My battery is dead

Thought I was having problems with my alternator that I just got replaced last month and come to find out it was just my battery. I'm such a non-grease monkey so any mechanic could rob me blind. So my buddy came over to take a look and told me I hadn't gotten ripped off, but I did need a new battery after he tested it with some new fangled device I had no idea what it was. At least I didn't have to take it in anywhere or get it towed. Saved me some time.