Sounds to me like maybe the previous owner may have been using an extended battery and it is throwing the gauge off. I would suggest a full charge while off (if that's possible with a Droid, I know my wife's usually kicks on as soon as it's plugged in). Even after the green light comes on, leave it on the charger for at least another hour as that comes on at 90%. Once charged, take it to full death (not good to do on a regular basis, but once won't hurt). Try to do this with as little use as possible. Then, take it back to full charge (off, if possible). This should help your phone to learn the full range of your battery and, hopefully get your gauges aligned. You may need to do a couple cycles from full to sub 10 - 15% after this but, once you get it set, you should be able to charge/discharge at will.
