Development of a Lithium-Ion Battery System - mediaTUMmediatum.ub.tum.de/doc/1161061/1161061.pdf · Development of a Lithium-Ion Battery System Modular Design Aspects and Effects

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

27.03.2012 Development of a Modular Lithium-Ion Battery System 2

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


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


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


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)



• 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


Impact of cell failure when 10% of the cells are defective


• 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



• 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


BMS slave board


• 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


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)


[Ref.: Darcy]


• 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


[Ref.: Tyco] [Ref.: Tyco]


• 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


surface temperature distribution [°C ]


• 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


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


[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


[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


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


Thank you for your attention!

Peter Keil

[email protected]

Lehrstuhl für Elektrische Energiespeichertechnik, TU München

Peter Burda

[email protected]

Lehrstuhl für Fahrzeugtechnik, TU München


mailto:[email protected]

mailto:[email protected]

References

Eric Darcy et al.: Cell PTC Device Characterization for 2008 NASA Aerospace Battery Workshop

Tyco Electronics Corporation: PolySwitch Resettable Devices – Fundamentals, 2011, p. 109


Documents

Development of a Lithium-Ion Battery System - mediaTUMmediatum.ub.tum.de/doc/1161061/1161061.pdf · Development of a Lithium-Ion Battery System Modular Design Aspects and Effects