Week 10 Thermal Management

Thermal management of battery systems in electric vehicles is critical for maintaining energy storage capacity, driving range, cell longevity and system safety. The thermal control will provide better cell/module temperature uniformity, less complicated design and a safer system. The heat generation phenomena and critical thermal issues of lithium-ion batteries along with thermal modelling of battery is discussed in paragraphs below.

Thermal model of battery pack

From the Simulink above is a Themal model of battery pack illustrating the effect of temperature submitted to a variable ambient temperature during a discharge and charge process. As shown from the Scope, the temperature dependent battery model performs close to reality, As the cell/internal temperature increases/decreases due to charge(or discharge) heat losses and ambient temperature variations, the output voltage and capacity also increase/decrease

Temperature effects are represented by these equations.

where:

• T_refis the nominal ambient temperature, in K.
• Tis the cell or internal temperature, in K.
• T_ais ambient temperature, in K.
• E/Tis the reversible voltage temperature coefficient, in V/K.
• α is the Arrhenius rate constant for the polarization resistance.
• β is the Arrhenius rate constant for the internal resistance.
• ΔQ/ΔTis the maximum capacity temperature coefficient, in Ah/K.
• Cis the nominal discharge curve slope, in V/Ah. For lithium-ion batteries with less pronounced discharge curves (such as lithium iron phosphate batteries), this parameter is set to zero.

Where:

CT, is heat capacitance

T, is inner cell Temperature

Ta, is ambient Temperature

RT, is convection resistance

Ps, is Power dissipated inside the cell

Thermal Runaway

Thermal runaway is a condition caused by a battery charging current or other processes that produce more internal heat than a battery can dissipate. It increases the temperature in a way that causes a further increase in heat and often leads to a destructive result.

The diagram above is showing battery temperature increased sharply and reached the maximum temperature T in just a few seconds.

Cooling materials and methods

Battery thermal management systems are used for cooling battery pack to achieve significant effect on the energy storage, durability, life cycle, and efficiency on an electric vehicle. Therefore it is necessary to choose a proper cooling method for a battery module/pack to maintain the required operating temperature. Use of nanofluids flowing in the corrugated mini-channel of the battery module cools down the cells, maintaining optimum temperature. It has been proven that, the best cooling performance is obtained with nanofluids as coolant showed 30% reduced the maximum temperature as compared with the traditional method. However, the pressure drop also increases. Besides, the nanofluids as coolant give the cooling capacity higher than that water as a coolant.

Another form of protection against high tempeature is to place protective material between battery modules. A thermal conducting film, thermal conducting plates, fans and liquid cooling systems are used to cool down the battery or to dissipate heat from a hot area.

Electric Vehicle battery pack

Installing a battery in a cool area or ventilated space, using temperature-compensated charging voltage, and installing an active or passive cooling system can avoid unnecessary high temperature.

A battery management system manages the battery within acceptable performance and safe operating conditions. It requires precise measurement of cell voltage, charge and discharge currents, and temperature. As all of these could cause a rise in cell temperature, cells are also equipped with a polyswitch that prevents excessively high currents out of the cell. Cells are equipped with a burst valve that opens when the pressure gets too high, or the battery is exposed to heat externally, to reduce the temperature and prevent thermal runaway.

Active thermal management relies on cooling technologies using a substance such as air, liquid and refrigerant cooling. It involves an external device that helps dissipate heat. Active methods are generally more expensive and complex than passive techniques.

Passive methods rely upon the thermodynamics of heat transfer conduction, convection and radiation. Passive battery cooling includes metal heat-sinks, PCMs and specialised heat-shields. These are typically less expensive than active technologies and are easier to put in place