Why is my computer shutting down when I unplug the power adaptor?

Hi all,

Yesterday I replaced the battery, hard drive, and the power adaptor on my laptop. Everything was going well until a few hours ago, when suddenly the computer would shut off when I unplugged the charger. I have tried resetting the SMC to no avail. I no longer have a warranty on this computer, and am not interested in buying a new one. Please help!