When i use my droid bionic as a modem using PDA.net my battery reads as charging but it continues to drop battery pct charge.
If you use your device while plugged into a USB, it could happen.
Check your apps, and turn off all the nonessential notifications and GPS/Wifi usage. Or at least adjust the frequency.
If I have 20+ apps, all checking notifications and synchs, I'd never get my device charged!!