computer won't power on with battery

I replaced the battery in my computer. I charged it until the orange light went off. I then unplugged the power adaptor and tried to power on the computer pressing the power on key. Nothing happened. I plugged the power adaptor back in and pressed the power on key. It came on with no problem.

Why does the power on key not work with a fully charged battery but work OK with the power adaptor connected? Could a wire have become detached on the motherboard when I removed the back to replace the battery?