This guide will help with the replacing of the internal battery. It might be useful if the power of the overall camera is not working, even with the external batteries replaced.
The internal battery is in the Digital AV Out compartment on the side of the camera.
Lift the rubber flap of the AV compartment.
Use the plastic opening tools to gently pry up the plastic that is attached to the battery.
Gently pull out battery with fingers and replace.