The transition towards new energy vehicles represents a pivotal strategy in addressing global energy and environmental challenges. Within this context, the performance, safety, and longevity of the vehicle’s core component—the traction battery pack—are paramount. The operational temperature of lithium-ion batteries is a critical factor influencing these attributes. Excessively low temperatures increase internal resistance and reduce discharge capacity, while high temperatures accelerate material degradation and pose severe safety risks such as thermal runaway. Consequently, an effective battery management system (BMS), particularly its thermal management subsystem, is indispensable for maintaining optimal operating conditions.
Among various thermal management strategies, liquid cooling has emerged as the predominant solution for high-power applications due to its superior heat transfer coefficient and temperature uniformity compared to air cooling. A typical liquid-cooled battery management system utilizes cold plates with internal flow channels attached to the battery modules or cells. Coolant circulated through these channels absorbs and removes the waste heat generated during battery operation. The central challenge in designing such a system lies in maximizing heat dissipation and temperature homogeneity while minimizing pumping power and system weight. This has directed significant research focus towards enhancing the heat transfer performance of the liquid cooling plate. Our research focus has shifted to optimizing the design of liquid cooling plates to improve thermal management efficiency. The primary technological pathways for enhancement revolve around three interconnected areas: optimization of the cooling channel’s geometry and layout, integration of internal and external heat transfer augmentation features, and the selection and optimization of advanced cooling media. The logical progression from a traditional flat cold plate to these enhanced configurations forms the core of contemporary research in BMS thermal design.
The performance of a cold plate is often evaluated using key thermal and hydraulic metrics. The primary thermal goal is to minimize the maximum cell temperature (\(T_{max}\)) and the temperature difference within the pack (\(ΔT_{pack}\)). Hydraulically, the pressure drop (\(ΔP\)) across the cold plate must be minimized to reduce parasitic pumping power. The overall efficacy can be assessed by a performance evaluation criterion (PEC) that balances heat transfer enhancement against friction increase. The Nusselt number (\(Nu\)) and friction factor (\(f\)) are fundamental parameters used in these analyses:
$$Nu = \frac{h D_h}{k_f}, \quad f = \frac{ΔP D_h}{2 ρ_f u_m^2 L}$$
where \(h\) is the convective heat transfer coefficient, \(D_h\) is the hydraulic diameter, \(k_f\) is the fluid thermal conductivity, \(ρ_f\) is the fluid density, \(u_m\) is the mean flow velocity, and \(L\) is the channel length. An effective enhancement technique increases the \(Nu\) number more significantly than it increases the friction factor \(f\).
1. Cooling Channel Structure and Layout Optimization
The design of the flow channel within the cold plate is the first and most direct approach to improving the thermal performance of the battery management system. The evolution has progressed from simple, serial paths to complex, biomimetic, and multi-branch architectures aimed at better flow distribution and thermal uniformity.
1.1. Conventional Channel Designs
Early designs primarily featured serpentine and parallel channel layouts. The serpentine channel forces the coolant to traverse the entire plate area in a single, winding path, which generally provides good cooling but results in a significant longitudinal temperature gradient and high pressure drop due to its long flow length. Parallel channels offer a lower flow resistance by dividing the flow into multiple shorter paths. However, they often suffer from flow maldistribution, where channels with lower flow resistance receive more coolant, leading to uneven cooling and localized hot spots. Optimization studies have focused on parameters such as channel number, width, and turning geometry. For instance, increasing the number of parallel channels can reduce the average temperature rise but may exacerbate flow imbalance. Optimizing the turn geometry in serpentine designs, such as using larger fillet radii, can reduce pressure losses and improve temperature uniformity by smoothing the flow.
| Channel Type | Key Characteristics | Advantages | Disadvantages | Typical Performance Trend |
|---|---|---|---|---|
| Serpentine | Single, continuous winding path. | Forces coolant over entire plate; good for high heat flux areas. | High pressure drop; significant inlet-to-outlet temperature gradient. | High Nu, high f. Performance sensitive to turn design. |
| Parallel (Straight) | Multiple independent straight paths from inlet to outlet manifold. | Low flow resistance (low ΔP). | Prone to severe flow maldistribution, causing thermal non-uniformity. | Potential for low Nu if maldistributed; low f. |
| U-type / Z-type | Variants with different inlet/outlet positions (side-by-side or diagonal). | Can offer compromise between flow length and distribution. | Distribution still non-uniform; thermal performance depends heavily on layout. | Moderate Nu and f. Outlet placement critical for ΔT. |
| Tree-shaped / Dendritic | Fractal-like branching network mimicking leaf veins or blood vessels. | Excellent flow distribution; minimized flow path length; reduced pumping power. | Complex design and manufacturing. | High Nu with relatively low f. Excellent temperature uniformity. |
1.2. Advanced and Biomimetic Channel Designs
To overcome the limitations of conventional layouts, researchers have turned to nature-inspired and advanced topological designs. Tree-shaped or dendritic channels replicate the efficient transport networks found in biological systems (e.g., leaves, lungs, circulatory systems). These designs utilize a branching pattern where a main channel splits into multiple smaller channels, ensuring that the flow path from the inlet to any point on the plate is nearly minimized. This principle, known as Murray’s Law, optimizes the channel diameters at each bifurcation to minimize flow resistance. For a battery management system, this translates to remarkably uniform cooling with a lower pressure drop compared to a serpentine channel of equivalent cooling capacity. The branching generates secondary flows at junctions, which further enhance mixing and heat transfer.
Another innovative approach is the use of wavy or corrugated channels. By introducing periodic undulations to the channel walls, the thermal boundary layer is periodically interrupted and reformed. This disruption prevents the boundary layer from becoming fully developed, maintaining a higher local heat transfer coefficient along the channel length. Furthermore, the waviness can induce Dean vortices or other secondary flow patterns that promote fluid mixing between the core and near-wall regions. The heat transfer enhancement factor for a wavy channel compared to a straight one can be expressed as a function of the wave parameters:
$$ \frac{Nu_{wavy}}{Nu_{straight}} = f(Re, A/λ, γ) $$
where \(Re\) is the Reynolds number, \(A\) is the wave amplitude, \(λ\) is the wavelength, and \(γ\) is the channel aspect ratio. Optimization studies show that there exists an optimal combination of \(A\) and \(λ\) that maximizes the Nusselt number augmentation while keeping the friction factor increase acceptable for the BMS.
2. Internal and External Flow Field Enhancement
Beyond optimizing the primary channel layout, integrating passive enhancement structures directly into the flow field is a highly effective strategy to boost the thermal performance of the cold plate in a battery management system. These structures aim to disrupt the laminar sub-layer, increase turbulence, and enlarge the effective heat transfer surface area.
2.1. Internal Flow Disruption with Protrusions and Fins
The insertion of various types of fins, ribs, or dimples on the channel walls is a classic heat transfer enhancement technique now widely applied to battery cold plates. These features work by:
- Breaking the Boundary Layer: Protrusions periodically restart the thermal boundary layer, maintaining a steeper temperature gradient near the wall.
- Increasing Surface Area: Fins directly augment the contact area between the coolant and the solid cold plate.
- Generating Secondary Flows and Vortices: Angled or shaped fins (like delta-winglets, V-shaped, or oblique ribs) create longitudinal vortices that sweep fluid from the core towards the wall and vice-versa, enhancing convective mixing.
The convective heat transfer rate with fins can be modeled considering fin efficiency (\(η_f\)):
$$ q = h A_b (T_b – T_f) + η_f h A_f (T_b – T_f) $$
where \(A_b\) is the base (un-finned) area, \(A_f\) is the fin surface area, \(T_b\) is the base temperature, and \(T_f\) is the fluid temperature. The design challenge is to maximize \(A_f\) and maintain a high \(η_f\) without causing an unacceptable pressure drop. Recent studies on cold plates for BMS have explored novel fin shapes like open Y-fins, airfoil-shaped fins, and staggered pin-fins within channels, showing significant reductions in maximum battery temperature and improved temperature uniformity compared to plain channels.
2.2. Secondary Flow Channels and Split-Flow Designs
A more integrated approach involves designing the cold plate with a primary channel for bulk flow and smaller secondary or split channels that guide coolant into specific regions or create intricate internal flow networks. For example, a cold plate might feature a main serpentine channel with numerous small branching channels (like a leaf vein pattern) that penetrate the areas directly beneath each battery cell. Another design employs a “double-layer” cold plate where the coolant flows in a lower layer of channels and is directed upwards through pin-fins or pedestals that make contact with the upper plate surface attached to the battery. These designs effectively shorten the heat conduction path from the battery to the coolant and provide a more targeted cooling effect, which is crucial for managing the high heat generation in a modern battery management system.
| Enhancement Type | Mechanism of Action | Impact on BMS Performance | Design Considerations |
|---|---|---|---|
| Longitudinal/Rectangular Fins | Increase surface area; guide flow. | Reduces average temperature; can increase ΔP if flow is constricted. | Fin height, thickness, and spacing. Risk of high pressure drop. |
| Oblique/V-shaped Ribs | Generate longitudinal vortices; enhance fluid mixing. | Significantly improves temperature uniformity; moderate ΔP increase. | Rib angle, pitch, and attack angle are critical parameters. |
| Pin Fins (Cylindrical/Conical) | Create turbulent wakes; provide extensive surface area. | Excellent for hotspot mitigation; often high ΔP. | Staggered array is more effective than inline. Diameter and height optimization needed. |
| Dimpled Surface | Create separation vortices and boundary layer reattachment. | Enhances local heat transfer with relatively low ΔP penalty. | Dimple depth and arrangement pattern. |
| Secondary/Split Channels | Redirect flow to critical zones; shorten conduction path. | Greatly improves spatial temperature uniformity. | Complex design; requires careful balancing of flow distribution. |
3. Coolant Selection and System Parameter Optimization
The choice of working fluid and the operational parameters of the cooling loop are fundamental aspects that interact with the cold plate design to determine the overall efficiency of the battery management system.
3.1. Advanced Cooling Media: Nanofluids and Dielectric Fluids
While water and water-glycol mixtures are standard coolants, research into advanced fluids aims to increase the effective thermal conductivity (\(k_{nf}\)) of the coolant itself. Nanofluids—base fluids with suspended nanoparticles (e.g., Al2O3, CuO, SiO2, carbon nanotubes)—have been extensively studied for this purpose. The enhanced thermal conductivity can be estimated by models like the Maxwell model:
$$ \frac{k_{nf}}{k_{bf}} = 1 + \frac{3(α-1)φ}{(α+2)-(α-1)φ} $$
where \(k_{bf}\) is the base fluid conductivity, \(α = k_p/k_{bf}\) is the ratio of particle to fluid conductivity, and \(φ\) is the particle volume fraction. For a BMS, using nanofluids can lead to a lower average battery temperature and more uniform thermal distribution. However, challenges include potential nanoparticle aggregation, increased viscosity (leading to higher \(\Delta P\)), erosion, and long-term stability. Dielectric fluids like mineral oil or engineered fluids (e.g., Novec) are also considered, especially for direct immersion cooling concepts, due to their electrical insulation properties, though their thermal performance is often inferior to water.
3.2. Optimization of System Operating Parameters
Even with a fixed cold plate design and coolant, the operating conditions of the battery management system cooling loop significantly affect performance. Key parameters include:
- Coolant Inlet Temperature (\(T_{in}\)): Lower \(T_{in}\) directly reduces the battery temperature but increases the energy cost for chiller operation. It is the most direct parameter affecting the maximum pack temperature (\(T_{max}\)).
- Coolant Flow Rate (\(\dot{V}\) or \(u_m\)): Increasing the flow rate enhances the convective heat transfer coefficient (\(h \propto u_m^{0.8}\) for turbulent flow) and reduces the coolant temperature rise across the plate, thereby lowering \(T_{max}\). However, the pressure drop increases sharply (\(\Delta P \propto u_m^{2}\)), leading to higher pumping power. An optimal flow rate exists that balances cooling performance with parasitic energy consumption.
- Flow Direction and Inlet/Outlet Configuration: In multi-channel plates, the arrangement (e.g., Z-type vs. U-type) affects flow distribution and consequently the temperature uniformity (\(\Delta T_{pack}\)). Counter-flow arrangements between adjacent channels can improve uniformity.
The optimization of these parameters is often multi-objective, aiming to minimize \(T_{max}\), \(\Delta T_{pack}\), and pumping power \(P_{pump}\) simultaneously:
$$ \text{Minimize: } [T_{max}(\dot{V}, T_{in}), \, \Delta T_{pack}(\dot{V}, T_{in}), \, P_{pump}(\dot{V})] $$
Modern BMS controllers dynamically adjust these parameters based on battery load, ambient conditions, and thermal state to achieve optimal operation.
| Coolant Type | Typical Composition | Advantages for BMS | Disadvantages / Challenges |
|---|---|---|---|
| Deionized Water | Pure H2O. | Highest specific heat capacity and thermal conductivity among common fluids; excellent heat transfer. | Freezing point at 0°C; requires corrosion inhibitors; electrically conductive (risk if leaked). |
| Water-Ethylene Glycol (WEG) | Mix of water and EG (typically 50:50). | Lower freezing point; anti-corrosion properties. | Lower thermal conductivity and higher viscosity than pure water, reducing heat transfer efficiency. |
| Dielectric Oils | Mineral oil, silicone oil, engineered fluids. | Electrically insulating; enables direct cooling concepts. | Generally poor thermophysical properties (low k, high ν); can be flammable; environmental concerns. |
| Nanofluids | Base fluid (water, EG, oil) + nanoparticles. | Enhanced thermal conductivity; potential for lower ΔT and higher heat removal. | Increased viscosity and density (higher ΔP); stability issues (settling, agglomeration); higher cost. |
| Phase Change Slurries | Fluid with suspended micro-encapsulated PCM. | High apparent heat capacity due to latent heat; can smooth out transient thermal loads. | Complex rheology; potential clogging; limited long-term cycle stability. |
4. Conclusion and Future Perspectives
The pursuit of enhanced heat transfer in liquid-cooled battery management systems has evolved into a sophisticated, multi-disciplinary endeavor. Significant progress has been made by strategically optimizing cold plate channel architectures—moving from simple serpentine designs to biomimetic tree-shaped and wavy channels that improve flow distribution and thermal uniformity. The integration of internal augmentation structures like oblique ribs, pin fins, and secondary flow networks has proven highly effective in disrupting boundary layers and boosting local convection, directly addressing the challenge of hot spots within the battery pack. Concurrently, the exploration of advanced coolants like nanofluids offers a pathway to increase the fundamental heat carrying capacity of the working fluid, while systematic optimization of operating parameters like inlet temperature and flow rate allows for dynamic, efficient control of the thermal state.
The overarching trend in BMS thermal design is the move towards integrated, multi-objective optimization. Future research will likely focus on the co-design of the cold plate structure with the battery cell/pack layout, leveraging advanced manufacturing techniques like additive manufacturing to create highly complex, topology-optimized cooling structures that were previously impossible to fabricate. The development of stable, low-penalty advanced coolants remains an active area. Furthermore, the integration of the thermal management system with the vehicle’s overall energy management, using predictive algorithms and real-time sensor data to pre-emptively adjust cooling power, will be crucial for maximizing both battery performance and vehicle range. Ultimately, through continuous innovation across these fronts, liquid-cooled battery management systems will become even more efficient, reliable, and compact, supporting the next generation of high-performance, fast-charging electric vehicles.