The diode board grounds through the negative side mounting bolts. I'm more familiar with the airheads that use the same Bosch charging system and that's how they work. I'm pretty sure my T3 and LM4 ground the same way. It wouldn't hurt anything to run a ground cable straight from the battery to the diode board, but making sure all ground connections are bright, shiny and clean would be good enough and all that's really needed.