Alisha Navarro:
Why don't you look at the Maxim App Note # 621. It describes a nice battery charger for a Lead/Acid type battery. Here is a link to the AN621.

Note that in general to fully charge a 12 volt battery and get maximum battery life you need to have a source supply voltage that is in excess of 14.5 -> 15 volts. Not 12 volts like you mentioned. The fully charged terminal voltage of a Lead Acid type battery is something like 13.48 volts. Thus to enter into the ending constant voltage charger mode for float holding on the battery you need a voltage source higher than that to regulate down to.

The Maxim app note referenced above describes a battery charger that uses a switch mode converter to convert a wide range input voltage to the appropriate level to charge the 12 volt battery. This will lead to a good efficient design. Simpler and lower cost designs will use a linear regulator type of circuit to provide the constant voltage mode portion of the charging cycle. These types generate increased heat that could be detrimental to long term system longevity. And certainly excess heat does raise the entropy of the universe a small amount.

Michael Karas