Electric Vehicle Safety Technical SymposiumNHTSA
May 18, 2012
Battery safety involves interactions from cell component to the battery system level.It might not be a stretch to state that seemingly unsafe cell system can be made safe by the use of a design or operating parameters that can ensure highly reliable and safe operation.The converse can also be true. An apparently robust cell system can fail due to the improper choice of module/pack system parameters.Hence, cell/battery manufacturers and vehicle OEMs work very closely together to ensure the development/deployment of safe battery system.
Battery safety is achieved through layers of protection:Individual cellsGroups of cells (modules)PackStructure and internal assembliesThermal ManagementHigh Voltage circuitsBattery Management & Control
Interactions within the battery system (especially under abuse/abnormal conditions) can be complex, but are managed using advanced engineering toolsDesign Failure Mode and Effects AnalysesFailure Tree AnalysesTest to failureAbuse testingFailures in one or more components within a pack can be isolated or contained to prevent cascading of failures.
Utmost attention is paid toward designing a cell that is safe.Infact, we can easily say thatmechanical, thermal and electricalsafety considerations dictate the selection of all cell components!RequirementsHighestThermal StabilityLarge Voltage Stability WindowKey Cell ComponentsCathodeSeparatorElectrolyteCase designMetal caseLaminated packaging
Several cathodes arenow inuseinlarge-formatbatteries.Spinel(LiMn2O4)NMC (LiNiMnCoO2)NCA (LiNiCoAlO2)LFP (LiFePO4).For design purposes, balance is sought among safety, power, energy, life and cost.
Current anodesoverwhelmingly are carbon based. There are subtle differences in their abuse-tolerance behavior depending on, among others, their surface area, etc. However, their overall response under abuse-condition is quite similar.Current electrolytesare all based primarily on organic carbonate solvents. Stability at the anode as well as at the cathode are considerations that play a key role in selecting the electrolyte system.
Separators not only have the role of keeping the anode and cathode apart from each other, they also need to be very robust with respect to mechanical/thermal stress in course of life.With this goal in mind, separators have been developed which are mechanically much more robust than their conventional counterparts.These separators have low shrinkage at elevated temperatures and are resistant to internal shorts.
Metal Can: Usually equipped with safety devices such as a safety-vents which become activated when certain amount of pressure is generated within the cells.Laminated Packaging: Quite a few manufacturers prefer the laminated packaging (or pouch) for a number of reasons, such as ease of manufacturing, lower cost, easier scale-up and most importantlyThe pouch design allows the release of gas at high pressure and temperature without the building-up to an unsafe level.
Rigorous DFMEA is carried out for cells under operating and abuse conditions. For example,This involves mechanical dimensions of electrodes, separators.Cathode to anode ratios, electrolyte amounts, etc…..Abuse- ConditionsThorough analysis is carried out to identify design parameters that could result in unsafe operating conditions.Extensive Review with OEMs.
Comprehensive testing is carried out to characterize and identify range of voltage, temperature, mechanical deformation etc. that might lead to unsafe battery conditions.Not only are USABC, SAE –standard tests are carried out, additional tests outside of these tests requested by the OEMs are also carried out.These tests are further augmented by UNDOT tests to simulate shipping conditions.
Modules/packs are built with the utmost attention for the physical/thermal/electrical protection of the cells to ensure safe operation as well as performance and life during the life of the vehicle.Cell/battery manufacturers work very closely with the OEMs to ensure that the battery is provided the best location from safety point of view.They are subjected to a wide array of mechanical, electrical and environmental validation tests developed in close cooperation with the OEMS.
Built to attain complete electrical isolation of the battery pack from the vehicle. An HVIL (High Voltage Interlock System) is used to prevent exposure of occupants to high voltages.High- voltage and low-voltage lines are separated for electrical safety.High-voltage(busbars, contactors, connectors)circuitsare made in such a way that they survive stress/abuse during vehicle life.BMS (battery management system) carries out critical safety functions such as charging and health managementOpens the contactors when voltage, current and temperature limits critical from safety points of view are violated.
While battery safety is better understood within the context of an overall battery system, focus of all cell design and development work is the manufacturing of abuse-tolerant cells and hence tremendous effort is devoted toward selecting cell materials that are safe.Cell/battery manufacturers and vehicle OEMs work in close cooperation with each other to develop abuse-tolerant cells and packs.Extensive validation tests in conjunction with the OEMs using industry-standard procedures to identify parameters that are critical for the safety of Li ion propulsion batteries.