I want to use my old iPhone 3GS as an Ipod. Nothing is working?

I'm tired of wasting battery on my iPhone 4 to listen to music, so I wanted to use my old iPhone 3GS as an iPod. I read you had to have a sim card inserted in the phone, regardless if it's activated. So I put the old one inside, but when I connect it to my computer, it won't charge, or sync with iTunes. It's definitely not my charger because both of my phones charge in a wall outlet, and my iPhone 4 charges through the computer. Just not the iPhone 3GS.... I'm going on vacations with long car rides and would like to use this. Help please!