Charging battery directly without plugged in

Zubair , it could be a problem with the charging circuit, but I'd start witha new battery and then the dock connector. If that fails check all the charging components. This could be caused by a failed/damaged charging coil. Check for any physical damage, or if you have the knowledge and tools, check for function. 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. All of these are pretty much available at mouser. com or digikey.com. Check the Diode D604, located just above L28. Hope this helps, good luck.

As for charging the battery separate from your phone, do keep in mind that it is easy to overcharge it. This could lead to some rather hot and explosive battery. Yes you could build a charger but make sure that you include a PMW IC to prevent overcharging. Something like this could work for you.

Hope this helps, good luck.