Ipad stops charging after it charges??? !!!

Hello, I have a problem and I would appreciate any help. I have an ipad (ipad 1) that I've been using for 2 years, it worked perfectly and the battery lasts for hours, suddently, the ipad battery doesn't charge anymore, when the battery drains, it is turned off and starts charging the battery, but just after it starts again (battery at 2%) it doesn't charge anymore, also the ipad is not recognized in iTunes when I connect it to the USB port. This are some things I've done:

- iTunes and iPad have the latest version.

- Turned off and on again the ipad.

- Hard reseted it (pressing and holding both buttons until it restarts).

- Tested all my ports, both in a PC and in a Mac.

- Tested with two USB cables. I have an Iphone, it charges, connects to iTunes with both cables, but not the Ipad.

- Tested connecting directly to a power source. Again, my iphone charges perfectly with both cables connected to a power source but not the Ipad.

- I turned off the iPad, but had it connected about 40 minutes to a power source, and after I turn it on again, the battery is charged at 4% (looks that it charges while the OS is turned off).

It is strange to me that when the battery is drained and the ipad turns off, the screen shows the icon of the battery, a plug + right arrow + thunder symbol, after I plug to a power source or a USB port, the plug icon disappears (sounds to me that it recognizes the power source) and it seems that the ipad starts charging because after 15-20 minutes the OS starts again, however, after it starts, the upper right side of the screen doesn't show the the charging symbol and there is not a "Not charging" message, just the icon of the battery with 2% left, until the battery is drained again.

Hope someone can help me with this. Thanks in advance...