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Development of a Lithium-Ion Battery System
Modular Design Aspects and
Effects of PTC Devices in High-Energy Cells
Peter Keil1, Peter Burda2
Technische Universität München
1Lehrstuhl für Elektrische Energiespeichertechnik
2Lehrstuhl für Fahrzeugtechnik
Agenda
• Concept of MUTE electric vehicle
• Selection of suitable lithium-ion cells
• Modular design aspects of battery system
• PTC devices in lithium-ion cells
• Conclusion
• The MUTE battery system
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MUTE electric vehicle
• Vehicle concept
– Lightweight sub-compact car (L7E) designed for inner cities / suburban areas
and as secondary car for rural areas
– Driving range: > 100 km
– Max. Speed: 120 km/h
– Traction Power: 15 kW at wheels
– Power Consumption: < 7 kWh / 100 km
– Focus on cost efficient solutions
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MUTE electric vehicle
• Requirements for Battery System
– Located in crash protected area behind passenger cabin
– Maximum weight: 100 kg
– Weight limitations lead to air cooling
– Maximum cycle depth: 80%
– Capacity at end-of-life: 80% of initial capacity
Initial nominal capacity: > 11 kWh
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Cell selection
• Criteria for selection process
– Specific energy, energy density
– Cell safety
– Production quality
– Costs and availability
– Geometry (prismatic, cylindrical, pouch-bag)
• Final decision
– Cylindrical 18650 cells due to their good availability
and mature production processes
– High-energy cells from Japanese manufacturer
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Modular Design Aspects
• Scalability on module level
– Capacity can be adapted by cell capacity
and the number of cells connected in parallel
– Voltage level is adjustable by changing the
number of cells in series
• Scalability on pack level
– Capacity and voltage level can be varied with the
number of modules connected in series/parallel
– Available space can be used efficiently in the
construction process of the vehicle
– It might be possible for the customer to influence battery
system (increase range by buying additional modules)
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Modular Design Aspects
• Durability of battery system
– Battery system design based on small
cells leads to a better fault tolerance
against premature capacity loss or
statistical failure of single cells
– Averaging of deviations caused by
production
– Defective parts can be repaired by
exchanging one single module instead
of replacing the whole battery system
=> cost reduction
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Impact of cell failure when 10% of the cells are defective
Modular Design Aspects
• Safe handling and maintenance
– Module voltage below 60V leads to safe handling
– Special High Voltage Training
• Not required for handling of single modules
• Only necessary for works on interconnected modules
• Safety in case of misusage
– Lithium-Ion cells: danger of thermal runaway under abuse conditions
– Modular battery pack with small 18650 cells increases safety by reducing
thermal interaction between neighboring cells and modules
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Modular Design Aspects
• Battery Management System (BMS)
– Versatile master-slave-architecture
– Decentralized cell-individual state determination
and cell balancing performed by BMS slave
– BMS master receives preprocessed information
of arbitrary amount of slaves for monitoring
battery safety and range estimation
– Cells and BMS of one module form fixed unit:
information concerning history of battery
module is coupled to module’s BMS hardware
=> simplifies evaluation of used batteries
for second life applications
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BMS slave board
Modular Design Aspects
• Battery modules for MUTE
– 112 cylindrical cells embedded in two supporting frames
– Spacers between cells separate cooling channels
– Interconnection of cells by spot welding
– Usage of high-energy 18650 cells with PTC device
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PTC Devices in Lithium-Ion Cells
• High-energy consumer cells often contain PTC device
– PTC device = Resistor with Positive Temperature Coefficient
– Used as safety device in high-energy cells
– PTC in cylindrical 18650 cells: small disc below the cap (positive pole)
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[Ref.: Darcy]
PTC Devices in Lithium-Ion Cells
• Working principle
– PTC consists of polymer with conductive carbon black particles
– High current (e.g. in case of short circuit) heats up PTC (Joule Heating: I²R)
– Heated polymer expands and conductive paths break down
– Resistance increases by several magnitudes
– Resistance of PTC drops again after reduction of current / temperature
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[Ref.: Tyco] [Ref.: Tyco]
PTC Devices in Lithium-Ion Cells
• Short circuit test of cylindrical 18650 cell
– Initial short circuit current: 50 A
– Limited current by tripped PTC: < 1,5 A
– Time-to-trip after short circuit: < 0,5 s
– Location of tripped PTC device becomes clearly visible in IR image
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surface temperature distribution [°C ]
PTC Devices in Lithium-Ion Cells
• Experiments with PTC devices
– Evaluation of PTC device behavior under
high current conditions
• Reversibility
– Resistance of PTC device increases from tripping event to tripping event
(initial resistance: 20 mΩ, after several tripping events: 30 - 40 mΩ)
– PTC device also becomes more sensitive and trips earlier
• Parallel connection several PTC devices
– Tripping of PTC devices connected in parallel shows an avalanche effect
– Group of parallel PTC devices trips simultaneously within several milliseconds
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Foto Testaufbau
PTC Devices in Large Battery Systems
• PTC device becomes instable if voltage exceeds limitations
– Danger of too high voltage at a tripped PTC when there is a short circuit of a
whole battery pack and whole voltage drops at one PTC device
• Expected failure mechanisms
– Voltage exceeds limits for a certain time: PTC can burn away
=> sparks and heat generated outside the area of active materials,
heat dissipated directly by thermal mass of cell
=> no current limitation any longer, same behavior as cell without PTC
– Continued current flow through tripped PTC: additional thermal losses
=> increasing PTC temperature can cause a melting of gasket seal
=> direct connection between outer can (-) and cap (+)
=> internal short circuit of cell => danger of thermal runaway
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[Ref.: Darcy]
PTC Devices in Large Battery Systems
• Evaluation of cells with PTC devices for MUTE battery system
– PTC device causes higher cell resistance, more thermal losses
– Additional protection in case of short circuit
– Current limitation provides more time for BMS to detect and react on short
circuit of battery system before a destruction of the cells occurs
– In case of high voltage drop at single PTC device,
there might be a failure of the PTC device,
but failure occurs outside the area of active material
– After PTC failure, behavior comparable to cell
without PTC device
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[Ref.: Darcy]
Conclusion
• Safety of lithium-ion cells with PTC device is better or at least equal
to cells without PTC device
• High energy cells with PTC devices can be used for MUTE EV
• Modular battery system based on cylindrical 18650 cells has been
developed
• Battery system shows a flexible and cost efficient solution for
prototype application as well as series production
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The MUTE Battery System
• Characteristics
– 11 Modules with 112 cylindrical 18650 li-ion cells
– Energy Content of 12 kWh
– Weight below 100 kg
– Specific energy
• Cell level: 208 Wh/kg
• Module level: 170 Wh/kg
• System level: 130 Wh/kg
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Thank you for your attention!
Peter Keil
Lehrstuhl für Elektrische Energiespeichertechnik, TU München
Peter Burda
Lehrstuhl für Fahrzeugtechnik, TU München
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