Why my Ipad2 can't continues to charge until 100%.

Try and calibrate your battery once a moth for accurate reporting.

Run it down from full charge until the iPad shut down.

Charging iPad battery

http://support.apple.com/kb/HT4060

http://www.apple.com/batteries/ipad.html

1. Try a reset (ignore)

2. Clean docking port with clean tooth brush

3. Try another wall socket

4. Try another cable (make sure it's an original Apple cable)

5. Try another charger (make sure it's an original Apple charger)