV, is critically susceptible to low-temperature environments. At sub-zero temperatures, the charging and discharging capabilities of lithium-ion batteries deteriorate significantly due to increased internal resistance and slowed electrochemical kinetics. This degradation manifests as reduced driving range, sluggish acceleration, prolonged charging times, and can even compromise battery safety and longevity. Consequently, effective thermal management, particularly low-temperature heating strategies, has become a pivotal research focus within the power battery domain. Among various heating methodologies, the heating film technology stands out for its simplicity, efficiency, and design flexibility. This article delves into the application of heating film technology within the thermal management system of vehicle power batteries, analyzing its impact on key performance metrics such as average temperature rise rate, temperature uniformity, and energy consumption, thereby providing a comprehensive theoretical and technical foundation for system optimization.
The thermal management system, governed by the sophisticated algorithms of the battery management system (BMS), is responsible for maintaining the battery pack within an optimal temperature window. The battery management system continuously monitors cell voltages, temperatures, and current. Upon detecting a low-temperature condition that necessitates heating, the BMS activates the heating circuit. Heating films, as an active heating component, have emerged as a mainstream solution due to their direct and controllable heat transfer mechanism.
Fundamentals of Heating Film Technology
Working Principle and Structural Composition
The operational principle of a heating film is grounded in Joule’s law and heat conduction. Fundamentally, it is a laminate structure designed to convert electrical energy into thermal energy uniformly. When integrated into the battery management system circuit and energized by the battery pack itself or an external source, current flows through a conductive resistive layer. The inherent resistance of this layer causes power dissipation in the form of heat. This generated heat is then conducted through the battery module casing or directly to the cell surfaces, raising their temperature.
The typical structure of a heating film comprises several functional layers, as summarized below:
| Layer | Material Examples | Primary Function |
|---|---|---|
| Conductive Resistive Layer (Heating Element) | Etched Metal Foil (Copper, Aluminum), Conductive Polymer (PI with metal coating), Carbon Nanotube (CNT) ink | Core layer that generates heat when current passes through. Its resistivity and pattern design determine the power density and heat distribution. |
| Electrical Insulation Layer | Polyimide (PI), Silicone Rubber | Encapsulates the heating element to prevent electrical short circuits and ensure user safety. Must have high dielectric strength and thermal stability. |
| Protective Layer / Overlaminate | Silicone Rubber, Fluoropolymer (e.g., PTFE), Polyester | Provides mechanical protection against abrasion, puncture, and environmental factors like moisture and chemicals. Enhances durability. |
| Adhesive Layer | Acrylic, Silicone-based Pressure-Sensitive Adhesive (PSA) | Bonds the heating film securely to the battery module or cell surface, ensuring optimal thermal contact for efficient heat transfer. |
| Electrodes & Leads | Tinned Copper, Nickel-plated strips | Provide low-resistance connection points for power input and integration with the battery management system harness. |
The heat generation rate (\(P\)) of the film is governed by Joule’s law:
$$P = I^2 R = \frac{V^2}{R}$$
where \(I\) is the current, \(V\) is the applied voltage, and \(R\) is the effective resistance of the film’s heating pattern. The battery management system regulates the applied voltage or duty cycle (in case of PWM control) to modulate this power.
Integration and Packaging Strategies
The integration strategy for heating films is highly dependent on the battery cell format and module design. The primary goal is to achieve efficient and uniform heat transfer while minimizing thermal gradients and mechanical stress.
| Cell Format | Preferred Heating Film Placement | Rationale |
|---|---|---|
| Prismatic (Square) Cells | On the large side faces of cells or modules (single-side or dual-side). | Maximizes contact area with the cell’s broad surface, which is often aluminum or steel casing with good thermal conductivity. Dual-side placement improves uniformity. |
| Pouch Cells | On one or both of the large flat surfaces of the cell stack or module. | Direct contact with the flexible pouch surface allows for efficient heat transfer. Care must be taken regarding pressure and potential cell swelling. |
| Cylindrical Cells (e.g., 21700) | Interstitial pads between cells, or on the module’s cold plate/bracket. | Direct wrapping is difficult. Films are often integrated into the module’s structure to heat the array from the perimeter or bottom. |
| Blade-type (Long Prismatic) Cells | On the top and/or bottom major surfaces of the cell or module. | Similar to large prismatic cells, this placement targets the largest surface area for heat conduction into the cell’s core. |
Advantages and Inherent Challenges
Heating film technology offers distinct benefits that make it attractive for automotive applications, but it also presents specific challenges that must be addressed by the battery management system and pack design engineers.
| Advantages | Challenges & Mitigation Strategies |
|---|---|
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System Design and Application Engineering
Defining Heating Film Specifications
The design process begins with defining the thermal requirements, which are then translated into electrical and physical specifications for the heating film. Key input parameters include:
- Battery Pack Thermal Mass (\(C_p\)): The total heat capacity of the system (cells, module structure, etc.).
- Target Heating Performance: Desired average temperature rise rate (\(\overline{TRR}\)), maximum allowable temperature spread (\(\Delta T_{max}\)), and target temperature (\(T_{target}\)).
- Operating Voltage Range: Defined by the vehicle’s electrical system (e.g., 300V – 450V).
- Environmental Conditions: Minimum ambient temperature (\(T_{amb,min}\)), desired heating time from \(T_{amb,min}\) to \(T_{target}\).
The required total heating power (\(P_{req}\)) can be estimated using a simplified energy balance, neglecting losses for initial sizing:
$$P_{req} \approx \frac{C_p \cdot (T_{target} – T_{amb,min})}{t_{target}}$$
where \(t_{target}\) is the desired heating time. A more accurate model used by the battery management system for control includes heat loss to the environment:
$$C_p \frac{dT}{dt} = P_{heater} – hA(T – T_{amb})$$
where \(h\) is the effective heat transfer coefficient, and \(A\) is the surface area for heat loss.
Based on \(P_{req}\) and the system voltage \(V\), the nominal resistance \(R_{film}\) of the heating film network can be determined:
$$R_{film} = \frac{V^2}{P_{req}}$$
The physical area and power density (\(PD\)) are then derived based on available space and thermal limits:
$$PD = \frac{P_{req}}{A_{film}} \quad \text{(typically 0.2 to 0.5 W/cm² for safety and uniformity)}$$
Low-Temperature Charging Strategy Governed by the BMS
The battery management system orchestrates a precise sequence to enable safe and efficient charging in cold conditions. A typical strategy involves the following states:
- Detection & Initiation: Following plug-in, the BMS performs system checks. If the lowest cell temperature \(T_{min}\) is below a pre-defined threshold \(T_{charge,allow}\) (e.g., 0°C to 5°C), charging is inhibited, and heating is initiated.
- Pure Heating Phase: The heating film is activated at full or controlled power. The battery management system monitors cell temperatures until \(T_{min}\) reaches a “heating stop” threshold \(T_{heat,stop}\) (often lower than the optimal charging temperature to conserve energy).
- Combined Heating & Charging Phase: Once \(T_{min} > T_{heat,stop}\), the BMS enables a low-current charge while continuing to heat. This phase carefully balances internal heat generation from charging (\(I^2R_{internal}\)) with the supplemental heat from the film.
- Pure Charging Phase: When \(T_{min}\) reaches the optimal charging temperature range (e.g., 15°C – 25°C), the BMS deactivates the heater and ramps up the charge current to its maximum allowable value for the given temperature and state of charge (SOC).
The control logic can be summarized by the following battery management system state transitions:
IF \(T_{min} < T_{charge,allow}\) THEN State = HEATING;
IF \(T_{min} \ge T_{heat,stop}\) AND State = HEATING THEN State = HEAT_AND_CHARGE;
IF \(T_{min} \ge T_{optimum}\) AND State = HEAT_AND_CHARGE THEN State = CHARGING;
Simulation Analysis and Performance Evaluation
Impact of Heating Film Power and Layout
Computational Fluid Dynamics (CFD) and thermal simulation are indispensable tools for evaluating heating film designs before physical prototyping. Simulations assess parameters like average temperature rise rate, temperature uniformity, and total energy consumption under various scenarios. For a representative battery pack with an initial temperature of -20°C, simulations were conducted for different heating film power ratings.
| Heating Film Power (kW) | Final Temp. Uniformity, \(\Delta T_{final}\) (°C) | Avg. Temp. Rise Rate, \(\overline{TRR}\) (°C/min) | Heating Time to 10°C (min) | Energy Consumed (kWh) |
|---|---|---|---|---|
| 1.13 | 9.62 | 0.27 | 110 | 2.07 |
| 1.38 | 8.42 | 0.46 | 84 | 1.92 |
| 1.46 | 6.80 | 0.55 | 55 | 1.59 |
The average temperature rise rate is calculated as:
$$\overline{TRR} = \frac{T_{target} – T_{initial}}{t_{heat}}$$
where \(t_{heat}\) is the duration of the pure heating phase. The results clearly demonstrate a trade-off: increasing the heating film power improves the \(\overline{TRR}\) and reduces both heating time and total energy consumption, while also enhancing temperature uniformity (\(\Delta T\) decreases). This counter-intuitive reduction in energy consumption with higher power is due to the significantly shorter heating time, which reduces the period of dominant heat loss to the environment. This highlights the importance of a properly sized heater controlled by an intelligent battery management system.
Optimization of BMS Control Strategy
Beyond hardware sizing, the strategy implemented in the battery management system software profoundly impacts performance. Simulations comparing different temperature setpoints for transitioning between heating states reveal critical insights. The evaluation starts from a more severe -30°C condition using a 1.46 kW film.
| Control Strategy ID | Pure Heat Stop Temp., \(T_{heat,stop}\) | Final Heater Off Temp., \(T_{optimum}\) | Total Charge Time (min) | Total Heating Time (min) | Final \(\Delta T\) (°C) | Total Heater Energy (kWh) |
|---|---|---|---|---|---|---|
| 1 | -20°C | 5°C | 212 | 72 | 8.81 | 2.08 |
| 2 | -20°C | 10°C | 188 | 79 | 9.06 | 2.30 |
| 3 | -20°C | 15°C | 174 | 86 | 9.45 | 2.54 |
| 4 | -20°C | 20°C | 160 | 94 | 9.77 | 2.78 |
| 5 | -20°C | 25°C | 140 | 103 | 9.69 | 3.06 |
Analysis of the “Combined Heating & Charging” phase is particularly revealing:
| Strategy ID | Heat+Charge Phase Duration (min) | \(\overline{TRR}\) in This Phase (°C/min) | \(\Delta T\) at Heater Off (°C) | Energy in This Phase (kWh) |
|---|---|---|---|---|
| 1 | 48 | 0.52 | 3.85 | 1.39 |
| 2 | 55 | 0.54 | 4.58 | 1.61 |
| 3 | 62 | 0.55 | 5.41 | 1.84 |
| 4 | 70 | 0.55 | 6.33 | 2.08 |
| 5 | 79 | 0.55 | 7.35 | 2.36 |
The key findings from this strategic analysis are:
- Total Charge Time vs. Energy Trade-off: A higher final heater-off temperature (\(T_{optimum}\)) allows for faster charging once heating stops, significantly reducing total charge time (Strategy 5 is 34% faster than Strategy 1). However, this comes at the cost of substantially increased heater energy consumption (47% higher for Strategy 5).
- Temperature Uniformity Degradation: Longer combined heating and charging phases lead to increased temperature spread (\(\Delta T\)) within the pack at the moment the heater turns off. This is due to the complex interaction between external heating, internal joule heating from the charge current, and varying thermal paths.
- BMS Strategy Optimization: The optimal strategy is not universal; it depends on user priorities. If connected to a grid (plugged-in), maximizing speed by using a high \(T_{optimum}\) may be preferred. For on-the-road scenarios, minimizing energy drain from the battery might dictate a lower \(T_{optimum}\). An advanced, predictive battery management system could dynamically select the strategy based on navigation data (e.g., proximity to a charger).
Conclusion and Future Perspectives
Heating film technology has proven to be a highly effective and versatile solution for addressing the critical challenge of low-temperature performance in electric vehicle power batteries. Its integration into the comprehensive thermal management system, governed by a sophisticated battery management system, enables precise control over battery temperature, directly impacting average warm-up rate, temperature uniformity, and overall energy efficiency. As demonstrated through simulation, both the hardware parameters (power, layout) and the software strategy within the BMS are levers for optimization, often involving trade-offs between speed, energy use, and uniformity.
Future advancements in this field are likely to focus on several key areas:
- Advanced Materials: Development of heating films with self-regulating Positive Temperature Coefficient (PTC) characteristics to intrinsically prevent overheating, and use of graphene or other nanomaterials for higher efficiency and better uniformity.
- Intelligent, Predictive BMS Algorithms: Leveraging cloud connectivity and vehicle navigation to pre-condition the battery using grid power before arrival at a charging station or before a scheduled drive, vastly improving user experience and preserving onboard energy.
- System Integration: Closer integration of heating films with other thermal management components, such as heat pumps and refrigerant-based cooling systems, to create a holistic, energy-efficient thermal management system that can shuttle heat from motors or electronics to the battery as needed.
- Enhanced Diagnostics: Integration of diagnostic features within the heating film circuit itself, allowing the battery management system to detect degradation or faults, such as delamination or local hot spots, proactively.
In conclusion, heating film technology, when skillfully engineered and seamlessly integrated with an intelligent battery management system, is a cornerstone for unlocking the full potential of electric vehicles in all climates. It ensures not only performance and range but also safety and battery longevity, contributing significantly to the widespread adoption and user satisfaction of electric mobility.