This is the Castle Creations Parallel Battery Wire Harness with Deans Plugs.
Connects two battery packs in parallel, enabling capacity (mAh) to be
doubled while the voltage stays the same
10 gauge wires
Parallel Battery Wire Harness with two Deans Male Ultra Plugs and one
Deans Ultra Female Plug
Length: 5.5" (140mm) not including connectors
Always use identically prepared packs with this harness. Both packs
must be fully charged and properly balanced. Both packs must be
made by the same manufacturer, have the same C rating, same number
of cells, and both must have the same capacity. Failure to observe
this warning will result in battery damage.