12V battery -> 12V diode -> capacitor
The Diode can be a typical Silicon diode. Voltage ratings are usually 400V or more.. no problem. The Current rating should be 1 Amp or more.. 3 Amp would be good. Capacitor charging current will be high.
1N4006 or 1N4007 1 Amp diodes easy to find
1N5408 3 amp diodes fairly common.
Examples: http://goo.gl/nbvdJ
DISCLAIMER: Mentioned stuff from my own shop...