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NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.
Fail Safe Design for Large Capacity Lithium-ion Batteries
NREL Commercialization & Tech Transfer Webinar March 27, 2011
Gi-Heon Kim
John Ireland, Kyu-Jin Lee, Ahmad Pesaran
Kandler Smith
Source: A123 Source: GM
NATIONAL RENEWABLE ENERGY LABORATORY
Challenges for Large LIB Systems
2
• Li-ion batteries are flammable, require expensive manufacturing to reduce defects
• Small-cell protection devices do not work for large systems
• Difficult to detect faults in large cells before catastrophe
0 50 100 150 200 250 3003.4
3.5
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4
Time [sec]
Volta
ge [V
]
Reference1Ah40Ah
Outer temperature
Inner temperature
Small cell
Large cell
blog.wellsfargo.com
engadget.com
Short
Short
NATIONAL RENEWABLE ENERGY LABORATORY
Barriers for Safe Large LIB Systems
3
1) Early Fault Detection • The faults leading to a field accident are believed to grow from a
latent defect over time • Signals of an early-stage-fault are difficult to detect in large
capacity LIB systems
2) Electrical Isolation of Fault • To limit further electric current feed energizing the fault • Neither active nor passive circuit breaker is directly applicable in
large capacity LIB systems
3) Suppression of Fault preventing Propagation • Pack fault response depends on pack integration characteristics
as well as individual cell safety characteristics • Using safe cells does not guarantee the safety of a pack
consisting of them
NATIONAL RENEWABLE ENERGY LABORATORY
“Fail-safe” concept differentiates:
• Power line (to be as conductive as possible) and • Balancing line (to be relatively resistive);
Electrical Current Paths in LIBs
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1) Battery Electric Power Delivery: Charging/Discharging
2) Balancing: Material Utilization
The proposed architecture facilitates early fault detection and isolation features…
NATIONAL RENEWABLE ENERGY LABORATORY
Fail-Safe Design
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AAI2 I1
Rf1
Rf2
Rf3
Rf4
AAI2 I1
• |Signal| >> |Noise|
• Known reference; (I2 – I1)ref = 0
• Single measure per module
I2 – I1
Viability of the proposed concept has been inves4gated against various design parameters and opera4on condi4ons
• Simula4on model • Ini4al tes4ng
Fault Detection
NATIONAL RENEWABLE ENERGY LABORATORY
AAI2I1
AAI4I3
Impact of ISC Resistance on Signal
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Fault Detection
Cell# 1
Cell# 3
Cell# 4
Cell# 2
Cell# 5
0 5 10 15 200
2
4
6
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Curre
nt [A
]
Time [sec]
I2-I1 +
I2-I1 -
0 5 10 15 200
2
4
6
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Curre
nt [A
]
Time [sec]
0 5 10 15 200
2
4
6
8
Curre
nt [A
]
Time [sec]
Rs= 100 mΩ
Rs= 500 mΩ Rs= 2 Ω
40Ah (20Ah+20Ah) Rf= 100 mΩ Module current : 0 A N in series : 5 Shorted Cell # : 2
Magnitude of signal varies with ISC resistance
Rs: 100 mΩ/ 500 mΩ/2Ω
NATIONAL RENEWABLE ENERGY LABORATORY
Experiments Confirmed Model Predictions
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Fault Detection
Rs= 200 mΩ Rs= 1 Ω
Experimental implementa4on reproduces the results
AAI2I1
AAI4I3
AAI2I1
AAI4I3
Cell# 1
Cell# 3
Cell# 2
-‐1
0
1
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0 20 40 60 80
Series1
Series2
0
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0 20 40 60 80
Series1
Series2
16Ah (8Ah+8Ah) Rf= 180 mΩ Module current : 0 A N in series : 3 Shorted Cell # : 3
Rs: 200 mΩ / 1Ω
Cur
rent
[A]
Time [sec] Time [sec]
I2-I1: + I2-I1: -
I2-I1: + I2-I1: -
0
0.1
0.2
0.3
0.4
0.5
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0.8
0.9
1
0 20 40 60 80
Series1
Series2
Current through parallel junc4ons
NATIONAL RENEWABLE ENERGY LABORATORY
Isolating the Fault
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Fault Suppression
Isolated Fault
AA
Rf1
Rf2
Rf3
Rf4
Reviving “Mitigation technologies valid with small capacity batteries”
Once the faulted electrode is electrically isolated, the subsequent behavior of the faulted electrode depends on the characteristics of individual unit, and not on the pack or module assembly characteristics. Therefore, designing safe packs can possibly be reduced to designing safe unit cells.
Multi-series-branch configuration of the proposed design still allows partial power delivery from the pack even after the local-shut down for fault isolation is executed.
NATIONAL RENEWABLE ENERGY LABORATORY
Summary & Next Steps
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• Proposed “Fail-safe design for large capacity LIB system” is promising to address the barriers identified: Early fault detection, Electrical isolation, Fault suppression.
• Simulation models have demonstrated the viability of the proposed concept against various battery design parameters and operation conditions
• Collaborate with others for further evaluation, development and embodiment of the design and the methodologies proposed • Develop/refine computational tools for specific designs • Build prototypes and test • Demonstrate safety improvement of the proposed design against
various failure modes of vehicle battery systems
NATIONAL RENEWABLE ENERGY LABORATORY
Acknowledgements
10
Vehicle Technology Program at DOE • Dave Howell • Brian Cunningham
NREL Energy Storage Task Lead • Ahmad Pesaran
NREL Colleagues • John Ireland • Matthew Keyser • Jeremy Neubauer
Thank you for your attention!