As part of the pre-flight inspection, one of the checks is to verify that the low voltage light is illuminated when switching just the battery on. This makes sense; there's a low voltage and the battery is being discharged because we're running everything off the battery, and the alternator is not providing any power.
However, as part of the startup - just before ignition - we switch both the battery and alternator switches on. At this point, the low voltage light is not illuminated, despite the alternator not providing power as the engine is not running. This doesn't make sense to me - though we've turned the ACU on, the alternator is not providing power. This seems like something that should trigger the warning light.
I've examined the C152 POH and it shows a basic electrical layout. I'm not sure as to the copyright of the handbook, so I won't post it here. It is available online for those interested, figure 7.7. Unfortunately, this did not help explain the behaviour I'm seeing.
I've reproduced this on two different 152s. Why does the warning light not detect the fact that the alternator is providing no power, or am I completely misunderstanding the function of this light?