What should we know about the liquid cooling system in electric car lithium batteries?

Thermal management systems are designed to maintain a battery within a temperature range suitable for battery operation; reduce the difference between the maximum temperature and the minimum temperature within the battery pack. The thermal management system of the electric car lithium batteries should be able to rapidly dissipate the heat in the battery box under a high-temperature working environment so that the battery can function at its optimal temperature range. Lithium-ion batteries should have a working temperature no greater than 0 °C and no lesser than 55 °C. Secondly, the uniformity of temperatures in a battery pack should be improved to minimize the temperature differences between individual batteries in different positions. Typically, the temperature difference between individual cells in the battery pack shall be less than 10C; Finally, a battery in a low-temperature environment should be heated to the appropriate temperature range as soon as possible. Generally, a temperature rise of over 15 °C should occur within 0.5 hours.

Liquid cooling system composition

The cooling liquid has a large thermal capacity and can take away the excess heat of the battery system through circulation, so as to realize the best working temperature condition of the electric car lithium battery pack. The basic components of the liquid cooling system include the electric water pump, electric core radiator (indirect cooling), temperature sensor, air conditioning system (compressor, condenser, evaporator), heater and liquid-liquid heat exchanger. In high-temperature conditions, the air conditioning system provides cooling; in low-temperature conditions, the heater is responsible for heating the coolant.

Heat transfer principle

Heat transfer media are different, so there are different forms of cooling, such as air cooling, liquid cooling and so on. In principle, we need to start with simultaneous interpreting of heat. There are three main forms of heat transmission: heat radiation, heat conduction and convection.

1. Heat radiation
Thermal radiation is conducted by objects whose temperature is higher than absolute zero. Thermal radiation does not require medium and contact and transfers heat outward in the form of electromagnetic waves.

2. Heat conduction
Heat is transferred from high to low temperatures through a medium. Heat conduction differs from thermal radiation in that it requires two conditions: temperature difference and medium.

3. Convection
Inside the fluid, the relative flow is driven by temperature differences.

Heat, inside the power battery cell, is mainly transmitted to the electric car lithium battery surface in the form of heat conduction, and then to the surrounding space in the form of radiation and convection. If a thermal management system is added to the system, the heat transfer process is partially changed. For example, in the indirect heat dissipation mode, the heat is mainly transferred from the battery surface to the radiator shell in the form of heat conduction, and then transferred from the shell to the radiator channel surface in the form of heat conduction; The heat is transferred to the coolant by heat conduction from the surface of the flow channel. The coolant transfers the heat inside the coolant by convection and transfers it to the outside of the battery pack following the forced flow of the coolant.

Thermal management of battery packs involves three measures: cooling the battery pack, low-temperature preheating the battery pack and thermal insulation of the battery pack.

Thermal management scheme of the battery pack

1. Cooling of the battery pack

The cooling function of the liquid cooling system is mainly realized by circulating low-temperature coolant. If the required heat dissipation power is relatively small and the heat capacity of the coolant itself is relatively large, it is not necessary to start the circulation process, which can meet the requirements of the set temperature range.

There are two main forms of electric car lithium battery pack cooling, direct cooling and indirect cooling. Direct cooling is that the cooling medium flows directly from the surface of the cell to take away excess heat; Indirect cooling is that the cooling medium flows through the channel of the pipe and radiator, and the radiator contacts the electric core to transfer the heat of the electric core to the cooling medium.

2. Low temperature preheating of the battery pack

Originally, the compressor can have a heating function, but its low-temperature heating effect is poor, and the power consumption is relatively large, which has a serious impact on the endurance capacity of the power battery; At the same time, when the temperature is too low, the discharge power of the battery pack is too low or simply lower than the minimum discharge temperature. Therefore, the preheating process before the vehicle starts is designed in the heat management strategy.

There are two basic forms of low-temperature preheating of a battery pack: internal heating and external heating.

Internal heating: use the AC power supply outside the battery pack to heat the battery electrolyte until it reaches the applicable temperature range of the battery pack. The component generating heat is the battery itself, so it is called internal heating.

External heating: the outside power supply is used to heat the medium other than the battery. The medium transfers heat to the battery and gradually increases the battery temperature until the battery is within the appropriate temperature range. The external medium includes air medium and liquid medium. The heating elements include PTC and heating film.

External heating is a common method. The general implementation form is that the battery pack is equipped with a heater, which does not use the power of the power battery, but in the shutdown state, connect the power supply other than the battery pack to supply power to the PTC or heating membrane. The external power supply is generally the electric energy from the large power grid. The heater can work according to the applicable maximum power without worrying about the waste of electric energy. The overall heating rate is relatively high.

3. Battery pack temperature keeping
These measures are not set for each vehicle with a thermal management function. After the vehicle is preheated and the battery pack enters the working state, the battery itself will generate a lot of heat. If it is not in an extremely cold environment and there is no need to stop for a long time, the operating temperature of the battery pack can be maintained by its own heating.

Main factors affecting cooling effect

1.  Coolant temperature
In the cooling process, the lower the temperature of the coolant, the lower the maximum and minimum temperature of the battery, but there is a big gap between the two. During heating, the higher the temperature of the coolant, the greater the temperature difference of the battery. With heating, the higher the temperature of the coolant, the greater the temperature difference between the coolant and the battery. In other words, the greater the temperature difference between the cells at different positions inside the battery pack, the greater the temperature difference between each cell.

This phenomenon is mainly related to the different influence degrees of temperature regulation of the electric core heating management systems at different positions. Some electric cores have a large contact area with the radiator, and some are relatively small; On the other hand, when the coolant circulates inside the battery pack, the temperature changes from inlet to outlet. In different positions, the temperature difference between the coolant and the cell with the same body temperature is different. Only accurate thermal design can solve this problem, rather than simply adjusting the coolant temperature.

2. Coolant flow
With the increase of coolant flow, the maximum temperature of the battery system decreases, but the temperature difference increases. After crossing a maximum temperature difference, the flow continues to increase and the temperature difference begins to decrease. The maximum temperature and temperature difference have been decreasing in one direction while the flow continues to grow.

But the flow cannot continue to increase. On the one hand, it is related to the amount of energy consumed, so it is inevitable to choose a flow with the best cost performance. On the other hand, maintaining a large flow for a long time is a test of the strength of the coolant circulation system. Consequently, the equipment’s service life may be reduced. Therefore, proper liquid flow is also crucial.

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