on a doozil there are 3 connections at the back - a power output, which charges the battery. An exciter connection which takes voltage from the battery via the battery warning bulb. This appears electrically as a ground, so that when the alternator is not charging, it lights the warning light. As soon as the alternator starts charging, a small back voltage is produced, which extinguishes the light. A pulsed output is also generated on the "W terminal. These pulses can be counted to drive an rev counter.