model checking battery configurations to detect thermal runaway
DESCRIPTION
Recent research in electrical engineering developed dynamically reconfigurable multi-cell Li-Ion batteries. Such batteries adapt to the voltage and electrical current requirements on the fly by changing the electrical configuration of cells. These batteries have potentially high efficiency and extended lifetime, but pose a challenge of creating and analyzing configuration schedulers to realize that potential. A completely different strand of research investigates chains of overheat reactions in batteries that cause fires and explosions, also known as thermal runaway. This research heavily relies on a fixed cell configuration to analyze heat propagation, and is not directly applicable to the reconfigurable batteries, thus rendering those batteries vulnerable to thermal runaway. In this talk I present an integration of results from those two areas of research, preserving independence of their technical details. I use model checking of dynamic battery configurations to discover configuration patterns that may lead to thermal runaway.TRANSCRIPT
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Model-Checking Battery Configurations to Detect Thermal Runaway
Ivan RuchkinDionisio De Niz
Sagar ChakiDavid Garlan
Software Research Seminar Institute for Software Research
April 28, 2014
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Driverless electric car created!
Image credit: machinespider.com
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Will it cross the intersection safely?
Image credit: thedetroitbureau.com
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No, its battery will catch fire!
Image credit: johndoug.smugmug.com
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Outline
● Battery for Electric Vehicles● Electrical Domain: Scheduling● Thermal Domain: Runaway ● Problem: Domain Separation● Solution: Model Checking
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Battery for Electric Vehicles
● Array of Li-Ion cells – NOT flooded lead-acid.● Rechargeable; powers vehicle movement.● Input & output – (voltage, current).
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Dynamic Reconfiguration
● Structure: reconfigurable matrix of cells.– Connected in series and parallel.
– Carry different charges.
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Outline
● Battery for Electric Vehicles● Electrical Domain: Scheduling● Thermal Domain: Runaway ● Problem: Domain Separation● Solution: Model Checking
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Electrical Domain: Scheduling● Goal – select charging and discharging cells to:
– Maintain required output
– Maximize battery lifetime
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Electrical Domain: Scheduling● Goal – select charging and discharging cells to:
– Maintain required output
– Maximize battery lifetime
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Electrical Domain: Scheduling● Discharge schedulers for n parallel cells:
– 1RR: discharge one most charged cell at a time, rotate every time interval.
– 1+1RR: discharge one cell till fully discharged, move to most charged one.
– nRR: discharge all n cells in parallel.– kRR: discharge k<n cells, where k is selected
adaptively based on charge.
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Electrical Domain: Scheduling● Discharge schedulers for n parallel cells:
– 1RR: discharge one most charged cell at a time, rotate every time interval.
– 1+1RR: discharge one cell till fully discharged, move to most charged one.
– nRR: discharge all n cells in parallel.– kRR: discharge k<n cells, where k is selected
adaptively based on charge.
● Takeaway: electrical modeling deals with cell charge, scheduling, and efficiency.
– Abstractions: electrical circuit, algorithms.
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Outline
● Battery for Electric Vehicles● Electrical Domain: Scheduling● Thermal Domain: Runaway ● Problem: Domain Separation● Solution: Model Checking
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Thermal Domain: Runaway
● Cells generate heat from charge and discharge.● At some temperature cells catch fire.● Reaction propagates to other cells – runaway.
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Thermal Domain: Runaway
● Cells generate heat from charge and discharge.● At some temperature cells catch fire.● Reaction propagates to other cells – runaway.
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Thermal Modeling for Batteries
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Thermal Modeling for Batteries
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Connection Patterns Affect Runaway
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Thermal Domain Summary
● Goal: seek battery designs resilient to runaway.● Abstractions:
– 3D physical models of cells.
– Lumped differential equations.
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Outline
● Battery for Electric Vehicles● Electrical Domain: Scheduling● Thermal Domain: Runaway ● Problem: Domain Separation● Solution: Model Checking
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Domain Separation
● Electrical domain:– Abstraction: circuits
– Decides configuration of cell connections
– Oblivious of heat (assumes any configuration is acceptable heat-wise)
● Thermal domain:– Abstraction: geometry
– Simulates heat propagation
– Cannot scale to dynamic scheduling(assumes fixed configuration)
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Domain Separation
● Electrical domain:– Abstraction: circuits
– Decides configuration of cell connections
– Oblivious of heat (assumes any configuration is acceptable heat-wise)
● Thermal domain:– Abstraction: geometry
– Simulates heat propagation
– Cannot scale to dynamic scheduling(assumes fixed configuration)
● Problem: is a battery with a concrete scheduler vulnerable to thermal runaway?
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Outline
● Battery for Electric Vehicles● Electrical Domain: Scheduling● Thermal Domain: Runaway ● Problem: Domain Separation● Solution: Model Checking
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Solution: Bridge Domains
● Electrical model picks a scheduler.● Thermal simulations encode a connection
pattern.● Model checker verifies if the scheduler respects
the pattern via exhaustive state search.
Electrical domain
Thermal domain
Model checking
Discharge scheduler
Acceptable
configuration patternSchedulerrespects
the pattern
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Model Checking: Abstraction
● Matrix of cells. Each cell x has:– charge(x) – a Boolean charge.
– group(x) – a serial group it belongs to.
– status(x) – discharging (d), charging (c), idle (i).
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Model Checking: Abstraction
● Encoding: charge as color, “group, status”:
1,d 1,d 1,i 1,d
2,d 2,i 2,d 2,d
3,d 3,d 3,d 3,i
● Matrix of cells. Each cell x has:– charge(x) – a Boolean charge.
– group(x) – a serial group it belongs to.
– status(x) – discharging (d), charging (c), idle (i).
1,i
3,i
2,i
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Model Checking: Transition
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Model Checking: Transition
● Step 1 (deterministic): – Scheduler forms cell groups
– Scheduler decides which groups to discharge
● Scheduler variants: – Fixed cell groups (FG) vs. group packing (GP).
– Weighted (W - pick most charged) vs. unweighted (U - pick sequentially).
● Step 2 (non-deterministic): – For all cells x:
● If status(x) == d then x.charge = 1 ++ x.charge = 0;● If status(x) == c then x.charge = 1 ++ x.charge = 0;
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Property Checked: Thermal Neighbors
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Property Checked: Thermal Neighbors
● Cells x1 and x2 are thermal neighbors if: – group(x1) = group(x2)
– status(x1) = d && status(x2) = d
– ||x1 – x2||1 <= Const [used Const = 2]
● TN(n) – number of cells with n thermal neighbors● LTL property “configuration is resilient to runaway”:
– G ( Σi (K(i)*TN(i)) >= 0 ), i = 0..3.
● K(i) are weights determined by thermal simulation.
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Property Checked: Thermal Neighbors
● Cells x1 and x2 are thermal neighbors if: – group(x1) = group(x2)
– status(x1) = d && status(x2) = d
– ||x1 – x2||1 <= Const [used Const = 2]
● TN(n) – number of cells with n thermal neighbors● LTL property “configuration is resilient to runaway”:
– G ( Σi (K(i)*TN(i)) >= 0 ), i = 0..3.
● K(i) are weights determined by thermal simulation.
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Validation: Effectiveness
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Validation: Effectiveness
● Implementation: Promela/Spin.● Effectiveness of bridging domains:
– Does model checker separate schedulers into those that satisfy the property & those that do not?
– Setup: 4x4 battery, K(0) = K(2) = K(3) = 2, K(1) = -1
● Result: – Schedulers with fixed groups satisfy the property.
– Scheduler with group packing doesn't.
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Validation: Scalability
● Scalability: does model checker handle practically sized tasks?– Electrical experiment in Kim09 has 4 cells.
– Thermal experiment in Pesaran13 has 3 cells.
Cells Time on FGU (s)
Time on FGW (s)
Time on GPW (s)
9 0.13 0.15 0.15
12 0.61 2.34 3.94
16 44.0 31.4 127
20 1060 619 MEMLIM
25 MEMLIM MEMLIM MEMLIM
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Conclusion
● Electrical and thermal domains are separated:– Checking thermal runaway vulnerability is not
feasible within either domain.
● Model checking of battery configurations:– Bridges the domains.
– Detects thermal runaway vulnerability.
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Battery Scheduling Architecture
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Related Work● Thermal abstraction on top of electrical● No reconfiguration and limited fidelity (only
convection)
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References
● H. Kim and K.G. Shin. Scheduling of Battery Charge, Discharge, and Rest in RTSS 2009.
● S. Ci, J. Zhang, H. Sharif, and M. Alahmad. Dynamic Reconfigurable Multi-Cell Battery: A Novel Approach to Improve Battery Performance in APEC 2012.
● G. Kim and A. Pesaran. Analysis of Heat Dissipation in Li-Ion Cells & Modules for Modeling of Thermal Runaway in LLIBTA Symposium 2007.
● A. Pesaran et al. Tools for Designing Thermal Management of Batteries in Electric Drive Vehicles in in LLIBTA Symposium 2013.