The battery was old and with little charging capacity and the front panel was scratched!
It took about 30 min to open, dismount, change the parts and put it all together again! Everything went pretty well, except for just one problem: The new front panel had holes that wouldn´t align with the holes of the metal frame, so I had to enlarge the holes with an swissblade knife! Also, the front panel replacement should be sold in a kit that includes an #00 philips screwdriver (or a plastic tool to use it as a screwdriver) to facilitate the process, since not everyone has this kind of tool ready at hand.
Just go for it! It´s easy and fun!