The simple way is to try to follow the procedures for taking out the battery case in reverse order:
Place yourself on the left side of the vehicle (in driving direction).
Make sure that the multi-plug is clean. Hold the battery case with the left hand at the handle. With the right hand, guide and support the back of the battery case. Invert the appropriate part of the opening in the handle over the mushroom head on the axle of the quick-release fastener. Now you move the back part of the battery case at the guide pins past into the framework triangle. In addition you must push the battery case forward on the crossbar as far as possible (in driving direction). Push carefully the entire case to the rear and pay attention to the fact that the two white guide pins should slide into the pilot holes.
Do not attempt to use extra force; this can lead to defects!
Check whether the battery case is sitting properly on the guide pins. Now close the quick-release fastener; the battery case is now correctly installed.
Attention: If a continuous audio signal sounds, when the battery case is switch on, then it does not sits properly on the guide pins, and the multi-plug at the back of the battery case does not give a perfect contact.