I replaced the battery on my 2002 Oldsmobile Alero. Now my defroster/heater selection knob doesn't work. The air is

I don't believe the battery would affect this. You might need a new heater duct solenoid. There is a tiny electromechanical device that selects which vents the air comes out by using electromagnetism to move a plastic door. The electricity strenthens the magnet causing it to pull a plunger into itself and the plunger is attached to the plastic door.

You might also have a problem with the cable. Some vehicles use the soldenoid to shift airflow and others use a plain metal cable that is attached to the knob on the vent selector. I recommend looking under the dash for wires that may lead to this solenoid to locate it. I would look for a cable coming out of the back of the heater vent selector. You can remove this panel usually with little effort to check and see if it is electronically controlled or mechanically controlled. This is an annoying problem that I have encountered with my vehicle. I just gave it more time to heat up and bought some heet window defroster until I had time to fix it.

Hope this helps,
Ben