Well if you use an LS t-connector alternator, you only need three wires
* positive to A/BAT and S
* negative to E
* 12v light between A and L
Per Basic Alternator Wiring

Don't forget a Fusible Link for safety. Won't blow like a fuse, but help prevent fires.
You can use a generic $3 link.

But you also need a small charged battery (or other 12V DC source) to initialize the charging. If the batteries to be charged aren't too run down they will be enough.