When I run a 12 v computer fan with a 9 v battery it runs fine, but when I added two 1.5 v batteries to the breadboard it slows down, the voltage is almost cut in half. What could be the cause of this?

 

If you connect a 3V battery to a 9V battery in parallel then the voltage becomes 3V and the 9V battery is shorted and will have a very short life.

You need to learn something about series and parallel.

 

Thank you. I have learned something today.

 

If you want the voltages to add you connect the batteries in series, plus to minus to plus to minus, etc.