Thermal Evaluation of a High-Voltage 

Ultracapacitor Module for Vehicle 

Applications

Jason Lustbader

(Jason_Lustbader@nrel.gov)

Charlie King

Jeff Gonder

Matt Keyser

Ahmad Pesaran

July 15th, 2008

Advanced Capacitor World Summit

San Diego, CA July 14-16, 2008

NREL/PR-540-43564
2

Outline

• Objectives

• Cell Testing

– Calorimeter testing

– Thermal imaging

• Module thermal testing

• Observed self-cooling

• Summary
3

Objectives

• Identify thermal issues of ultracapacitor 

cells and modules over a range of vehicle 

duty cycles to understand and minimize 

thermal impacts

• Identify improvements for ultracapacitor 

thermal management
4

Cell Description: 

Maxwell BOOSTCAP 3000-P 

• Voltage Range = 0 V – 2.7 V

• Crated = 3000 F

• Toperating = -40 C to +65 C

• m = 0.55 kg

• Carbon electrodes

• Aluminum current collectors

• Organic electrolyte (Acetonitrile)
5

Calorimeter Description

• Cavity dimensions: 21 x 20 x 39 cm (WxHxL)

• Heat rate detection: 0.015 W to 100 W

• Minimum detectable heat effect: 15 J (at 25oC)

• Baseline stability: ± 10 mW 

• Temperature range: -30oC to 60oC

• Large conduction calorimeter that measures 

heat generation and heat capacity 
6

Calorimeter Results: Heat Generation and Efficiency

Current Square Wave, 5 Cycles, Ttest = 30 C, Single BCAP3000-P Cell

At 200 A, Requiv = 0.000350 Ω
7

Calorimeter Results: 

Heat Capacity, Ttest = 30°C, Single BCAP3000-P Cell

0

500

1000

1500

2000

2500

0 0.5 1 1.5 2 2.5 3 3.5 4

Time (hours)

Heat Rate (mW)

Tm

Q C removed

p ∆

=

Starting Cell T= 43.9oC

Ending Cell T = 29.5oC

Average Cell T = 36.7oC

Mass = 0.545 kg

Energy = 8453 J

Heat Capacity = 1079.6 J/kg-K

Three test average results

Heat capacity deviation < 1%
8

Thermal Imaging
9

Thermal Imaging: Single BCAP3000-P Cell 

200 A, Square Wave Cycle, Tambient = 24 C

+ + +

- - -
10

Thermal Imaging: BCAP3000-P Series String of 5 Cells

200A, Square Wave Cycle, Tambient = 22°C

Cells coated for uniform emissivity

+

-

Aborted test, terminal cell heating 

+

-

All images when target temperature was equal to 40°C

Switched positive end cells, retorqued 

+

- Improved uniformity over aborted case

Large bus bar on positive terminal

-

+

Improved uniformity over re-torqued case

1.

2. 3.
11

Thermal Imaging: BCAP3000-P Series String of 5 Cells 

200 A, Square Wave Cycle, Tambient = 22°C, Cells Switched 

+

-
12

Module Thermal Testing Facility Description

• ABC-1000 bidirectional 

programmable power supply

– 420V, 1000A, 125 kW

• Environmental chamber

– 64 ft3

– -45 C to 190 C 

• Independent data acquisition 

system
13

Maxwell Module BMOD0063-125 V 

• Early module design

• 48 cells 

• Crated = 63 F

• 0 V – 125 V

• Toperating = -40 C to +65 C

• Imax, cont = 150 A (Trise<=15 C)

• Vfan = 13.8 V, Ifan = 6.55 A 

• In chamber air flow 

~ 244 CFM 

• All clearances were greater 

than specified minimums Heat Sink

Cooling 

Air Flow

Cooling 

Air Flow

Heat Sink
14

Module: Internal Thermocouple Locations

Cell Detail Instrumentation

Bus Bar Detail

Top Face

Bottom Face

Top & Bottom Face

Detailed

Air Flow

•Thermal interface pad bottom

•Thermal interface pad top

Indicates positive terminal up

Heat Sink

Heat Sink

Thermal Pad

Thermal Nut

Balance Bar

Bus Bar
15

Module: Other Instrumentation

• Voltages for every cell (48) attached to 

bus bars

• Current

• Airflow

– Mapped flow as a function of pressure drop 

along fins

– Used in-chamber pressure drop to estimate 

flow during chamber tests

External Thermocouple Locations 
16

Thermal Performance Test Cycles

• 20 A charge to 120 V immediately before cycling

• 120 minutes continuous cycling

• Square wave cycle

– 60 V to 120 V

Square wave cycle

• Proprietary Oshkosh Heavy Hybrid cycle: Irms ≈ 225 A

• Light-Duty HEV test cycle: Irms ≈ 90.4 A
17

Light-Duty HEV Test Cycle

• NREL analysis shows significant HEV fuel savings are 

achievable with “low”-energy Ucap energy storage*

• Power profile obtained from simulation to cycle this module:

* Pesaran, A.; Gonder, J.; Brooker, A. “Factors & Conditions for Widespread Use of Ultracapacitors in Automotive 

Applications.” Proceedings of Advanced Capacitor World Summit 2007; July 23-25, 2007, San Diego, CA. 

• Vehicle Assumptions

– Midsize car

– Parallel HEV 

configuration

– Vehicle mass = 1675 kg

– Engine = 110 kW

– Motor = 25 kW

– US06 cycle 

– 80 Wh operating 

window 

– ~10% improvement in 

simulated fuel economy 

over comparable 

conventional vehicle on 

same drive cycle -60

-40

-20

0

20

40

60

80

100

0 100 200 300 400 500 600

Time (s)

Speed (mph), Power (kW) or Energy (Wh)

Driving Speed (mph)

Ucap Power (kW)

Net Energy Use (Wh)
18

Module: Cell Terminal Temperatures

150 A, Sq Wave, Ttest = 30 C

Offset due to fan heating

Module positive 

terminal cell heating

Temperatures close to steady state 

after 2 hours of cycling

Preheating due to initial 

charge prior to cycling
19

Module: Selected Temperatures

150 A, Sq Wave, Ttest = 30 C

Positive terminal lug hotter than cell, 

indicating lead wire heating

Coolest cell at air inlet 

corner (side cooling effect)
20

Module: Selected Temperatures

Light-Duty HEV Test Cycle, 12 cycles (120 min), Ttest = 30 C

• Less than an 8 C rise after 120 minutes of cycling

• Irms = 90.4 A 
21

Module: Cell Terminal Temperature Rise Over Ambient*

Ttest = 0 or 30 C

* Average over last five minutes of cycling, 115-120 min
22

Module: Cell Terminal Temperature Rise Over Ambient*

Ttest = 0 or 30 C

Reduced fan, 221 CFM

* Average over last five minutes of cycling, 115-120 min

244 CFM
23

Module: Cell Terminal Temperature Rise Over Ambient*

Ttest = 0 or 30 C

* Average over last five minutes of cycling, 115-120 min

244 CFM Reduced fan, 221 CFM
24

Module: Cell Terminal Temperature Rise Over Ambient*

Ttest = 0 or 30 C

* Average over last five minutes of cycling, 115-120 min

244 CFM Reduced fan, 221 CFM

HH
25

Module: Cell Terminal Temperature Rise Over Ambient*

Ttest = 0 or 30 C

* Average over last five minutes of cycling, 115-120 min

244 CFM Reduced fan, 221 CFM

HEV

HH
26

Module: Estimated Temperature Distribution*

150 A, Sq Wave, Ttest = 30 C 

• Average over last five 

minutes of cycling

• Bus bar temperature 

used when no cell data 

were available

• Pos and neg cell 

terminals averaged

• Missing data averaged 

and/or estimated

Positive Terminal Negative Terminal

Temperature peak 

down centerline

* Average over last five minutes of cycling, 115-120 min
27

Positive terminal heating

Negative terminal

Module: Center Line Temperatures* 

150 A, Sq Wave, Ttest = 30 C

* Average over last five minutes of cycling, 115-120 min
28

Module: Cell 17 Detail, Exit Side Center

150 A, Sq Wave, Ttest = 30 C

• Capacitor midpoint significantly hotter than terminals, may be good place for 

maximum temperature measurement

• ~40% temperature drop across fin

• ~17% temperature drop from terminal to thermal pad

• ~ 5% of the temperature drop across thermal pad (≈0.85°C)

Heat Sink

Thermal Pad

Thermal Nut

Balance Bar

Bus Bar

Final Average Temperature*

* Average over last five minutes of cycling, 115-120 min

-

+
29

Module: Observed Self-Cooling, Cell Temperatures 

Ttest = 30 C, Fans Off, Full Discharge

•5.5 A charge to 125 V, open-circuit rest for 5 hrs, 20 A discharge to 0 V

•Observed cell self-cooling to below ambient temperature
30

Module: Observed Self-Cooling, Fans Off

Cell 17 Detail (Air Exit Center Cell), Ttest = 30 C

• Cooling trend from cell midpoint to outside environment

• Cell surface midpoint cools ~ 0.82 C

• Requiring ~ 483 J

Heat Sink

Thermal Pad

Thermal Nut

Balance Bar

Bus Bar

Heat Sink Heat Sink

5 sec moving average taken at

cell midpoint minimum
31

Investigation of Self-Cooling,

Endothermic Calorimeter Response on Discharge: 

BCAP 3000-P Cell, Ttest = 30 C

• Preliminary data (insufficient 

rest periods)

• Endothermic response 

measured

• Lead heating will decrease 

measured endothermic 

response for the inner 

chamber

• On the order of 345 [J] of 

cooling for a 1.3 V discharge

• If linear with voltage change 

– ~628 [J] of cooling on full 

discharge 

– giving ~1.06 C of cooling for 

a cell

• Consider the heat gain from 

the environment and 

additional thermal mass in the 

module (terminal nuts, bus 

bar, and heat sink)

Insufficient 

rest periods

Charge and clamp Lead heating

Discharge from 

Vmax to 0.5 Vmax

Test description: charge at 5.5 A to 2.6 V, clamp 

voltage for 3 hrs, discharge to 1.3 V at Idis, rest 3 hrs

Possibly caused 

by self-discharge

10 A 20 A 30 A 50 A 70 A 90 A

Preliminary Data

Idis
32

Explanation of Self-Cooling, Reversible Heat Effect:

Entropy Theory Compared to Measurement

C: Cell capacitance [F]

e: Elementary Charge [C]

k: Boltzmann constant [J/K]

Q: Heat [J]

S: Entropy [J/K]

t: time [s]

T: temperature [K]

U: Potential [V]

VH: Helmholtz Layer Volume [cm^3]

Vo: Total electrolyte Volume [cm^3]

Charged State Discharged State

Reversible heat, entropy model*

Simplified 

Helmholtz 

Layer 

Assumption

* Schiffer, J., et al. (2006). "Heat generation in double layer 

capacitors.” Journal of Power Sources 160(1): 765-772.

• Assume

– Entropy model suggested by 

Schiffer et al.

– A specific capacitance of 

6.5-30 μF/cm2

– Vo ≈ 200 cm3

– dHelmholtz = 0.8 nm

– T change can be neglected 

for entropy calculation

• Found

– Qdischarge = -203 [J] to -515 [J] 

• For a 1.3 V discharge

• T = 30 C

– Agrees reasonably well with the 

Q ~ -345 [J] measured in the 

calorimeter

dt

dU

V

V

e

Ck T

dt

ds T

dt

dQ

o

measrev H















 −=−= 2 ln ,
33

Conclusions: Cell

• Thermal efficiency decreased 

approximately linearly with current

• Heat generation increased approximately 

with the square of current

• Thermal imaging showed that 

– Positive cell terminals tend to heat faster, 

possibly because of cell construction

– Cell terminal connections are important for 

thermal performance
34

Conclusions: Module

• With ~80% of rated unrestricted air flow, the tested module was less 

than 2.8 C above its rated 150 A continuous current temperature

• Vehicle environmental temperatures (-30 C- 52 C*) and power 

demands are highly variable, requiring an understanding of 

ultracapacitor temperatures as a function of these variables

• The current level must be limited to prevent cells from reaching high 

temperatures that reduce life and reliability 

• The module had less than an 8 C rise after 120 minutes of 

continuous cycling on a simulation based HEV US06 drive cycle

• Peak temperatures occurred near the module center and at the 

module positive terminal

• Understanding module temperature distribution is critical to design 

effective thermal management systems and properly locate sensors 

• Preferential cooling of the module centerline and reduction of lead 

wire heating would be beneficial

• Capacitor self-cooling was observed on discharge both at the module 

level and in the calorimeter

*USABC FreedomCAR requirement
35

Acknowledgements

• Supported by the Vehicle Technologies 

Program of the U.S. Department of Energy

• Review and Technical Discussions

– Oshkosh Corporation: Michael Bolton, Lorenzo Pisan

– Maxwell Technologies: John M. Miller, Uday Deshpande, Jeremy 

Coperthwaite

– JME: John R. Miller, Arkadiy Klementov

– NREL: Bob Rehn, Ken Kelly, Rob Farrington, Barb Goodman, Terry 

Penney