The voltage regulator maintains a constant voltage level by controlling the output of the alternator. If you're alternator is failing to charge your battery or run the electrical systems of your vehicle, the internal diodes of the regulator may be faulty. A good way to check for a faulty voltage regulator is to use a digital multimeter on the battery, with the engine running, to read the charging voltage. If voltage is not constant around 13 to 14 volts, chances are the voltage regulator has gone out. Replace yours to restore proper operation to your charging system.