Advanced Mitigating Measures for

the Cell Internal Short Risk

by

Eric Darcy1 and Kandler Smith2

1. NASA-JSC/NREL; 2. NREL

for the

Electric Aircraft Symposium

23 April 2010

Rohnert Park, CA

NREL/PR-5400-48673
Outline Mitigating Measures For Cell Internal Shorts

• Why are internal cell shorts a concern?

• Fault tree example for cell internal shorts

• Measures to mitigate cell internal shorts

– Design

– Manufacturing

– Operations

– Testing

– Analysis

• Conclusions
Why are Li-ion cell internal shorts a concern?

• Li-ion cells provide the highest 

specific energy (>180 Wh/kg) and 

energy density (> 350 Wh/L) 

rechargeable battery building 

block to date with the long life 

necessary for vehicles

• Electrode/electrolyte thermal 

instability and flammability of the 

electrolyte of Li-ion cells make 

them prone to catastrophic 

thermal runaway under some rare 

internal cell short conditions.

• These incidents are estimated at 

a 1 in 1-10 million probability with 

COTS cells in consumer 

applications*

– Can we lower that probability?

•B. Barnett, et. al., Power Sources Conference, Philadelphia, July 2008

•C. Mikolajczak, et. al., IEEE Portable 2007 Conference, Orlando, 2007
• ~100 incidents since 2001

• ~10 billion cells since 2001

1 in 100 million probability

CPSC Record of Relevant Field Failures

Date Recall # Description Incidents qty Injury qty Recall qty

May-05 05-179 Apple iBook laptop batteries, LG Chem cells (Taiwan/China) 6 0 128,000

Jun-05 05-204 Hi-Capacity ® laptop batteries, China/Korea/Taiwan 6 10,000

Apr-06 06-145 HP Compaq laptop batteries, unknown cell 20 1 15,700

Aug-06 06-245 Apple Powerbook, Dell laptops, Sony cell (Japan) 9 0 1,800,000

Jul-07 07-267 Toshiba laptop batteries, Sony cell (Japan) 3 0 1,400

Oct-08 09-035

Dell, HP, Toshiba laptop batteries, Sony cell (Japan) made from 0ct 04 

to Jun 05 19 2 100,000

totals 63 3 2,055,100

All injuries were minor burns

Above list contains only recalls since 2001 caused by cylindrical Li-ion cell issues

Total worldwide Li-ion cell demand from 2001 to 2008 was 14.5 billion cells*

Latest one (Oct 08) reported with cells made 4 years earlier and indicates a long 

latency.

*Institute of Information Technology, Ltd
Example Fault Tree for Cell Internal Shorts

Fire/Violent 

Rupture

Due to 

cell internal 

short

Separator

Failure

Insulator

Failures

Crimp Seal

Failure

Native or FOD

Contamination

Jellyroll top

insulator

Jellyroll bottom

insulator

Jellyroll outside

wrap insulator

Positive tab

insulator

Crimp seal

+ tab insulator

JR top

insulator

JR bottom insulator

JR wrap
Manned Spacecraft Battery Approval Flow Chart

2-Fault Tolerant?

Approved!

Rejected!

Risk mitigated by defined 

process?

Wrt leakage

Wrt cell internal shorts

Most battery designs are not strictly 2-FT compliant

 Triple containment is required for leakage

 Triple isolation/insulation is required for cell internal shorts

Within safe voltage, energy, 

energy density, surface/volume 

ratio, and chemistry
Building Block Cell is 18650

Consumer Electronics Industry Standard COTS

• Eighty 18650 cells in 16p-5s topology 

are used in the new spacesuit battery

Graphic and photo courtesy of ABSL

Note the bank-to-bank

separator/insulator
Spacesuit Battery “Brick”

Graphic courtesy of ABSL
Final Assembly Steps of Spacesuit Battery

Graphic and photos courtesy of ABSL
Mitigating Measures Applied to Spacesuit Battery – Design, Fab, & Ops

• Design

– Selected mature cell designs in production 

since 2003 – not highest Wh/L models

– Fully characterized for another program in 

2004

– Excellent calendar life proven with lots 3 and 

4 years old

– A p-s topology and charger design allows insight into cell bank balance every charge

• Manufacturing

– Completely automated cell production line at 

both LLB cell vendors (Japan & Canada)

– Fab rates > 1 million cells per month with very high performance uniformity

– Date code of both cell lots is April 07

– Selected cell designs are not subject to any 

CPSC recalls

• Operations

– Tighter voltage window (3.2V to 4.12V/cell) 

than consumer applications (3.0V to 4.2V) 

puts less stress on the jellyroll as do C/8 

charge/discharge rates

– Low cycle life expected (~25) thru 2020, vs

>500 for consumer electronic applications

– Majority of calendar life at 30% vs 100% 

SoC for laptops

– Much lower operating temp (10 to 41°C) vs

(0 to 55°C for laptop batteries)

Cross sections of candidate Li-ion cell designs
Mitigating Measures Applied to Spacesuit Battery – 100% Testing

– Flight COTS cells are rigorously screened to detect manufacturing flaws • Mass, OCV (after > 24 months storage), AC impedance, visual examination

• Thermal cycling, vacuum cycling with residual gas analysis (leak check)

• Initial charge input (after long storage), capacity performance (2 cycles), DC resistance

• Screen out all cells that outside ± 3 sigma and store

• 100% X-ray examination for proper jellyroll alignment

• Repeat OCV, capacity cycling, AC impedance, DC resistance prior to selecting cells for 

brick assembly

– Flight 80-cell bricks are rigorously tested to detect assembly flaws • Visual, Mass, OCV, AC impedance, insulation resistance, fit check with housing

• 15 depress/repress cycles with final one holding vacuum for 3 days (& at 35°C)

• 5 thermal cycles with 3 hr dwells at the extremes (-14 to 45°C) 

• 2 Capacity cycles at room temperature

– one at high rate (C/2 charge to 21V with 1A taper, C/2 discharge to 15V)

– one at mission rate (5A charge to 21.5V with 1A taper, 3.8A discharge to 16V)

– Flight LLBs will be rigorously tested to detect assembly flaws • OCV, CCV, Thermistor Check, Insulation Resistance, Bonding, Mass, and Visual Inspection

• Thermal cycling with 3 hr dwells at the extremes (-14 to 45°C) 

• Random vibration at 0.06 g2/Hz for 1 minute/axis (0.03 is mission, 0.04 is standard 

workmanship level)

• 1 hot and 1 cold mission simulation cycle (charge at ambient, discharge in vacuum)

• 1 autocyle (discharge, charge, discharge, recharged to 30%) with the LIB Charger GSE
Typical X-ray – COTS 18650

Note the proper jellyroll anode overlap alignment
Example of an X-ray Reject – COTS 18650

Note the telescoping jellyroll anode overlap misalignment

• Insufficient anode overlap can lead to Li plating

• This s/n cell had passed all other cell acceptance tests
18650 Initial OCVs after 17 months at as received SoC

9 rejects

Average 3.8046

sdev 0.00197

sdev% 0.052%

Min 3.7771

Max 3.8072

Range 0.0301

Range% 0.792%

 -3sigma 3.7986

 +3sigma 3.8105

Range 0.0118

Range% 0.311%

Lo Rejects 9

Hi Rejects 0

Total Rejec 9

Count 1057

Reject% 0.85%

6σ range is 11.8 mV (0.311% of mean)

9 outliers out of 1057 cells tested
18650 Initial OCVs after 17 months at as received SoC

6σ range improves from 11.8 mV (0.31%) to 7 mV (0.18% of mean) after removing 9 

outliers (not shown). No more outliers appear with this tighter 6σ range

Average 3.8047

sdev 0.00117

sdev% 0.031%

Min 3.8016

Max 3.8072

Range 0.0056

Range% 0.146%

 -3sigma 3.8012

 +3sigma 3.8082

Range 0.0070

Range% 0.184%

Lo Rejects 0

Hi Rejects 0

Total Rejects 0

Count 1048

Reject% 0.00%

Standard dev is at 1.2 mV (0.031%)

Excellent cell-to-cell OCV uniformity after 17 months at ~45% SoC
OCV Retention on Large Cell Designs

• OCVs measured after 

2 years of storage on 

small 18650 cells vs

< 3 months on large 

(15-55Ah) aerospace 

cell designs

• Large cell designs 

found more likely to 

have self-discharge 

outliers than small 

COTS cells

OCVs during receiving inspection

3.35

3.4

3.45

3.5

3.55

3.6

3.65

3.7

3.75

3.8

3.85

0 5 10 15 20

2Ah 18650

55Ah cell

44Ah cell

30Ah cell

52Ah cell
COTS 18650 Initial Charge Capacity

Note: Test performed in Sep 08 and cell date code is Apr 07

(17 months at 50% SoC, RT and 0°C)

Same 9 rejects as low OCVs

Average 1.2245

sdev 0.01368

sdev% 1.117%

Min 1.2138

Max 1.4602

Range 0.2464

Range% 20.123%

 -3sigma 1.1835

 +3sigma 1.2655

Range 0.0821

Range% 6.704%

Lo Rejects 0

Hi Rejects 9

Total Rejects 9

Count 1057

Reject% 0.85%

6σ range is 82 mAh (6.7% of mean)
18650 Discharge Capacity (C/10 to 3.0V)

Standard deviation of 5 mAh (0.23%) is very tight

Average 2.1589

sdev 0.0050

sdev% 0.233%

Min 2.1380

Max 2.1821

Range 0.0441

Range% 2.044%

 -3sigma 2.1437

 +3sigma 2.1740

3sRange 0.0302

3sRange% 1.400%

Low Rejects 2

High Rejects 2

Total Rejects 4

Count 857

Reject% 0.23%
Comparing Large vs Small Cell

• Comparing C/10 discharge 

capacity variations at room 

temperature, all on BOL 

cells, n=20, except for 

2.1Ah cell where n=857

• Large cell standard 

deviation ranges from 

0.64% to 1.05% of mean 

(vs 0.23% for 18650 cells)

• Small cell designs are more 

uniform discharge capacity 

performers

0.00%

1.00%

2.00%

3.00%

4.00%

5.00%

6.00%

7.00%

StDev%

Range%

3sRange%
18650 Self-Discharge

7-day at 4.2V self-discharge rate measured by difference in capacity delivered

Average capacity lost to self-discharge is 17.8 mAh w/ standard deviation = 2.7 mAh

Average 0.0178

sdev 0.0027

sdev% 15.149%

Min -0.0313

Max 0.0220

Range 0.0533

Range% 300.093%

 -3sigma 0.0097

 +3sigma 0.0258

3sRange 0.0162

3sRange% 90.895%

Low Rejects 3

High Rejects 0

Total Rejects 3

Count 857

Reject% 0.35%
Soft Short (Small COTS Cell)

3.02

3.04

3.06

3.08

3.1

3.12

3.14

3.16

0.00 5.00 10.00 15.00

N2-592-S-21

N2-592-S-22

N2-592-S-23

N2-592-S-24

N2-592-S-25

N2-592-S-26

N2-592-S-27

N2-592-S-28

N2-592-S-29

N2-592-S-30

3.02

3.04

3.06

3.08

3.1

3.12

3.14

3.16

0.00 5.00 10.00 15.00

N2-592-S-31

N2-592-S-32

N2-592-S-33

N2-592-S-34

N2-592-S-35

N2-592-S-36

N2-592-S-37

N2-592-S-38

N2-592-S-39

N2-592-S-40

V

o

l

t

s

,

V

V

o

l

t

s

,

V

Days Days

14-day OCV bounce back after deep discharge (constant voltage to 3.0V)

Very uniform OCV bounce back performance
Soft Short (Large Cell Design)

3.025

3.03

3.035

3.04

3.045

3.05

3.055

3.06

0.00 5.00 10.00 15.00

N2-596-Y-1

N2-596-Y-2

N2-596-Y-3

N2-596-Y-4

N2-596-Y-5

N2-596-Y-6

N2-596-Y-7

N2-596-Y-8

N2-596-Y-9

N2-596-Y-10

3.025

3.03

3.035

3.04

3.045

3.05

3.055

3.06

3.065

3.07

0.00 5.00 10.00 15.00

N2-596-Y-11

N2-596-Y-12

N2-596-Y-13

N2-596-Y-14

N2-596-Y-15

N2-596-Y-16

N2-596-Y-17

N2-596-Y-18

N2-596-Y-19

N2-596-Y-20

V

o

l

t

s

,

V

V

o

l

t

s

,

V

Days Days

4 cells out of 20 had declining OCV between days 10 and 14

14-day OCV bounce back after deep discharge (constant voltage to 3.0V)
Mitigating Measures - Testing

• Perform rigorous cell 

acceptance screening

– Serialization and visual

– OCV after long storage period

– AC impedance

– Insulation resistance 

– Mass

– Dimensional (or use of go/no go 

gauges are acceptable)

– Capacity cycling with DC internal 

resistance (for secondary only)

– Thermal cycling with leak detection

– Vibration (can be done at battery 

level)

– Pressure cycling with leak detection

– Self-discharge at 100% SoC (can be 

replaced with long OCV stand test

– Soft short at <0% SoC ( for 

secondary only and can be 

replaced with long OCV stand)

– X-ray inspection (optional)

– Reject all ± 3 sigma outliers

• Perform rigorous cell 

qualification testing of each lot

– Capacity performance

– Environmental exposure

• Thermal cycling

• Shock & Vibration

• Repress/Depress

– Capacity performance

– Mission simulation life 

– OCV vs SOC, temperature

– Calendar life and self-discharge vs

SOC and temperature

– Abuse Tolerance

• Electrical, mechanical, and thermal abuse

– NDE (CT scan) and DPA 

– Cell Production Line Audit (if 

possible prior to committing to 

cell buy)

– Archive a quantity of cells for each 

lot
What to look for in CT scan?

Scan courtesy of Exponent

• Proper alignment of jellyroll contents

• Consistent active material coatings

• Lack of contamination, high density particles show up as bright specs
18650 Weld Splatter Finding

Images courtesy of Exponent
What to look for in DPAs?

• Consistent mechanical 

alignment

– Anode overlapping cathode

– absence burrs

– No separator tears or 

wrinkles

• Lack of contamination

– Heat effective zone halos

– No foreign or native 

delamination debris

• No Li deposits or plating

• Consistent active material 

coating with smooth edges

• Solid weld connections 

without splatter

Photos courtesy of Exponent
Cell Production Line Audits

• Audit cell manufacturers to identify processes that present high risk to 

generate latent cell defects and evaluate current measures taken to 

mitigate them

– 2 day cell production line visits with technical battery experts

– Metallic particle generation prevention and contamination control

– Periodic particle contamination sampling of key processes 

– Humidity control of dry rooms and incoming materials

– Real-time process monitoring and implementation of statistical process control

– Defective part removal, isolation, and destruction

– Inventory control

– Product returns and failure investigation

• Deliverables

– Presentation detailing findings

– Action item list with recommendations
Analyses

• Thermal analysis by NREL

– Complete model of spacesuit battery

– Includes cell electrochemical and PTC 

device electrical and thermal 

characteristics

– Validated by external short circuit 

testing

– Demonstrating tolerance to short circuit 

external to battery

– Projecting catastrophic thermal 

runaway for a small range of short 

circuits internal to battery but external 

to cells

• Probability Risk Assessment by SAIC

– Considered manufacturing history and 

quality of the 18650 cell design along 

with its reject rate during our 

acceptance tests

– Predicts that chances of cell failure in a 

battery at 1 in 160,000 over a 5-year 

service life Graphics courtesy of NREL
ABSL experiment: Bank 3 short through external resistor

MODEL VALIDATION FOR PACKEXTERNAL SHORT

80 cell battery in 

test enclosure

10 mΩ resistor
Model vs Test Article

D

E

F

G

Brick Temperature Sensor Locations

A

B

Center-most 

cell

C

Cell Temperature Sensor Locations
Model Validation – First 6000 seconds

Temperature (

oC)

Time (s)

Symbols: ABSL test data

Lines: NREL model prediction

Center cell

Edge cell

GRP

Al plate-c

Al plate-p

Corner cell

Box
e.g. bank 3 short caused by conductive debris between banks 3 and 4

External Short Now Positioned 

Internal to Pack

Bank 1

+

Bank 2

-

Bank 3

+

Bank 4

-

Bank 5

+
4 V

8 V 16 V

12 V

.... ....

Cartoon of Shorted Middle Cell Bank

Short runs through cell can of cell from adjacent bank 4

Note that 3-layer (Kapton-Nomex-Kapton) bank-to-bank

insulator/separator is omitted for clarity

Cell Bank 3 Short path
Overview of Bank 3 Short Results

• Catastrophic thermal runaway 

is predicted at 20 mΩ

• Maybe also at 10m Ω

Bank 3

Cell 42 Cell 56

Rshort Short Condition

(SOC0 = 100%)

Cell 42 Tmax (Bank 

3)

Cell 56 Tmax

(Bank 4)

10 mΩ External-to-pack, earth 97oC @ 6000-s 75oC @ 6000-s

Internal-to-pack, earth 150oC @ 16-s 146oC @ 16-s

Internal-to-pack, space 153oC @ 16-s 147oC @ 16-s

20 mΩ Internal-to-pack, space 525oC @ 110-s 522oC @ 110-s

30 mΩ Internal-to-pack, space 595oC @ 240-s 591oC @ 240-s
Bank 3 short from 100% SOC: 10 mΩ vs. 20 mΩ

35

• 10 mΩ: • Bank 3 PTCs trip quickly and 

uniformly due to high in-rush 

current causing PTC self-heating

Cell Current (A)

Time (s) Time (s) Cell Current (A)

20 mΩ:

Bank 3 PTCs trip slowly, at 

different times dependent upon 

bank 3 temperature distribution

Cell 42 PTC trips at 8-s

Remaining bank 3 PTCs 

trip at 16-s

Cell 42 PTC trips at10-s

Remaining bank 3 

PTCs trip between 

60-s and 110-s
Bank 3 short from 100% SOC: 10 mΩ vs. 20 mΩ

36

• 10 mΩ: • Bank 3 PTCs trip quickly and 

uniformly due to high in-rush 

current causing PTC self-heating

Temperature (

oC)

Time (s) Time (s) Temperature (

oC)

20 mΩ:

Bank 3 PTCs trip slowly, at 

different times dependent upon 

bank 3 temperature distribution

All bank 3 PTCs 

trip by 16-sec

All bank 3 PTCs 

trip by 110-sec

Cell 42 Cell 42

Rest of bank 3 Rest of bank 3
Analysis Findings

• Model agrees fairly well with external short test data

• Relocating short from pack-external to pack-internal 

will cause substantial additional heating of cells that 

can lead to cell thermal runaway

• Large sensitivity to Rshort

• Negligible sensitivity to earth/space BCs on transient 

response (thermal mass dominates)

• Additional heat sinking external to battery box won’t help

• Thermal runaway predicted for Rshort ≥ 20 mΩ

• Cleanliness during battery assembly is critical

• Use of high temperature tolerant, electrically 

insulative materials will also prevent collateral 

propagation of short circuits inside battery packs

37
Conclusions

• A portfolio of mitigating measures are necessary to ensure safety

– Selecting a mature cell design produced in large volumes with an 

absence of CPSC recalls is a very prudent measure

• Commercial competition for runtime pushes every higher energy density 

(Wh/L) into same volume (i.e, 18650 standard)

• All inert cell components are targeted for diets or elimination, such as thinner 

can and separator thickness, weakening design robustness

– Small COTS cell designs made using highly automated processes 

typically yield unsurpassed performance uniformity, which indicates tight 

process tolerances

• OCV retention after long rest periods and OCV bounce back after deep 

discharge are excellent discriminators of defective cells

– NDE and DPA sampling and cell production line audits, targeted on 

defect and contamination control, are critical for assessing cell 

production quality

– Operating cells within positive margins of voltage, current, temperature 

is also very important for life and safety

– Detailed thermal models can predict short circuit vulnerabilities of a 

battery design

• Example; certain external cell shorts that are internal to battery assembly are 

predicted to lead to cell thermal runaway in spacesuit battery

• Implies similar vulnerability to cell internal shorts, but verification needed
Acknowledgements 

• Gi-Heon Kim, Larry Chaney, and Ahmad Pesaran at 

NREL for their thermal analysis contributions 

• ABSL, Exponent, and Mobile Power Solutions for 

providing key data for this study