With the IP67 standard becoming mandatory for passenger vehicles, the range of cooling methods available for power lithium-ion battery systems has been severely narrowed. Among the more mature cooling methods, air cooling has been largely excluded from passenger vehicle battery pack applications, except through methods to combine it with other heat transfer techniques. Furthermore, the demonstration effect of Tesla has made water cooling no longer a research topic but a key focus for rapid commercialization.
This article focuses on a key aspect of the liquid cooling system for power lithium-ion batteries: the liquid cooling plate. The first half covers basic knowledge of liquid cooling plates, while the second half discusses the typical usage of liquid cooling plates in current vehicle models.
The term "liquid cooling plate" doesn't seem to have a universally accepted meaning. However, considering its specific application in power lithium-ion battery packs, we can define it as follows: In a power lithium-ion battery system, excess heat is generated during battery operation. This heat is transferred through contact between the battery or module and the surface of a plate-shaped aluminum component, and is ultimately carried away by the coolant flowing through the internal channels of the component. This plate-shaped aluminum component is the liquid cooling plate.
General requirements for liquid cooling plates
It has a large heat dissipation capacity, which can promptly dissipate excess heat generated during the operation of power lithium-ion batteries and prevent excessive temperature rise;
High reliability is essential because most products operate under harsh conditions in road vehicle environments, including vibration, shock, and alternating high and low temperatures. Power lithium-ion batteries, with voltages often reaching hundreds of volts, pose a serious problem due to coolant leakage. Even with a coolant possessing excellent insulation properties, external impurities can immediately reduce its insulation performance. Therefore, the reliability of the cold plate seal is crucial.
The heat dissipation design is precise, preventing excessive temperature differences within the system. This is due to the inherent performance requirements of lithium-ion batteries, as battery performance and aging are closely related to operating temperature.
There are strict requirements on the weight of the cold plate, which stems from the pursuit of energy density in power lithium-ion battery systems. A cooling system that severely reduces the system's energy density is simply unacceptable to both customers and designers.
Liquid cooling systems utilize the high heat transfer coefficient of liquids, relying on liquid flow to transfer high amounts of heat. This is one of the most effective heat dissipation methods currently available, capable of dissipating hundreds to thousands of watts of heat. The manufacturer's standard piping liquid cooling plates, by placing coolant pipes in direct contact with the base plate of the equipment being cooled, reduce the number of heat exchange interfaces between the equipment and the coolant, thereby maintaining minimum thermal resistance and improving performance.
This manufacturer categorizes liquid cooling plates based on their manufacturing process, primarily including: vacuum brazing cold plates, friction stir welding cold plates, exposed tube cold plates, and aluminum/copper plate long-hole drilled cold plates. Their respective advantages and disadvantages are shown in the table below.
Typical parameters:
Example of a liquid cooling plate product from a certain manufacturer.
This company categorizes cold-rolled steel plates based on the most prominent characteristics of the products, resulting in three types.
Type 1 emphasizes heat dissipation performance. A finned structure is used in the fluid path to increase the contact area with the coolant, thereby improving heat transfer performance. The product features a vacuum brazed construction and can be supplied with customized configurations.
Type 2 emphasizes low pressure drop. The liquid cooling plate uses specially manufactured CNC-milled microchannels to form fluid channels on the base plate. It has excellent heat dissipation performance under low pressure drop conditions, thereby reducing the cost of the fluid circulation system.
Type 3 emphasizes the embedded pipe structure. Pipes are embedded in the base plate to form a mechanically robust cold plate. Surface-extended liquid cooling plates utilize thicker and denser pipes to increase the surface area in contact with the coolant, thereby improving heat transfer performance.
Example of a cold-rolled steel plate product from Company B
This product is lightweight overall, but it cannot bear weight itself.
Typical process of liquid cooling plate
The manufacturing process of liquid-cooled heat sinks is more complex than that of conventional air-cooled heat sinks. Liquid cooling requires higher reliability in its processes, therefore only manufacturers with strong technical expertise can provide reliable technical support. The general manufacturing processes for liquid-cooled heat sinks include the following.
Pipe embedding technology
The embedded tube process is the most commonly used manufacturing process for liquid cooling heat sinks and liquid cooling plates. Generally, it involves embedding copper tubes into an aluminum substrate. This involves using CNC milling to create grooves in the aluminum substrate, then using a stamping machine to press the pre-bent copper tubes onto the aluminum substrate, followed by brazing, and finally, the product is manufactured into a water-cooled plate.
There are generally three types of buried tube liquid cooling plates: shallow buried tube liquid cooling plates; deep buried tube liquid cooling plates; welded tube technology; and double-sided clamped tube technology liquid cooling plates. The manufacturing processes for these three types are largely similar, and the production difficulty is also comparable. Some liquid cooling principles originally designed for high-power switching devices can also be applied to the cooling systems of power lithium-ion batteries.
Shallow buried pipe process: suitable for single-sided installation. After the copper pipe is flattened, it is milled at the same time as the aluminum plate. This fully utilizes the high thermal conductivity of the copper pipe to carry away heat, and the lightweight aluminum is used to reduce weight and control costs.
Deep-buried pipe process: The filler is imported from the United States with high thermal conductivity epoxy resin. When the temperature difference requirement of the cooled device is not high, it can be installed on one or both sides. Because the copper pipe thickness has not been re-produced and the filler is used to protect the safety of the supply, it is especially suitable for cold plates with refrigerant as the medium.
Welded pipe process: Suitable for copper plate + copper pipe, thereby reducing the thickness of the plate and achieving a weight reduction effect.
Double-sided clamping process: Components can be installed on both sides, the process is simple and low-cost; aluminum plate + aluminum tube + copper tube + stainless steel tube.
Profiles + Welding
Liquid-cooled radiators are produced based on profiles. These radiators come in many shapes and types, including plate type, channel type, and modular type. The general manufacturing principle is to produce and weld the profiles and connectors to form a complete liquid-cooled radiator.
The cold plate flow channels are straightened using an extrusion process, and then the circulation is opened through machining. Sealing is typically achieved using friction welding, brazing, or other welding techniques. This process is efficient and low-cost; however, it is not suitable for applications with excessive heat dissipation density, nor is it suitable for applications where too many screw holes on the surface restrict the water channel flow or reduce reliability. Key applications include: water-cooled heat dissipation and heating devices for power lithium-ion batteries, water distribution boxes, and integrated heat dissipation products for standard power modules.
Machine production + welding
Water-cooled plates are machined, and the internal flow channel size and path can be freely designed. They are suitable for thermal management products with high power density, irregular heat source layout, and limited space. They are mainly used in the design of heat dissipation products in the fields of wind power converters, photovoltaic inverters, IGBTs, motor controllers, lasers, energy storage power supplies, and supercomputing servers, but are less commonly used in power lithium-ion battery systems.
Microchannel radiators are also a type of radiator manufactured using a combination of machine production and welding processes. Their manufacturing is more complex than that of other radiators. Microchannel radiators are generally used in machines with high heat dissipation power and concentrated heat dissipation. Because the microchannel design has wider and more even water channels, it can quickly remove the accumulated heat.
However, the manufacturing process of microchannel liquid cooling radiators is also quite complex. Generally, microchannels are produced by machine and then welded using friction welding, which also results in higher manufacturing costs.
Die casting + welding
Die casting is a mature and widely used molding method. With the rapid development of new energy vehicles, it has become the preferred method for mass production of motor controllers, power lithium-ion battery pack trays and heat sinks. However, it is necessary to control die casting impurities and porosity in the process. Conservative methods such as using sealing rings or friction welding are required to improve reliability and prevent leakage.
Die casting followed by welding ensures good process control and stability, enabling mass production. In addition to friction welding, some water-cooled plates also employ brazing or vacuum brazing.
This type of water-cooled plate can be integrated with the die-cast battery pack housing; the lower water-cooled plate of the Audi Q7 PHEV is an example of this. A finished sample was already on display at the Beijing Auto Show a couple of days ago.
Typical vehicle water-cooled plate
In power lithium-ion battery systems, there are many ways to remove heat from the cell surface. Based on their application, they can be divided into cell-level water-cooled plates integrated inside the module and module-level water-cooled plates designed outside the module. Below are several images from the WeChat official account "Power Lithium-ion Battery Thermal Management Technology" to illustrate the usage of water-cooled plates in practical cases.
Module-level water-cooled plate
As a whole, the water-cooled plate is used in one or more battery modules. Since the water-cooled plate is a component of the entire battery pack, rather than a component of the battery module, we will place it under the title of "module-level water-cooled plate".
Audi Q7 PHEV battery pack
Mercedes-Benz SmartGen3 battery pack
Chevroletbolt 2017 Battery Pack
Chevroletbolt 2017 liquid cooling plate (actual product)
BMW i3 liquid cooling system
BMW i3 liquid cooling plate (actual product)
bMWi8 battery pack and cooling plate
Cell-level water-cooled plate
Water-cooled plates or sheets of dielectric material with good thermal conductivity are sandwiched between the battery cells to become part of the module, in order to achieve better heat dissipation. We put this type under the title of "water-cooled plate inside the module".
Exploded view of Volvo XC90T8 battery pack module
GMVolt module structure
GMVolt cooling structure
Tesla Model S module
Tesla cylindrical battery water-cooling plate patent description
Observing the use cases, we can see that: for square batteries, liquid cooling systems mostly use module-level water cooling plates, which are generally placed at the bottom of the battery box; for pouch batteries, liquid cooling is mostly done by integrating small water cooling plates inside the module, or by integrating aluminum plates in the module and then setting module-level liquid cooling plates on the outside of the module; for cylindrical batteries, the serpentine tube, spearheaded by Tesla, is the main form of liquid cooling heat sink.
