This is the Castle Creations Parallel Battery Wire Harness with Deans Plugs.
FEATURES: Connects two battery packs in parallel, enabling capacity (mAh) to be
doubled while the voltage stays the same
10 gauge wires
INCLUDES: Parallel Battery Wire Harness with two Deans Male Ultra Plugs and one
Deans Ultra Female Plug
SPECS: Length: 5.5" (140mm) not including connectors
COMMENTS: 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.