Hi there,
Battery charging isnt as simple as it seems at first. The first thing you have to know is what kind of battery chemistry you are dealing with. This is extremely important as the charge circuit must be designed according to the type of battery (NiMH, NiCd, Li-ion, Lead Acid, etc.). In fact, if more than one type is to be charged with the same charger then you may have to design two or more algorithms into the charger. That's just the way it must be.
In general though the ac is stepped down and then a dc switching converter is used in some fashion, or even a linear in some cases.
So do begin with, you should state the battery chemistry(s) you will be working with and we can go from there.