Battery problems

[corredor]

I bought two Blackberry Curves 8530 for Virgin Mobile USA from Walmart (no longer available from Virgin Mobile website) and both seem to have battery problems. I thought the one I bought for my friend had a defective battery, but now the one I bought for myself, which I have been using only two days, is also having a battery problem. I wonder if it is my mistake: I had the phone plugged in overnight but turned on because I also had the alarm set, and also, I used the phone today to download Facebook and some other applications. My phone went dead around 6 pm today. Could it be that I should have turned my phone off while charging overnight, or is it the downloading I did that caused the battery to run out; could it be that Walmart is selling defective phones; or is it that these phones just have a short battery life?