Does anyone know how to make a 12v fan charge a battery?
(media.scored.co)
If you spin the motors with a voltmeter, you should be able to see DC volts. These are probably permanent magnet DC motors so the only way to regulate the voltage will be how fast you spin the motors. If you can't get enough, you can wire the motors in series to double the voltage output. The easiest thing would be to get a buck boost converter that outputs around 14 volts to charge your battery.