Thermal management of electric vehicle batteries involves both cooling and heating to maintain optimal battery temperature. Various cooling methods exist, including air cooling which uses cabin air or an independent system, liquid cooling which circulates coolant, and direct refrigerant cooling. Heating is mainly done through resistive heaters. Proper temperature control maximizes battery life and performance by keeping it within its optimal operating range.

1. Electric
Vehicles in
America
Battery Thermal
Management –
Hardware and System
Layout Overview
Alfred Piggott EE3120
4/17/2011

2. 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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3. 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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4. 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.
Figure 1: Li-ion Battery Capacity and Temperature (www.mpoweruk.com)
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.
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5. Transferring heat out of the battery starts with the cell geometry. Typical battery cell geometries are
cylindrical, prismatic and pouch style. (Figure 2)
Figure 2: Typical Battery Cell Geometry (www.behrgroup.com)
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 3: Transferring heat from the battery (www.behrgroup.com)
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6. Figure 4: Round Battery Coolant Manifold (Tesla Motors Patent Application US 2010/0104938 A1)
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.
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7. 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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8. Independent Air Cooling System
The Independent air cooling system uses preconditioned air from the cabin plus another evaporator
dedicated to the batteries. (Figure 6)
Figure 6: Independent Air Cooled System (www.behrgroup.com)
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
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9. 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.
Figure 8: Liquid Cooled Battery Pack (http://gm-volt.com)
Figure 7: Liquid Cooled Battery Pack (http://gm-volt.com)
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
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10.  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 10: Evaporator Plate (www.behrgroup.com)
Figure 9: Direct Refrigerant System (www.behrgroup.com)
Battery Cells Evaporator
Plate
Figure 11: Evaporator Plate Battery Pack (www.behrgroup.com)
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11. An example of a direct refrigerant system is the 2009 Mercedes S400 BlueHybrid. (Figure 11)
Figure 11: Direct Refrigerant System (www.behrgroup.com)
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
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12. 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)
. mC p (T final − TInitial )
Q= (Equation 1)
Time
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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13. 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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14. 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>.
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