A single pole, double throw switch mounted on the handlebar would also solve the problem.
Connect switch Common to lights
Connect battery to one of the switched contacts
Connect permanent magnet DC generator (or AC dynamo) to the other switch contact
When speed is low and lights are dim or dark, switch the battery in.
Leave switch connected to Generator while not riding, prevents battery discharge.
You should fuse the + battery wire near the battery.
Jon
Connect switch Common to lights
Connect battery to one of the switched contacts
Connect permanent magnet DC generator (or AC dynamo) to the other switch contact
When speed is low and lights are dim or dark, switch the battery in.
Leave switch connected to Generator while not riding, prevents battery discharge.
You should fuse the + battery wire near the battery.
Jon