The above sketch and notes below come from Tony Hillyer, for many years senior designer at Sturmey-Archer.

When closing switch S with the cycle stationary, the battery pack (3 x R20) supplied the filament lamps. As the cyclist increased speed, the dynamo would generate, sharing the load until at 9mph the dynamo took over. Reducing speed the opposite occurred. There was always a light.

Although R20 primary cells are not rechargeable, we found with the aid of bleed resistance R, a small current flowed through the batteries in the reverse direction, giving a longer life from the batteries. Maybe by depolarising the cells.