I was given an old Powerbook 15" aluminum version, which I used a few times, then left idle for several months. When I got it out again, I plugged in the power cord, which did not light up orange, like it's supposed to. After a look at the apple discussion boards, it appears that a common cause of this problem is that the power adaptors tend to go bad. So I tried out another adaptor - it worked fine, and charged the battery all the way up. Looks like I'll be getting a new one.

Apple sells the regular power adaptor AND the universal power adaptor by Kensington for $79.00. Just search the apple.com store for "power adaptor".