Metallography of Battery Resistance Spot Welds

摘要: Li-ion cells provide an energy dense solution for systems that require rechargeable electrical power. However, these can undergo thermal runaway, the point at which cell becomes thermally unstable and results in hot gas, flame, electrolyte leakage, some cases explosion. The heat fire associated with this type of event is generally violent subsequently cause damage to surrounding system or present a dangerous risk personnel nearby. space flight environment especially sensitive risks particularly when it involves potential within habitable volume International Space Station (ISS). In larger battery packs such as Robonaut 2 (R2), numerous are placed parallel-series configurations obtain required stack voltage desired run-time meet specific power requirements. This raises second less obvious concern batteries certification use: joining quality resistance spot weld component wires/leads tabs, bus bars other electronic components assemblies. Resistance welds materials evaluation, visual inspection, conductivity (resistivity) testing, destructive peel metallurgical examination accordance applicable NASA Process Specifications. Welded cross-sectioned ensure they free cracks voids open any exterior surface. Pore contained zone but not surface, determined have sharp notch like characteristics, shall be acceptable. Depending on requirements, constructed aluminum canisters while others steel. schedules must developed certified each possible combination. canisters' positive terminals were difficult due bi-metal strip comes ultrasonically pre-welded by manufacturer. was further complicated maximum electrode force limited low-electrode prevent deflection during welding. Other comprised smaller diameter cylindrical steel inherently capable handling greater from electrodes. Allowing higher-electrode forces aids greatly insuring consistent network weld. Overall lessons learned: developing good jigs critical insure parts electrodes planer one another location sites remains accurate repeatable; maintaining strict control over critical--materials hardness chemical composition schedule accuracy die used stamp projections worth investment; proper seasoning producing welds--once been properly seasoned, cleaning/dressing should avoided until absolutely necessary.