When working with your MusicPad, there are a series of icons which are displayed that are designed to tell the user the status of the battery charge and if the MusicPad is plugged in to a power outlet. The following is a complete list of all these battery related icons as displayed in the upper left hand corner of the MusicPad and what status they represent:

Green Plug shaped Icon: this means that the batteries are fully charged and the MusicPad is plugged in to a power outlet.
Yellow Plug shaped Icon: this means that the batteries are approximately at the half charged state, and the MusicPad is plugged in to a power outlet.
Red Plug shaped Icon: this means that the batteries are nearly empty and the MusicPad is plugged into a power outlet.
Red solid battery shaped Icon in 4.0.6 software. Green Battery shaped Icon in 3.1.5 software: this means that the batteries are fully charged and the MusicPad is not plugged in to a power outlet.
Yellow half-full Battery shaped Icon: this means that the batteries are somewhere between approximately 20% charged and 80% charged and the MusicPad is not plugged in to a power outlet.
Red outlined or "empty" Battery shaped Icon: this means that the batteries are less than 20% charge left and the MusicPad is not plugged in to a power outlet. In most cases, this indicates that the MusicPad has somewhere between 15 and 30 minutes of time left depending upon the exact temperature of the room, the Brightness setting of the touch-screen display and whether the MusicPad is a low or high resolution unit.