Jim. Disconnecting just the battery will still give you headlight and rear light directly fed. Normal wiring means with no battery you lose horn and rear brake light. The voltage will be unregulated, so no heroics otherwise possible blown bulbs. Depends on your ride, daylight use simply disconnect dynamo connections, as suggested by BM. Battery will feed lights, brake light and horn as normal and all will work until battery exhausted.
Swarfy.