There are some battery chargers that you can buy which have some built in smarts but the standard ones are basically a large transformer and some large rectifier diodes. The main issue with the basic charger design is that if they are left connected to the battery for a long duration they will overcharge the battery. Instead of paying the big bucks have a look at the design that Razor made, he has also provided the full schematic so that you can easily build your own.
“Device consists of two main parts; power supply with current and voltage regulation (2 x LM317) and voltage monitoring circuit with switching relay. The charging procedure starts when voltage drops below 12.6V, and finishes when it gets to 13.8V. These values are not fixed and can be adjusted with trim-pots. The maximum charge current can also be adjusted with DIP switches.”