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Principles of power battery cooling system

Battery Colling System
Introduction

It is well known that the performance of the power battery is closely related to its temperature. The appropriate operating temperature can increase battery activity and make full use of electrical energy.
When the battery works in a high or too high-temperature environment for a long time, the lifespan will be significantly shortened, and even serious damage will occur. When the operating temperature of the battery is too low, the active substance is significantly reduced, the internal resistance increases, the discharge capacity decreases, and the energy utilization rate decreases.

The power battery must maintain an appropriate operating temperature to ensure the normal operation of the battery and extend its service life. In principle, the power battery unit is operational in the range from -40°C to +55°C (actual battery temperature). Therefore, at present, the power battery units of new energy are equipped with cooling devices.
The power battery cooling system includes air conditioning circulating cooling, water cooling, and air cooling.

1. Air conditioning circulating cooling

In high-end electric vehicles, the power battery has a refrigerant circulation circuit connected to the air conditioning system. The power battery unit is cooled directly through the coolant, and the coolant circulation circuit is connected to the refrigerant circulation circuit through the coolant refrigerant heat exchanger (i.e., the cooling unit).

Therefore, the refrigerant circulation circuit of the air conditioning system consists of two parallel branches. One is for cooling the interior space of the car, and the other is for cooling the power battery unit. Each branch has an expansion and globe combination valve. The two independent cooling systems are shown in the following figure.

power battery cooling system

How cooling Works

The electric coolant pump transports coolant through the coolant circulation circuit. As long as the temperature of the coolant is lower than that of the battery module, the battery module can be cooled only by using the circulation flow of the coolant. The increase in the coolant temperature is not enough to keep the temperature of the battery module within the expected range. Therefore, it is necessary to reduce the temperature of the coolant with the help of the coolant refrigerant heat exchanger (i.e., the cooling unit). This is the interface between the power battery coolant circulation circuit and the refrigerant circulation circuit of the air conditioning system.

If the expansion and cut-off combination valve on the cooling unit are opened, the liquid refrigerant will flow into the cooling unit and evaporate. This can absorb the heat of the ambient air, so it is also a coolant that flows through the coolant circulation circuit. The electric air conditioning compressor (EKK) compresses the refrigerant again and transports it to the capacitor, where the refrigerant becomes liquid. Therefore, refrigerants can absorb heat again.

How cooling Works

In order to ensure that the coolant channel discharges the heat of the battery module, the whole plane of the cooling channel must be pressed on the battery module with a uniformly distributed force. The compression force is generated by the spring strip embedded in the coolant channel. The spring strip is adjusted according to the geometry of the battery module and the lower half of the shell. The spring strip of the heat exchanger is supported on the lower part of the shell of the high-voltage battery unit, thus pressing the coolant channel onto the battery module. As shown in the figure.

How cooling Works

The rated power of the electric coolant pump in the coolant cycle circuit of the power battery unit is 50W. The electric coolant pump is fixed by a bracket on the cooling unit, which is installed in the rear right corner of the power battery

electric coolant pump

The coolant expansion box and coolant pipeline are shown in the following figure:

coolant

2. Water Cooled

The water-cooled power battery cooling system uses a special coolant to flow in the coolant pipeline inside the power battery, transmitting the heat generated by the power battery to the coolant, thus reducing the temperature of the power battery.
The following is a case study of the power water-cooled cooling system for electric vehicles.

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The cooling system is divided into two independent systems

  • Inverter/drive motor cooling system and
  • High-pressure battery pack cooling system.

The structure of the power battery cooling system is shown in the figure below, which mainly consists of an expansion tank, hose, cooling pump, battery cooler, etc

 power battery cooling system

The cooling system uses the principle of heat conduction to cycle through the coolant in each independent cooling system circuit to keep the driving motor, inverter, and power battery pack at the optimal operating temperature. Coolant is a mixture of 50% water and 50% organic acid technology. The coolant needs to be replaced regularly to maintain its optimal efficiency and corrosion resistance.

  • Expansion Water Tank

The expansion tank is equipped with a pressure relief valve, which is installed on the inverter tray. The overflow pipe is connected to the outlet pipe of the battery cooler, and the outlet pipe is connected to the cooling water pipe tee. The exterior of the expansion tank is marked with “MAX” and “MIN” scales, which is convenient to observe the coolant level.

  • Hose

The rubber coolant hose transmits the coolant between the components, and the spring clamp fixes the hose to each component. The power battery cooling system hose is arranged in the front compartment and under the rear floor assembly.

  • Cooling Water Pump

The cooling fluid pump of the power battery cooling system is installed by mounting a bracket and fixed to the body chassis by 2 bolts, through which the high-pressure battery pack cooling system is recycled.

  • Battery Cooler

The battery cooler is a key component of the power battery cooling system. It is responsible for maintaining the power battery at an appropriate operating temperature to optimize the discharge performance of the power battery. The battery cooler mainly consists of a heat exchanger, an expansion valve with a solenoid valve, a pipeline interface and a bracket. The heat exchanger is mainly used for heat exchange of power battery coolant and refrigerant in the refrigeration system, transferring heat from the power battery coolant to the refrigerant.

The cooling fluid cycle of the power battery cooling system is shown in the following figure:

cooling fluid cycle of the power battery cooling system

The system control chart is shown in the following figure

system control chart

 (Source: Internet, copyright belongs to the original work)

As shown in the previous two pictures, BMS is responsible for controlling the electric pump. The electric pump will be turned on when the temperature of the high-pressure battery pack rises to 32.5°C and closed when the temperature is below 27.5°C. BMS sends a signal requiring the battery cooler expansion valve to close and the pump to operate. ETC receives a signal from BMS to open the so-lentoid valve. ETC first opens the solenoid valve of the battery cooler expansion valve and sends a start signal to the EAC. During normal work, when the coolant temperature of the high-pressure battery pack is above 30°C, ETC will limit the refrigeration capacity of the passenger cabin. The coolant temperature is above 48°C. ETC will turn off the refrigeration function of the passenger cabin, except for the defrost mode.ETC only controls the coolant temperature. BMS controls heat exchange inside the coolant and the BMS high-pressure battery pack. When the vehicle enters fast charging mode, ETC will be awakened by the gateway module. At this time, the high-voltage battery pack cooling system will enter a normal state

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3. Air-cooled

The air-cooled power battery cooling system uses a cooling fan to inhale air from the inside of the car into the power battery box to cool the power battery and the control unit of the power battery

Toyota Prius, Camry (hybrid version), Corolla dual engine, and Levin dual engine all use air-cooled power battery cooling systems. The component composition and schematic are shown in the following figure

The air inside the car flows down through the intake pipe located on the rear window sill decorative panel and flows down through the power battery or DC-DC converter (hybrid vehicle converter) to reduce the temperature of the power battery and DC-DC converter (hybrid vehicle converter). Air is discharged from the car through the exhaust pipe.

GAC Trumpchi AG electric vehicles also adopt an air-cooled power battery cooling system. The assembly diagram of its power battery heat dissipation system is shown in the figure below.

The air inside the car flows down through the intake pipe on the decorative panel of the rear window sill, flowing down through the power battery to reduce the temperature of the power battery, and then passes through BMS, total plus and negative relays, and other electrical components. After lowering its own temperature, the air is removed from the car through the exhaust pipe.

The cooling fan is a DC low-voltage fan equipped with an independent DC-DC converter. When the cooling fan is working, the current flows out of the power battery through the DC-DC converter to convert 350V DC high voltage to 12V-16V DC low voltage and is provided to the cooling fan

The cooling diagram of power battery A and power battery B is shown in the following figure

Power battery A and B cooling path: The air inside the car flows down through the in-take pipe located on the rear window sill decoration board and flows down through the power battery to reduce the temperature of the power battery. Then it passes through BMS, total plus and negative relays, etc. After lowering the temperature of the electrical components, the air is pumped out by the cooling fan and vented. The tube is discharged from the car. This cooling method has an average cooling effect and is generally used for low-power and low-heat power battery cooling

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