They have a special regulator inside the alternator to trigger it to charge. It has to detect automatically when to bring the alt "online" to charge, and "offline" when the engine is not running so it will not run down the battery.

You have to make sure you run a fairly large wire for these as was mentioned, and in some installations they can be finicky to get them to charge. What this usually means is you may have to rev the engine a little bit to get it to start charging after you just start the engine.