Common problem, as said you also will find you have no battery warning light on the dash, theres a thin blue wire on the back of the alternator this brakes away from the connector. Usually when you turn ignition a +12V feed is sent to this terminal this in turn excites the field windings which starts the charge process, when the engine is running the terminal changes from being a ground to +12V hence extingushing the light. If you rev to 4000 this is enough to start the charge process.

Martin