Brief introduction of liquid cooling plate application in power battery system

With the requirement of the passenger car IP67 becoming necessary, the range of cooling methods available for the power battery system is severely narrowed. Among the more mature cooling methods, air cooling has been basically excluded from the application of passenger car battery packs in addition to the method of using other heat transfer means. Coupled with the demonstration effect of Tesla, water cooling is no longer a pre-research topic, and it has become the focus of commercialization as soon as possible.

This paper mainly deals with a point in the liquid battery liquid cooling system, liquid cooling plate, the basic knowledge of the first half of the liquid-cooled plate, the second half of the current typical model of the liquid-cooled plate application.

Liquid-cooled plate, there seems to be no uniform definition. We only define the application of the liquid-cooled plate of the power battery pack. Let's define it for the time being: In the power battery system, the battery works to generate excess heat, and the heat passes through the battery. Or the module is transferred in contact with the surface of the plate-type aluminum device, and is finally carried away by the coolant passing through the internal flow path of the device. This plate type aluminum device is a liquid cold plate.

General requirements for liquid-cooled panels

The heat dissipation power is large, and the excess heat generated during the working process of the power battery can be derived in time to avoid excessive temperature rise;

High reliability, working in road vehicle environment, vibration, shock, high and low temperature alternating environment, most of the products are harsh working conditions, and the power battery voltage is hundreds of volts, coolant leakage is a serious problem, even if you use Coolant with good insulation performance, but the insulation performance will be reduced immediately after encountering external impurities. Therefore, the reliability of cold plate sealing is very important;

The heat dissipation design is accurate, avoiding excessive temperature difference in the system. This is due to the performance requirements of the lithium battery. The performance and aging of the battery are closely related to the working temperature.

There are strict requirements on the weight of the cold plate. This is due to the pursuit of energy density in the power battery system, and the cooling system that seriously degrades the system energy density, which is unacceptable to both customers and designers.

The liquid cooling system utilizes the characteristics of large liquid flow heat transfer coefficient, and relies on liquid flow to transfer high heat. It is one of the most effective heat dissipation methods at present, and can dissipate hundreds of watts to kilowatts of heat. The manufacturer's standard pipeline liquid cooling plate is directly placed in contact with the bottom plate of the equipment to be cooled by placing a coolant pipe, which can reduce the number of heat exchange interfaces between the equipment and the coolant, thereby maintaining the minimum thermal resistance and improving performance.

The manufacturer divides the type of liquid-cooled plate according to the type of liquid-cooled plate. The main ones are: Vacuum Brazed Water Plate, FSW Coldplate, ExposedTubeColdplate, and Several liquid cooling methods such as Aluminum/CopperPlate LongHoleDrilled. Their respective advantages and disadvantages can be found in the table below.

Typical parameters:

A manufacturer's liquid cold plate product example

This home is divided into three types according to the most prominent characteristics of the product.

Type 1, emphasizing heat dissipation, the use of a fin structure in the fluid path increases the contact area with the coolant, thereby improving the heat transfer performance. The product has a vacuum brazed construction and is available in a custom configuration.

Type 2 emphasizes low pressure drop. The liquid-cooled plate uses a specially-made CNC milling micro-flow path to form a fluid passage on the bottom plate. Under low pressure drop conditions, it has excellent heat dissipation, which reduces the cost of the fluid circulation system.

Type 3 emphasizes the structural form in which the pipe is embedded. The pipe is embedded in the bottom plate to form a cold plate with strong mechanical properties. The surface extension liquid cold plate uses a thicker and more densely packed tube to increase the area, thereby expanding the surface area in contact with the coolant, thereby improving the heat transfer performance.

An example of a B-type cold plate product

This product is lighter in weight but cannot bear its weight.

Liquid cold plate typical process

The liquid cold plate production process is more complicated than the general air-cooled heat sink. The liquid cooling heat dissipation has high requirements on the process reliability, so the manufacturers with strong technical precipitation can provide reliable technical support. The general liquid cold plate production technology has the following several techniques.

Buried tube process

The buried pipe process is the most widely used liquid-cooled radiator liquid-cooled plate manufacturing process. Generally, the aluminum substrate is buried in a copper tube, that is, the aluminum substrate is machined by a CNC, and then the punched copper tube is bent. It is pressed onto an aluminum substrate, brazed and then processed into a water-cooled plate.

The buried tube type liquid cooling plate generally has three forms: one is a shallow buried liquid cold plate; the other is a deep buried liquid cold plate; the third is a welded pipe process; and the fourth is a double-sided pinch process liquid cold plate. The three forms of crafts are not much different, and the difficulty of processing is the same. Some of the original liquid cooling principles designed for high-power switching devices can also be used for reference in power battery cooling systems.

Shallow tube process: It is suitable for single-sided installation. After the copper tube is flattened and the aluminum plate is milled at the same time, it is beneficial to the high thermal conductivity of the copper tube to take away the heat. The use of aluminum lightweight plays the role of weight reduction and cost control.

Deep buried pipe process: The filler is made of high thermal conductivity epoxy resin imported from the United States. When the temperature difference of the cooling device is not high, it can be installed on both sides, because the thickness of the copper pipe is not processed twice, and the filler protection can provide application, safety, especially suitable for cold plates with refrigerant as medium.

Welded pipe process: suitable for copper plate + copper pipe, in order to reduce the thickness of the plate to reduce weight.

Double-sided pinch process: two-sided mounting device, simple process and low cost; aluminum plate + aluminum tube & copper tube & stainless steel tube.

Profile + welding

Liquid-cooled heat sinks processed on the basis of profiles. These heat sinks are more shaped and have more types, such as plate type, channel type, and combined type. The general production principle is based on profiles. Processing and welding, the profile and the joint pipe are combined into a whole liquid cooling radiator.

The cold plate flow path is directly formed by the extrusion process, and then the cycle is opened by the machining method, and the welding process is usually performed by friction welding, brazing welding, etc., the process has high production efficiency and low cost; and is not suitable for excessive heat dissipation density. The application is not suitable for applications where the surface has too many screw holes to limit the water course or reduce reliability. Mainly used in: power battery water cooling heat sink, water separator and standard power module integrated heat sink products.

Machining + welding

The water-cooled plate adopts the machine-added mode, and the internal flow channel size and path can be freely designed. It is suitable for thermal management products with large power density, irregular heat source layout and limited space. It is mainly used in: wind power converters, photovoltaic inverters. The design of heat dissipation products in the fields of IGBTs, IGBTs, motor controllers, lasers, energy storage power supplies, and supercomputers is less used in power battery systems.

The microchannel heat sink is also a heat sink manufactured by combining machining and welding processes. It is more complicated to manufacture than other heat sinks. Microchannel heat sinks are generally used on machines with large heat dissipation and concentrated heat dissipation. The way of the channel is because the water channel is wider and more uniform, and it can quickly take away the concentrated heat.

However, the micro-channel liquid-cooled heat sink manufacturing process is also relatively complicated, generally using a machined microchannel, and then using a friction welding process for welding, and the manufacturing cost is also high.

Die casting + welding

The die-casting process is a very mature and widely used forming method. With the rapid development of new energy vehicles, it has become the first choice for mass production of motor controllers, power battery pack trays and heat-dissipating boxes, but it is necessary to control die-casting impurities in the process. Problems such as stomata, conservative use of sealing ring method or friction welding all need to improve the reliability of the process to avoid water leakage.

Die-casting and re-welding, good process control, stable process, and batch delivery capability. In addition to the friction welding process, some water-cooled plates are also brazed or vacuum brazed.

This kind of water-cooled plate can be considered together with the battery-packed die-casting box. The AudiQ7PHEV lower-layer water-cooled plate is such a usage. At the Beijing Auto Show two days ago, the molded sample display has been seen.

Typical model water-cooled plate

In the power battery system, there are many ways to take heat away from the surface of the battery. Only in the scope of action, it can be divided into a battery-level water-cooled plate integrated inside the module and a module-level water-cooled design outside the module. board. The following are a number of pictures from the public number of "power battery Thermal Management Technology" to illustrate the application of water-cooled panels in actual cases.

Module level water cooled plate

As a whole, the water-cooled plate acts on one or more battery modules. As the component of the whole battery pack, instead of the components of the battery module, we put it under the head of the module-level water-cooled plate.

AudiQ7PHEV battery pack

Mercedes-Benz SmartGen3 battery pack

ChevroletBolt2017 battery pack

ChevroletBolt2017 liquid cold plate physical

BMW i3 liquid cooling system

BMW i3 liquid cold plate physical

Battery pack and cold plate for BMWi8

Battery level water cooled plate

A water-cooled plate or a sheet of dielectric sheet with good thermal conductivity is sandwiched between the cells to form a part of the module for better heat dissipation. This type of material is placed under the head of the water-cooled plate inside the module.

VolvoXC90T8 battery pack module explosion diagram

GMVolt module structure

GMVolt cooling structure

Tesla ModelS Module

Tesla cylindrical battery water-cooled plate patent description

Observing the application case, we can see that the square battery liquid cooling system mostly uses the module level water cooling plate, and is generally placed at the bottom of the battery box; the soft pack battery liquid cooling is the form of integrated small water cooling plate inside the module. There is also an integrated aluminum plate in the module, and a module-level liquid-cooled plate is arranged outside the module; the cylindrical battery, the serpentine tube headed by Tesla is the main liquid cooling radiator.

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