Plugging the battery into the main drive unit.
This connector on the battery: (can be a different color)
Plugs into the port inside the battery compartment:
Be sure the plug is tightly seated as shown below
It is a tight fit. It is difficult to grasp the plug to unplug it - it is fine to pull the wires, they are firmly attached to the connector plug.
Always unplug the battery when not in use. Do not leave battery connected to the drive when not in use.