Wouldn't charge...

Before my speakers wouldn't work, but I fixed it by resetting my "PRAM." The day after that I tried to charge my mac, it said is was charging and all that, but it drained my battery. So as ever I charged it again and it worked, but then later it just wouldn't charge. I asked someone I know that also has a mac, and the same thing happened to him. He said that you just have to wait a couple of days and it will charge again. Luckily he was right and now it charges again. Even though it works know I would still like to know why. It wasn't my charger either, because it was putting out volts on a voltmeter.

If this has happened to you and you know why please reply. I also hope that this will help anybody else that is having this problem.