Can I do an alternator check at idle in a C152?
The problem is the light might come on on its own at idle, which is normal.
The warning light comes on when the system voltage is below 12.5 volts on a 14 volt system, and 25 volts on a 28 volt system. Automotive alternators (which most Lycomimg powered light aircraft have) need to spin at at least 2000 rpm to make adequate charging current to maintain system voltage under load.
The belt drive on the engine that overdrives the alternator relative to engine rpm may or may not maintain this rpm with the engine idling, so the bus voltage of the system may be just over 12.5/25 vdc or just below it, depending on the loads on the system (like radios and lights, etc.) and the alternator's condition.
The 152's POH specifically warns you that the light may come on after extended idle periods with electrical loads on the system, so this is to be expected, and it means that a test done with the switch can't be trusted to give a valid result (the light could be on because of &quot;normal&quot; idle low voltage, not because you switched it off).
Doing it at runup rpm just makes sure the alternator is spinning well above this marginal operating condition so it's producing normal charging current when you switch it off.