For a few bucks you can get a bridge rectifier from RadioShack and a 7V Zener diode (online ‘cause RadioShack will not have it) and try the circuit below for a lead-acid battery. If you are using different batteries, get a Zener diode that matches the required charge voltage.
Try the following link for the Zener diode:
7V Zener Diode: KELVIN.com
NOT-SO-SMALL-PRINT: I have never tried the circuit below & I only know enough about electronics to be dangerous. You’ve been warned. Good luck, and let us know if it works.