Maybe your battery no longer holds charge after you drained it? I completely drained mine about 5 times before it would no longer hold a charge. When it did go, there wasn't really anything on (except my obdpro) that would drain it, it just died. Now I have a optima yellow-top and I must say, I really like it so far.
If you take it down to an autozone or a les schwab, they can test out your battery and get you a new one.