I would say the maximum allowed voltage drop is zero, if the car is well-designed. Well-designed here means that the alternator has a separate sense wire that senses the voltage at the battery terminals. Unfortunately, many cars such as my old 1989 Opel Vectra have alternators that sense the voltage at the alternator output. This eliminates one wire, saving a very tiny amount of money. However, the impact on battery longevity is great especially if you use electrical accessories a lot. So, in the long run this doesn't save money at all. This is especially troublesome as accessories such as rear window heater and seat heaters are often used directly after starting the car, at the time the battery needs additional charge. The float voltage of 12V lead-acid battery is 13.5V - 13.8V at room temperature. If your voltage at the battery terminals is below 13.5V, then it isn't even keeping decent charge, much less charging the battery. Note that cold temperatures need a different float charging voltage. Note that the air conditioner in most cars uses no electricity. It runs directly on mechanical power from the engine. So, it doesn't matter whether you have AC on or off. However, Toyota hybrids have electric air conditioning, but I believe it is run from the high-voltage system and not from the 12V system.