I tried to connect a 12V fan and a 3V LED to a 9V battery. But I did not get the required power for the fan. So I connected 2 batteries in series. When I connect the fan alone, it draws the required power. But when I connect the LED along with it, the fan turns slower than before. However, I would like to maintain the speed of the fan. This is for a project to make a unit that looks like fake flames. So I need the fan to maintain some good RPM so that the cloth that I will attach later flutters well and looks like a flame. I am new to this. Please help.
It seems that you are trying to connect the led directly to the 12V thereby reducing the voltage due to the internal resistance of the two batteries. That is not the way. Remenber also that 2 batteries of 9 V can make the ventilator turning but also there the voltage will drop. If this is the case place a resistor in series with the led. Let us say put some 560 ohm in series with the led and try again. It is better to look for a proper 12V supply.