Week 10 Thermal Management

Electric vehicles use large batteries to store energy. The energy flowing into the battery pack as it is charged either from regenerative braking or from the grid and discharged from the pack to power the vehicle and its accessories is measured by electrical current and voltage. The flow of current causes heating in the battery cells and their interconnection systems are proportional to the square of the current flowing multiplied by the internal resistance of the cells and the interconnect systems. The higher the current flow the more the heating effect will be.

The performance of Lithium-Ion battery cells is greatly impacted by their temperature, they do not perform well when too cold or too hot, which can lead to permanent and extreme damage of the cells or accelerated degradation. So in addition to cooling, heating of the cells may also be required at lower ambient temperatures to prevent damage during fast charging when the cells are too cold; this is because the internal resistance of the cells rises when they are cold.

There are 3 common battery thermal management methods used today:

1. Convection to air either passively or forced.
2. Cooling by flooding the battery with a dielectric oil which is then pumped out to a heat exchanger system.
3. Cooling by the circulation of water-based coolant through cooling passages within the battery structure.

The Battery Thermal Management System should be equipped with four essential functions to ensure the right operation conditions of the battery pack:

1. Cooling:

Due to inefficiency, battery cells will not only generate electricity but also heat. This heat should be moved from the battery pack when the battery temperature reaches the optimum temperature or even in advance. Thus, a cooling function is required in BTMS.

2. Heating:

In cold climates, battery pack temperature probably falls below the lower temperature limit. Hence, a heating function, such as a PTC heater, is required to assist the battery pack to reach the proper temperature range in a shorter time.

3. Insulation:

In extreme cold or hot weather, the temperature difference between the inside and outside of the battery pack is much larger than that in mild weather. Battery temperature will thus fall (cold) or rise (hot) sooner out of the proper temperature range. To prevent this, good insulation can slow down the falling or rise of battery temperature, especially when the vehicle is parked outdoors.

4. Ventilation:

Ventilation is required to exhaust the hazardous gases within the battery pack. In some systems, such as air systems, this function is combined with cooling and heating functions.

Air Cooling and Heating:

Air systems use air as the thermal medium. The intake air could be direct either from the atmosphere or from the cabin and could also be conditioned air after a heater or evaporator of an air conditioner. The former is called a passive air system and the latter is an active air system. Active systems can offer additional cooling or heating power. A passive system can offer some hundreds of watts cooling or heating power and active system power is limited to 1 kW. (Valeo, 2010)

Because in both cases the air is supplied by a blower, they are also called forced air

Systems. The Air system offers full functions of heating, cooling, and ventilation, but the exhaust air cannot be returned to the cabin again. In some cases, a heat recovery unit (air-air heat exchanger) is mounted after the battery pack in order to recover the heat from the exhaust air. It can prevent a mixture of exhaust air with intake air and at the same time provide extra saving potential.

Liquid Cooling and Heating:

Besides air, the liquid is another heat transfer fluid to transfer heat. There are generally two groups of liquids applied for thermal management systems. One is dielectric liquid (direct-contact liquid) which can contact the battery cells directly, such as mineral oil. The other is conducting liquid (indirect-contact liquid) which can only contact the battery cells indirectly, such as a mixture of ethylene glycol and water. liquid systems can also be categorized into either passive systems or active systems. In a passive liquid system, the heat-sink for cooling is a radiator. This system has no ability to heat. Heat transfer fluid is circulated by the pump within a closed system.

In an active liquid system, there are two loops. The upper is called the primary loop and the lower the secondary loop. The primary loop is similar to the loop in a passive liquid system, where the heat transfer fluid is circulated by the pump. The secondary loop is actually an air conditioning loop (A/C loop). The upper heat exchanger instead of being a radiator works as an evaporator (EVAP) for cooling operation and connects both loops. During heating operation, the 4-way valve will be switched, and the upper heat exchanger works as a condenser (COND) and the lower heat exchanger works as an evaporator. The heating operation loop is also called a heat pump loop.

Direct Refrigerant Cooling and Heating:

Similar to active liquid systems, a direct refrigerant system (DRS) consists of an A/C loop, but DRS uses refrigerant directly as heat transfer fluid circulating through a battery pack.

PCM:

During melting, heat is absorbed by PCM and is stored as latent heat until the latent heat is up to the maximum. The temperature is kept at the melting point for a period and the temperature increase is delayed. Therefore, PCM is used as a conductor and buffer in battery thermal management systems. Also, a PCM is always combined with an air cooling system or liquid cooling system to manage the battery temperature.

Battery cooling in Tesla Model S:

• The Tesla models S battery cooling system consists of a patented serpentine cooling pipe that winds through the battery pack and carries a flow of water-glycol coolant, thermal contact with the cells is through their sides by thermal transfer material. Again this will remove heat from the side of the cells rather than from the tabs, and overheating a Tesla battery pack under hard driving is easy to do.