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The optimum performance of a li-ion cell lies at a temperature range between 10° and 25°C. The higher the temperature at which the battery is stored, the more quickly it will self-discharge. In most instances, temperatures below freezing won't significantly damage lithium-ion batteries as they don't contain water, but low temperatures also lower the battery‘s performance.
For best efficiency, li-ion batteries should be stored in a well-ventilated, dry area kept around 25°C degrees Celsius. They should be stored away from direct sunlight, heat sources, and water. Batteries should be stacked so that they're stable and won't be bumped, knocked over, or otherwise damaged.
The sensitivity to temperatures above or below the optimum is also the reason why manufacturers have an eye on thermal management that ensures the best performance despite changing weather conditions or sporty driving styles.
Because of the construction of a traction battery, that consists of several hundred battery cells, the temperature distribution within the battery pack is irregular. The need for temperature management therefore is unquestionable, but how to set it up to keep all battery cells at the same temperature always is a question that manufacturers explore in their research and development with the help of intelligent and comprehensive data acquisition solutions.
Today’s state-of-the-art thermal management systems for e-vehicle traction batteries usually integrate a heat exchanger into the battery system to keep all battery cells at an almost constant temperature.
The integration of a heat exchanger is important, as a large temperature gradient between the cells will cause the cells to age at different rates. Otherwise, some battery cells would be getting warmer than other cells and, therefore, these cells will age faster than the cells that remain colder. If there is a large gradient occurring, the whole battery pack’s life span is very likely to be shortened.
Consequently, developers of a thermal management system are aiming to maintain a temperature difference between the battery cells, this ranges from about 2–3 °C from the coolest cell to the warmest cell. In the worst case, a temperature difference of as much as 6–8 °C between the battery cells is accepted but this is usually the case for larger battery packs.
Apart from the temperature difference, which the temperature management of a battery is supposed to compensate, there are other factors that play a role in the development of the system. Besides from the battery cell itself and the size of the battery pack, the technical specifications of a thermal management system depend on – at least two other aspects: The duty cycle in which the battery is used and the region where it is deployed.
For the design process of a battery’s thermal management system, it is important to consider all these factors since each individual factor contributes to the heat generation of a battery cell.
One of the questions that are crucial for the development a thermal management system is, how often and how fast or slow a battery pack will be possibly charged and discharged. As the generation of heat depends on the underlying need for the availability of electricity, the knowledge about the so-called “duty cycle” is important. Battery packs that are to be used in a high-power application, will naturally generate more heat than in a renewable energy storage application.
Another important aspect is the region in which a traction battery operates as part of the e-vehicle. In geographical regions with very high ambient temperature, batteries will begin their operating cycle at already elevated temperature levels and age faster. This fact has to be considered during the development of a thermal management system.
The third factor that influences the thermal management technology of e-vehicle batteries is the chemical components of a battery cell itself. Depending on the chemical composition of the li-Ion battery cell, its performance varies under high loads and temperature operations.
Duty-cycle, geography and cell chemistry define the vantage point for decisions, which materials and strategy to choose in thermal management of a traction battery. Additional influencing parameters are given by the physical characteristics of heat transfer in close connection to the placement of the battery pack within the vehicle. The three different types of heat transfer that must be considered are conduction, convection, and radiation, and their occurrence is determined by the source of heat and the different ways of its distribution by the surrounding materials.
While conduction refers to a direct transfer of heat energy from two objects that are in direct contact, convection occurs when heat is directed through a liquid medium to a heat-sinking device. In contrast to this, a radiative heat transfer refers to heat energy that is generated through electromagnetically thermally charged particles of matter that radiate from one source to another, generally through air. All three methods of heat transfer must be considered in the battery system design, but conduction and convection will have the greatest impact on the thermal system design.
For an active thermal management, it is therefore important to prevent heat distribution or to dissipate the heat through different mediums such as air, liquid, or refrigerant. These are pushed through the pack and over the cells to lower the temperature.
The two most common methods for this are using air or liquid as a coolant. When using air for active thermal management, the system generally integrates a fan, ducting, and heat transfer plates. The benefit of a system that cools with air is, that it can be relatively effective in responding to rapid changes in temperature and has a lower weight than a liquid-cooled system. A second benefit is that the cooled air can directly flow across the cells, dissipating the heat. The disadvantages of air as a coolant are that it is not as effective as liquid. Furthermore, depending on the air-flow design, it can cause irregular heat dissipation. While the battery cells at the beginning of the airflow are effectively cooled, the air itself warms up and does not have the same cooling effect as at the beginning of the cooling circuit. This uneven cooling can cause the cells to age at different rates, thereby reducing the life of the pack.
In contrast, the advantages of a cooling system with liquid are that it is quite an effective medium for quickly transferring heat away from the cells. With a heating element in the system, the liquid can also be used to keep the battery cells warm during cold weather conditions. A thermal management system with liquid as a coolant tends to be a heavier because of its greater mass and poses the risks of leaks in the battery pack, which are two disadvantages that developers must face.
A passive thermal management on the other hand, reduces the evolving temperature of the battery cells without conducting a coolant like air, liquid, or any other cooling medium into the battery pack.
This is possible by means of construction and the choice of selected materials that prevent an increase of heat distribution in and between the battery cells. The use of aluminium or metal housings transfers the heat of the cells to the material, disperses it and radiates it from the battery pack housing into the environment.
A different method of passive cooling by pack housing design is to add fins on the surface of the enclosure. These force an airflow over the pack when the vehicle is moving and thereby dissipating the heat from the pack.
A different choice of material that supports cooling the battery cells passively is a phase changing material (PCM). Phase changing means that this material assumes different aggregate states depending on the ambient temperature. It can go through multiple physical phases when heated, generally from a solid to a liquid. In the design of a passive thermal management system, the PCM is generally a block of solid material, often based on wax and graphite. It is machined or molded and placed in between the cells. As the cells heat up, the PCM absorbs the heat and disperses it by softening or melting. Because a lot of energy is needed to initiate this phase change, the use of a PCM is an effective cooling method.
As the construction of e-vehicles and the electrical performance of battery packs is subject to constant progress, the development of thermal management systems is also undergoing an evolution. Because of this, during the thermal system design and in testing thermal management of batteries, an accurate temperature measurement is needed. As the most accurate location to measure the temperature of a lithium-ion cell is inside the cell, it is, however, also the most difficult location to install a temperature sensor, unless it is designed into the cell itself.
To face the challenge of temperature measurement, one of the most challenging issues in the field of thermal management system development, typically cell measurements are taken from the surface.
For all further studies, analyses, and development steps, a safe, accurate, and easy-to-use temperature measurement system is a crucial tool to decide on the right material and design for active and passive thermal management of an e-vehicles traction battery.