Is it Safe to Recharge this battery?

I have a Nikon D40 dSLR and i needed to shoot something really quick before so I put the battery in the charger for 20-30 minutes a couple weeks ago and when i turned it on just now it says its full on the meter. Would it be fine if i were to stick it back in the charger to give it a full charge?

I have an important shoot coming up and i'm afraid it could die on me and i have no back up battery or camera.

It's ok to do once or twice. However, if you make it a habit, the battery will lose its ability to hold a full charge, and might need deep recycling to bring it back up to snuff.

Yes you can
watch the charger light
the manual will tell you what to look for
on - off - green - red - etc.