Week 10 Thermal Management

Lithium-ion cells performance depends on both the temperature and the operating voltage. Lithium-Ion cells work well when cells operate within limited voltage and temperature. Otherwise, damage will occur to the cells and will be irreversible.

The BTMS 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 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 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 rising of battery temperature, especially when the vehicle is parked outdoors.

4. Ventilation: Ventilation is required to exhaust the hazardous gases within battery pack. In

some systems, such as air systems, this function is combined with cooling and

heating functions.

Different thermal management techniques are used in battery thermal management systems as follows:

Air Cooling and Heating :

Air systems use air as the thermal medium. The intake air could be direct either from atmosphere or from the cabin and could also be conditioned air after a heater or evaporator of an air conditioner. The former is called passive air system and the latter is 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 an active system power is limited to 1 kW.

Liquid Cooling and Heating:

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. Depending on

the different liquids, different layouts are designed. For direct-contact liquid, the normal

layout is to submerge modules in mineral oil. For indirect-contact liquid, a possible

layout can be either a jacket around the battery module, discrete tubing around each

module, placing the battery modules on cooling/heating plate or combining the battery

module with cooling/heating fins and plates. Between these two groups,

indirect contact systems are preferred in order to achieve better isolation between

battery module and surroundings and thus better safety performance.

In passive liquid cooling, the liquid that is pumped to battery pack exchanges heat with the ambient outside air. In active cooling, the liquid exchanges heat with secondary air conditioned circuit.

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 battery

pack.

PCM- Phase Changing medium

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 melting point for a period and

the temperature increase is delayed. Therefore, PCM is used as conductor and buffer in

battery thermal management systems. Also, a PCM is always combined with air cooling system or

liquid cooling system to manage the battery temperature.

Thermo-electric Module :

To improve cooling/heating power of passive air systems, there are two possible

upgrades. One is through thermo-electric modules, which will be introduced here. The

other is a heat pipe .

Thermo-electric module can convert electric voltage to temperature difference and viceversa. Here the former effect is adopted. That means it transfers heat through the module

by consuming electricity directly.

Two fans are installed to improve heat transfer by forced convection. To combine a

passive air system with thermo-electric module, the combined system is able to cool

down the battery even lower than the intake air temperature, but the power is still

limited to around some hundreds of watts and less than one kW. It’s easy

to switch between cooling and heating operation. To achieve that, the poles of

electrodes need to be reversed.

Heat Pipe:

Besides thermo-electric modules, a heat pipe is another way to upgrade passive air

systems. The flat copper envelope of the heat pipe was under partial vacuum. The capillary structure is made of sintered copper powder. The heat pipe uses water as the working fluid. Water on the evaporator side will absorb heat and become vapour lower than100°C due to low pressure inside. Water on the condenser will dissipate heat to the surrounding and become liquid again. This cycle repeats again and again.

Note that, in comparsion to thermo-electric, a heat pipe is more reliable, because there

are no moving parts and no energy consumption. However, a heat pipe is unable to heat

the battery due to its fixed structural layout.

PTC Heater- Positive temperature coefficient:

PTC thermistors have many self-heated applications by utilizing their own voltagecurrent or current-time characteristics. One of the applications is as a self-regulating

heater known as a PTC heater. The temperature of a PTC heater can be kept at a fixed

point through adjusting the resistance of PTC heater automatically.

Based on the above mentioned methods, researchers have come recommend two cooling systems:

Combined Liquid System (CLS) :

The first is the liquid cooling system, which has four working modes:

• Bypass with heater working. - Used to heat the battery pack.
• Bypass without heater working.
• Passive cooling system and active cooling system.

The cooling system takes advantage of both passive cooling system and active cooling system. Passive cooling system has simple structure and is able to dissipate heat under normal conditions with low energy consumption. Under extreme condition, active cooling system has good thermal performance to keep battery temperature in the required range.

PCM Model (CLS+PCM):

The other preferred system is the combination of PCM material and CLS. PCM layers

are inserted into battery pack. It uses the advantage of PCM to have good thermal

performance, and CLS compensates for the limited operating temperature of PCM.