“Switchers almost universally require a load to work”
Wow! I connected a 12 volt muffin fan to a little car battery charger. Fan didn’t work but it the charger works on car batteries
Do you suppose that muffin fan looked like “nothing” to the charger?
That should work, assuming the fan wants to see 12 VDC and not AC. (AC would be rare) I’ve powered muffin fans to blow away solder fumes on my bench with wall warts plenty of times. I usually power a 12 volt fan with a 9 volt supply so the thing isn’t so screaming loud.
Without knowing anything at all about your situation, yeah it’s possible that the fan isn’t enough of a load to turn on a switcher. A car battery is practically a dead short, especially when mostly discharged. Pick up your little charger. If it’s really lightweight, it’s likely a switcher and it may need to see a much larger load than a little fan. Those fans use about 1 amp at 12 volts. A fully charged car battery draws 2-3-4 amps. Should be enough to start the switcher but if it isn’t, it isn’t. You might have to place a 10-12-20 ohm resistor across the fan wires but said resistor would have to be a biggy, like a 20-25 watt one. Does the fan work? Maybe try placing it across your car battery.
Most of the newer car battery chargers won’t work unless they detect some voltage from a battery.
This means that they won’t work to charge a completely dead car battery.