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In order to join a positive terminal to a negative terminal, two dissimilar metals, copper and aluminum need to be
connected. A123 recommends that the copper and aluminum straps should be joined together using a welding a
process. A123 joins the two types of metals together using an ultrasonic welding method. The copper bus bar is
pre-tinned to resist corrosion and to assist in the bonding to the Al bus bars.
Figure 28 – Two types of bus bars ultrasonically bonded in the center to each other
APR18650 and ANR26650 Cell Interconnects
The cylindrical cells can be interconnected by welding nickel or copper alloy straps directly to the cells’ end
terminals. The ends of the A123 cells have either a solid nickel disk or one plated with nickel to which the straps
can be directly welded.
For low current applications, a thin strap (< 0.07 mm) can directly spot welded to the surface with 1.3 mm
electrodes. However, high current applications will require thicker straps (~0.3mm). Direct spot welding is less
effective with these thick straps, because the current cannot focused in a small enough spot to generate enough
heat to reliably weld the two metals together. To concentrate the current in a small spot, a protrusion is stamped
in the strap when the strap is die cut. Additionally a slit is cut between the spots where the two welding
electrodes will contact the strap during welding. This forces more current to flow down through the spots rather
than short circuiting straight across the interconnection straps. The figure below shows schematically, how the
welding current flows down through the electrodes, and gets concentrated in a small area defined by the
protrusions as it circulates across the surface the surface of the cell and back through the other electrode.
Cell
Welding Electrodes
Nickel disk
Interconnection
Strap
Protrusion
Welding
Current
Nickel disk
Cell
Interconnection
Strap
Welding Electrodes
Protrusions
Figure 29 – Schematic of cylindrical cell welding process