The rectifier/controller changes the voltage from the alternator into the DC for the on board computer, 12V, and keeps the voltage within a certain range. This keeps the battery charged and ensures that bulbs do not blow etc. This piece is exposed to extreme temperature loading and can often become damaged. If the rectifier/controller is defective, the battery is constantly empty and on some ignition systems, the ignition no longer works, bulbs go or there is a flickering from the light.