An alternator configured for use in a vehicle comprises a stator having a plurality of stator windings. A rotatable field coil is positioned adjacent to the stator within the alternator. A field driver circuit is configured to deliver an electric current to the field coil. A voltage sensor is configured to detect a battery voltage. A controller in the alternator is configured to execute either a first field current control program or a second field current control program depending at least in part upon the detected battery voltage. The first field current control program is configured to control the electric current delivered to the field coil in a 12 volt vehicle power system. The second field current control program is configured to control the electric current delivered to the field coil in a 24 volt vehicle power system.