Bring to autozone for a free full charge, then go get a maintain

All major parts stores like autozone, kragen, advanced auto, pepboys, etc provide free battery charge, typically in an hour. After that, go buy a battery maintainer, also called float charger. Next time, if you are not going to drive it for a week or longer, hoop the maintainer to your battery, when you return, it'll be full. This maintainer will prolong your battery life.
Click here for maintainer intro

Repeated deep drain will kill your battery (4-5 times that's it).