0The Ideanator 9 years ago Technically you can, but only up to the batteries voltage. Capacitors are sorta like buckets and voltage is like the water level. If you're starting off with a 9v battery, the necessary circuitry to boost 9v to what you require would probably cost too much, and so I would recommend starting with mains voltage and a beefy bridge rectifier and smoothing cap as a rudimentary 120v or 240v power supply. The rule of thumb for "standard" cap usage is to pick a voltage rating that is double what you need. I'm guessing that your caps are rated at 400v, so you'll want around 200v maximum (240v is fine) for your charge voltage.For charging speed you'll need higher current for faster charge times which is another area where mains voltage would come in handy.