Need to investigate Netfabb. Don't know if it. I guess it's a mesh checker.

If you learn to model cleanly, you won't have to repair anything you build in Blender. You can just export your .stl or .obj files. Like anything, if you model sloppy, well, then you'll have errors. I'd bet that you could have gotten a clean model in Sketchup. Your part was not that complex. You had some back surfaces that were coincident with the bottoms of your curved battery slots, meaning, that they occupied the exact same place in space. That confuses a model that's supposed to be hollow and have an inside. You might want to try again in Sketchup and be very careful that your back surface doesn't intersect or lie exactly on the back or your curved battery slots.