Beaner now your asking har questions.
So to give a good answer I need to know what type of fan you have. Part numbers are better for me. I look up the prints.
You need to use ohm's law
v=i/r i=v/r r=v/i I'm old school so i is amps
I don't know the numbers of the fans so I'll use numbers from the air.
Your fan at
[email protected] amp then is balanced. Runs smooth and last it's lifetime.
If your fan 12vdc will be 1 amp because the resistance stayed the same. You can't change that. It's the windings of the fan. The fan try's to stay balanced so it uses the same power.
Now the pressure is 1/2 24 - 12 = 12vdc and runs half speed. Because it has to make up for power to run it uses 2 times the current. And that is more heat in the windings of the fan ( The resistance ). Heat is what burns up most electrical devices and I don't mean that in a smart ass way. Most heat is generated when we turn things on. The surge. Turning things on and of quick is very effecive killer.
I hope this help's