change the battery but it doesn't recharge itself

cristian, this could be because of multiple issues. First it may be the battery not taking a charge. Second it could be your charger or charging cable. try some other ones. Third it could be your dock connector. If the previous is okay, change the dock connector. Here is the guide for that. If that is not working you want to start by checking 3 inductors. For that, of course, you need a multimeter, or at least an ohm meter, to check for continuity. I hate to refer to those as fuses, but they most certainly control part of the charging circuit. First one is reference designator L3, it is a FERR-120-OHM-1.5A inductor. L4, right below it , has the identical value. Next would be the charger inductor L28 2.2UH-1.8A-155MOHM. If all of those components test okay, you must consider a failed U2 charging IC. It is a BGA IC and will require special tools and skills to replace.