The diodes for rectification (converting AC current to DC) are mounted on heat sinks, in packs of three. One heat sink has three positive diodes and the other has three negative diodes.
The positive diode heat sink is insulated from the frame , and is connected through the output terminal to the battery positive or B plus terminal.
The negative diode heat sink is connected to the frame, and this allows the return circuit, via the battery negative terminal to be completed.
