Yes, the standard design was marginal with no reserve....
The original dynamo is rated at 22 amps. Four sealed beam headlights draw 16 amps on their own. Add side lights, panel lights and ignition, and the total will be around 20 amps, maybe a bit more. With a little inefficiency in the dynamo, the battery will drain flat over a few hours. It happened to me on the CT Round Britain Reliability Run.
Nigel