Shown below is the MGA battery charging circuit diagram. This circuit is drawn as the car was originally equipped with positive earth electrical system and two 6 volt batteries. One 12 volt battery may be substituted in place of the two 6 volt batteries. To switch the electrical system over to negative earth, reverse the polarity of the battery (batteries), and momentarily short across the A and F terminals of the control box to re-polarize the generator field coils.