we change alternaters on focus's all the time.they are rubbish.the alternator is controlled by pcm and told to charge when requested.the smart charge generator in the back is dormant until pcm asks it to charge.normally it packs up and cant work as the request and stop is so frequent.
i would imagine cars could run without a charge light as the alternator has its main lead from battery then the 3 pin plug,1 wire is a fused battery feed,one is signal to and from pcm and the other pin is the light request wire.
hope this helps a litle bit mate as from the amount we have changed and reading up on etis,thats my understanding of 'smart charge' alternators