I want to make some 12v fans out of computer box fans with 3 18650 Li
batteries to use for ventilation over the
bed at night. One concern is that Li
batteries don't like to be completely discharged. Is there some simple way of preventing this so that they run for a while when I go to
bed but shut off when the state of charge drops to, say, 30% or similar? I don't have 12 v in the
cabin and don't want to run an extension cord. I'm familiar with, but not expert at,
electronics. I thought about zener diodes but am not sure the
battery voltage drop off is right. Perhaps a simple timer switch that could be programmed for a couple of hours. Are these available
battery operated? I know the AC versions are common. Also, must be adult-programmable. (A countdown timer is best. I never could figure out how to program my energy saving thermostat switch.)