how much time it will take to fully charge a 48 V 35 Amp hr battery using a 100 V DC dynamo running at an average speed of 20 km/hr
Typical bike dynamo has 6W nominal power.

48V 35Ah battery has 1680Wh (watt hours). For normal bike dynamo that would be

1680Wh/6W = 280h

However if you use bigger dynamo - wikipedia says:


  A trained cyclist can produce about 400 watts of mechanical power for an hour or more, but adults of good average fitness average between 50 and 150 watts for an hour of vigorous exercise. A healthy well-fed laborer over the course of an 8-hour day can sustain an average output of about 75 watts.[1] The yield of electric power is decreased by the efficiency of the human-powered generator.


So, trained cyclist:

1680Wh / 400W = ~4h

Healthy well-fed laborer:

1680Wh / 75W = 22,4h

Notice, thats just a generator powered by human legs, not bike that moves and charges battery at the same time. There are also power losses and limited battery efficiency. You may need to add about 50% mechanical energy for power losses on dynamo, charging circuit and battery chemical/thermal losses.