Motorola RIZR Z6tv

Has anyone found some inconsistencies with this when charging the phone, particularly when charging the phone while it's on? Sometimes, it will indicate that it's fully charged after I've had it plugged in for like 30 min, when it was dead when I plugged in in. Or it will say it's fully charged, and when I unplug it, it goes down by 1 or 2 bars. But then I shut the phone off, plug it in, wait for it to say it's fully charged, and then turn the phone back on and it says fully charged like it should. It just seems to be inconsistent, like it's not showing the correct battery status.