That is not bad, but DEPENDING on how hard the alternator was charging at the time. Drop is caused by current flow. If the battery was "up" and normal, and the alternator was merely providing basic ignition and no heater or lights, that might be "a lot." If the alternator is at full tilt, I would consider 1V about the limit maybe as much as 1.5 if no system upgrades have been done. And even that varies with alternator---------35--45---65A etc
