Electric Vehicles in America

Battery Thermal Management – Hardware and System Layout Overview

Alfred Piggott EE3120 4/17/2011

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Table of Contents

Summary .............................................................................................................................................................. 3

Battery Thermal Management .......................................................................................................................... 3

Battery Cooling ................................................................................................................................................... 4

Types of Battery Cooling Systems ................................................................................................................... 6

Air Cooled Systems ....................................................................................................................................... 6

Cabin Air Cooled System ............................................................................................................................. 7

Independent Air Cooling System ................................................................................................................ 8

Refrigerant Chilled Coolant System ............................................................................................................ 8

Direct Refrigerant ........................................................................................................................................ 10

Battery Heating ................................................................................................................................................. 12

Types of Battery Heating ................................................................................................................................ 12

Resistive Heater (Joule Heaters) ................................................................................................................ 12

Alternating Current Heating ...................................................................................................................... 12

Conclusion ........................................................................................................................................................ 13

Bibliography ...................................................................................................................................................... 14

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SUMMARY

Thermal management of EV (Electric Vehicle) and HEV (Hybrid Electric Vehicle) batteries allows

for the optimization of battery life and performance. Various battery heating and cooling methods

exist to control battery temperature within an optimal range, each with their own advantages and

disadvantages.

BATTERY THERMAL MANAGEMENT

Thermal management of an electric vehicle battery falls into two categories, battery cooling and

battery heating. Both heating and cooling are needed to maintain the vehicle battery in an optimum

temperature range which will maximize both performance and battery life.

A general trend for all battery chemistries is discharge time (Capacity) increases as temperature rises

above 25°C (77°F), discharge times decrease as temperature falls below 25°C (77°F). Charge times

increase as temperature drops below 25°C (77°F), and decrease as temperature go above 25°C (77°F).

Battery life increases as temperature drops below 25°C (77°F), battery life decreases as temperature

go above 25°C (77°F) (1)

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Lithium Ion (Li-ion) battery chemistry is dominating current production and new development

electric vehicles as well as consumer electronics. Li-ion operating temperature ranges from -10°C to

40°C (14°F to 104°F) below -10°C performance is significantly degraded and above 40°C the life of

the battery is reduced. The effect of battery temperature can be seen as the area under the curves in

figure 1.

Not only is absolute temperature important to performance and battery life but temperature gradient

within the battery cells is also important to control and is limited to 5° to 10° C. Temperature

gradient between the cells is limited to 5°C (7)

BATTERY COOLING

Although the energy conversion process from chemical energy to electric energy is around 95%

efficient, significant heat is generated in the battery. The heat generated in the battery is due to I2R

losses and enthalpy changes caused by chemical reactions in the battery. The rate of heat generation

is dependent on the battery chemistry and construction, Initial and final state of charge, battery

temperature and charge and discharge rate and charge and discharge profile. (7) If this heat generated,

in the battery is not dissipated at the same rate it is being generated the battery temperature will

increase.

Figure 1: Li-ion Battery Capacity and Temperature (www.mpoweruk.com)

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Transferring heat out of the battery starts with the cell geometry. Typical battery cell geometries are

cylindrical, prismatic and pouch style. (Figure 2)

Cylindrical battery cells are undesirable compared with Prismatic or Pouch style. This is owed to an

unfavorable surface to volume ratio compared with Prismatic and pouch style. The Prismatic and

pouch style have surfaces favorable for contact to heat conducting elements of the battery cooling

systems.

There are several ways (Figure 3 and 4) to transfer heat from the battery cells. When ease of

assembly, cooling effectiveness and packaging space are considered, Base/head cooling and

conductor cooling are most favorable. (8)

Figure 2: Typical Battery Cell Geometry (www.behrgroup.com)

Figure 3: Transferring heat from the battery (www.behrgroup.com)

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TYPES OF BATTERY COOLING SYSTEMS

Air Cooled Systems

There are two types of air cooled battery cooling systems, Cabin Air Cooled systems and

Independent Air Cooled Systems.

Figure 4: Round Battery Coolant Manifold (Tesla Motors Patent Application US 2010/0104938 A1)

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Cabin Air Cooled System

The Toyota Prius is an example that uses a Cabin Air Cooled system. This system uses

preconditioned air from the vehicle cabin cooling system to cool the battery pack (Figure 5). A

cooling fan draws air from the vehicle cabin. The air flows over the surface and / or through

channels in the battery and is exhausted to the outside of the vehicle.

Figure 5: (Cabin Air Cooled Battery Pack) www.autoshop101.com

Advantages

May use less energy since the cooling or heating air is already conditioned

May be lower cost than a liquid cooling system due to less complexity

Disadvantages

May be effective in mild climates, but not enough capacity in harsh hot or cold climates

Lower convection coefficient of air compared to liquid means less responsive system

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Independent Air Cooling System

The Independent air cooling system uses preconditioned air from the cabin plus another evaporator

dedicated to the batteries. (Figure 6)

Advantages

Batteries can be cooled below the cabin air temperature

The larger temperature difference between the air and battery compared with Cabin air

cooling will make the system more effective at removing heat

Disadvantages

Higher Cost than the Cabin Air Cooled system

Higher complexity than the Cabin Air Cooled system

More mass than a Cabin Air Cooled system

Refrigerant Chilled Coolant System

The Chevy Volt System (4) (Figure 7) uses a Refrigerant Chilled Coolant system. Coolant is circulated

by an electric auxiliary water pump through thin plates located between each cell (5) (Figure 8). A

three way control valve allow the coolant to either be cooled by an ambient air cooled radiator, an

Figure 6: Independent Air Cooled System (www.behrgroup.com)

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air conditioning chilled loop or bypass the cooling loops and keep circulating coolant around the

battery. The later of the loops contains an electric heater for battery heating.

Advantages

More precise thermal management of the battery than an air system

Reduced battery warranty over an air cooled system

High performance compared to air cooled systems

Disadvantages

Complexity due to many parts and functions

Higher Cost

Increased Mass may reduce fuel economy

Figure 8: Liquid Cooled Battery Pack (http://gm-volt.com)

Figure 7: Liquid Cooled Battery Pack (http://gm-volt.com)

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Lag in cooling response due to thermal mass of the coolant compared with Direct

Refrigerant Systems

Larger packaging space compared to Air Cooled Systems

Direct Refrigerant

Direct refrigerant systems connect an evaporator plate in parallel with the current evaporator of the

vehicle air conditioning system. (Figure 9,10,11) The evaporator plate(s) makes direct contact with

the battery cells and transfer heat from the battery cell to the refrigerant.

Figure 9: Direct Refrigerant System (www.behrgroup.com)

Figure 10: Evaporator Plate (www.behrgroup.com)

Evaporator

Plate Battery Cells

Figure 11: Evaporator Plate Battery Pack (www.behrgroup.com)

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An example of a direct refrigerant system is the 2009 Mercedes S400 BlueHybrid. (Figure 11)

Advantages

Packaging Space is relatively small

Lower complexity than a Chilled Coolant System

Disadvantages

Battery cooling only possible when the AC system is running unlike the Chilled Coolant

System

No technology available to integrate battery heating into the refrigerant circuit

Figure 11: Direct Refrigerant System (www.behrgroup.com)

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BATTERY HEATING

As mentioned previously, at low temperature, battery performance drops significantly. The

mechanism of this performance drop is increased viscosity of electrolyte in the battery. The viscosity

limits the flow of current in the battery and has a dramatic effect on battery capacity (2). Battery

heating becomes the solution in cold weather.

The amount of power required to heat the battery can be calculated knowing the amount of heat

required to change the battery from an initial temperature to a final temperature, the mass of the

battery, the specific heat capacity and the desired amount of time in which the heating is to take

place. (Equation 1)

TimeTTmC

Q Initialfinalp )( −= (Equation 1)

TYPES OF BATTERY HEATING

Resistive Heater (Joule Heaters)

The dominant heating method for electric vehicle batteries is resistive heaters. For chilled Coolant

Systems like the Chevy Volt, the resistive heating element is placed in the flow path of the coolant.

Air cooled systems rely on cabin heat to warm the battery. Direct Refrigerant may not provide a

heating system. Having no heating system or a low capacity air system may be okay for mild Hybrid

vehicles because loosing some functionality of the electric motor or regenerative braking will not

cease operation of the vehicle. In a fully electric vehicle this may not be acceptable.

Alternating Current Heating

One way to heat a battery that is still in the concept stages is heating the battery with alternating

current. One study (6) used alternating current to heat the battery pack and compared it with four

other heating methods. The other methods mentioned were heating the battery pack with external

electric heaters (Joule Heaters), heating each cell with electric heaters, using hot fluid to heat the

battery pack and using hot fluid to heat each cell. To perform the heating on a pure electric vehicle,

a 100 amp current was used at 60Hz. For an HEV, a 60 amp, 10 kHz current was used to prevent

.

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damage to smaller and lighter power electronics. The power required to heat a 40 kg battery from -

30°C to 0°C in 2 minutes was 9.76 kW for a 100% efficient process or 19.52 kW for a 50% efficient

process. The study concluded using alternating current was the most effective and used the least

amount of energy compared to the other studied methods.

CONCLUSION

To optimize the performance and life of electric vehicle batteries, engineers have devised many

different systems for thermal management. Each system has advantages and disadvantages. Which

system, combination or systems or future technology that is utilized will depend on the higher level

goals and targets of the vehicle for which they will be employed.

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BIBLIOGRAPHY

1. "Temperature Effects On Battery Performance & Life." Http://www.discover-

energy.com/files/shared/Discover_temperature_effects_charging.pdf. 2009. Web. 11 Apr. 2011.

<www.discover-energy.com>.

2. Stuart, T. A., and A. Handeb. "HEV Battery Heating Using AC Currents." Http://www.utd.edu. University of

Toledo, Toledo, OH, USA, Lake Superior State University, Sault Ste. Marie, MI, USA, 29 Sept. 2003. Web.

11 Apr. 2011. <http://www.utd.edu/~axh059000/publications/JPS_battery_heating.pdf>.

3. "Battery Life and How To Improve It." Electropaedia, Energy Sources and Energy Storage, Battery and Energy

Encyclopaedia and History of Technology. Woodbank Communications Ltd. Web. 12 Apr. 2011.

<http://www.mpoweruk.com/life.htm>.

4. WopOnTour. "The Chevrolet Volt Cooling/Heating Systems Explained." GM-Volt: Chevy Volt Electric Car Site.

Dr. Lyle J. Dennis, Dec. 2009. Web. 13 Apr. 2011. <http://gm-volt.com/2010/12/09/the-chevrolet-volt-

coolingheating-systems-explained/>.

5. "Dana Battery Cooling Technology Featured on All-New Chevrolet Volt -- MAUMEE, Ohio, Feb. 9, 2011

/PRNewswire/." PR Newswire: Press Release Distribution, Targeting, Monitoring and Marketing. Dana Holding

Corporation, 9 Feb. 2011. Web. 13 Apr. 2011. <http://www.prnewswire.com/news-releases/dana-battery-cooling-

technology-featured-on-all-new-chevrolet-volt-115630804.html>.

6. PESARAN, Ahmad, Andreas VLAHINOS, and Thomas STUART. "Cooling and Preheating of Batteries in

Hybrid Electric Vehicles." National Renewable Energy Laboratory. 16 Mar. 2003. Web. 14 Apr. 2011. <

http://www.nrel.gov/vehiclesandfuels/energystorage/pdfs/jte_2003-633_sw_ap.pdf>

7. Behr. "Li-ion Battery Cooling: More than Just Another Cooling Task." Www.behrgroup.com. Behr. Web. 15 Apr.

2011.

8. Heckenberger, Thomas. "Lithium Ion Battery Cooling: More than Just Another Cooling Task." Behr. Behr, 20 May

2009. Web. 16 Apr. 2011. <www.behrgroup.com>.

9. Pesaran, Ahmad A., and Matthew Keyser. "Thermal Characteristics of Selected EV and HEV Batteries."

National Renewable Energy Resource Laboratory. 9 Jan. 2001. Web. 17 Apr. 2011. <http://www.nrel.gov>.