Battery thermal management – learn how it protects battery

Whether it is an electric battery or a motorcycle battery, people’s requirements for energy density and charging time are getting higher and higher.

An effective battery thermal management system is of great significance for improving the overall performance of electric batteries or motorcycle battery pack. This article will compare and analyze battery thermal management function and its application in different types of battery.

Functions of battery thermal management

Heat dissipation: When the temperature is too high, the capacity of the battery will decay, and the risk of thermal runaway will increase. Therefore, when the temperature is too high, heat dissipation is required.

Heating: When the temperature is too low, the performance of the battery will decay. If the battery is charged at this time, there is a risk of thermal runaway due to internal short circuit caused by lithium precipitation. Therefore, when the temperature is too low, heating or heat preservation is required.

Temperature consistency: most air conditioners have the functions of frequency conversion and surround blowing, the purpose is to maintain the consistency of temperature in two dimensions of time and space. Similarly, power batteries such as ev battery cell also need to reduce the temperature difference in space as much as possible, and try to ensure that the temperature difference of the battery cells is as small as possible.

Different types of battery thermal management

● Cylindrical cell
Take the Tesla cylindrical battery as an example. The inside of the cooling pipe is divided into four channels. In order to prevent the temperature from gradually rising during the flow of the coolant, resulting in poor heat dissipation at the end, the thermal management system adopts a two-way flow field design.

The two ends of the cooling pipe are both The mouth is also the liquid outlet.Between batteries and between batteries and pipes, materials with electrical insulation but good thermal conductivity (such as Stycast 2850ct) are filled.

Here are some advantages:

(1) Change the contact form between batteries and heat dissipation pipes from line contact to surface contact, increasing heat transfer efficiency;

(2) Promote heat exchange between batteries, which is conducive to improving the temperature uniformity between single batteries

(3) Increase the overall heat capacity of the battery pack, thereby reducing the overall average temperature rise.

● Square cell 
Take the BMW 3 Series as an example. BMW uses a water-cooled condenser, and the condensed refrigerant passes through the pipeline and is connected in parallel.

All the way to the evaporator of the passenger compartment, all the way around a long pipeline to reach the battery pack, enter the battery refrigerant direct cold plate, and then the superheated refrigerant and the superheated refrigerant of the evaporator of the passenger compartment merge together and enter the compressor.

BMW controls the refrigerant direct cooling plate of the battery pack with a thermal expansion valve, and controls the thermal management of the battery cold plate by calibrating the thermal expansion valve.

It is also possible to use an electronic expansion valve to control more accurately, but the cost is slightly higher, which also shows that BMW has a relatively strong control ability in the system.

● Pouch cell
Cadillac CT6 PHEV, Weilan 5 EREV and 6 EV models all use soft-packed ternary lithium batteries provided by LG. Combined with GM’s self-developed power battery thermal management system control technology and strategy.

The purpose is to keep the temperature of the pouch cells in different positions inside the power battery assembly as consistent as possible. The soft-pack battery has no hard shell, and can more fully exchange “cooling capacity” and “heat” with the attached liquid cooling plate.

The 6 PTC module for the power battery of the EV model is set at the front end close to the power battery cable interface, and only a short pipeline can be used to introduce the heated coolant to preheat the battery cell at low temperature. The water-cooled plate control module is directly associated (fixed) with the distribution valve of the air-conditioning pipeline.

Once the high-temperature heat dissipation function of the power battery thermal management system is activated, the “cooling capacity” output from the electric air-conditioning compressor passes through the distribution valve and is directly connected to the water-cooled plate control module. Perform a “cold” exchange.

Comparative analysis of battery thermal management

Similarities:

1. Improve heat dissipation efficiency by increasing heat dissipation channels;

2. By using high thermal conductivity medium, the thermal conductivity rate is improved;

3. Through active cooling to heat conduction and heat dissipation

Difference:

1. The heat conduction medium used is different;

2. The heat dissipation path and heat dissipation channel are different;

3. The heat dissipation area and effective heat dissipation area ratio are different;

4. The cooling medium is also different;

5. The layout of the cold plate is different;

6. The effect of the radiator is different.

Battery thermal management design

Step 1: Decompose customer input to confirm thermal management system goals and requirements
Clarify the cell cooling requirements, heating requirements, temperature difference requirements and insulation requirements.

Step 2: Test and estimate the heating power of the module
According to the customer’s input, it is converted into the heating power of the charge and discharge rate of the battery under different working conditions. Determine the internal resistance at which SOC, which temperature, and which charge-discharge rate is determined according to the factors affecting the heating power.
Step 3: Select the heat transfer medium, preliminary design of the thermal management system
Thermally conductive materials mainly focus on thermal conductivity, density, flame retardancy, insulation performance, thermal stability, etc.

Step 4: Carry out theoretical calculation and simulation analysis of the thermal management system according to the heat conduction path of the module
According to the boundary input, the simulation of flow field and temperature flow field is carried out, including pressure, velocity, flow, and temperature under different working conditions.

Step 5: Experimental verification of the thermal management system
Verify the simulation results, understand the real performance of the battery thermal management, and compare the gap between simulation and experimental results.

Step Six: Optimizing the thermal management system
According to the analysis of the results, the thermal management design is further adjusted, and a thermal management optimization scheme is proposed.