It is handy to have a small battery charger for your automobile, especially if someone parks it in your garage for the night with a door just slightly ajar. In that case, the dome light will remain on all night and the battery will be very low in the morning. I made this charger when I ordered some electronic parts and received a 120 to 12 volt AC transformer with center tap. Sending it back would have cost as much as the transformer. Similar transformers can be found at places like Radio Shack. The transformer puts out about 3 Amps., so it is ideal as a trickle charger. A charge will require about 12 hours. But, it has gotten me out of several jambs. Some friends have also borrowed it when their batteries were dead and it worked for them. The center tap allowed the output to be either 6 or 12 volts. At the time I made this charger I helped to maintain an older farm tractor with a 6 volt electrical system.
http://www.instructables.com/id/Auto_battery_charger_for_6_or_12_volt_sytems/