Brexit,actually the UK has come out rather well.

[Danidl]

Superficially that could theoretically be legal, but it's questionable whether such a system could be tested using the EN15194 standard, so might not be able to be approved.

More importantly though, given the legal restrictions on our very limited power motors, the drive power downtime could halve the existing power and more than halve the drive experienced. I doubt a future limited to a true 100 watts of drive to the road would be popular with e-bikers.

The simpler solution now that high current delivery batteries are much lighter and smaller is two batteries. One on charge from the small generator and one delivering current, the two automatically switching roles at optimum intervals. Since lithium batteries are suited to cyclic charging, this is entirely practical.
.

... Still carrying on my argument, there is no particular requirement that the duty cycle be 50% the charging voltage could be much higher as it is average charge to which the battery responds. The motor must be constrained to the 48v to meet the EN standard.

The bike would still meet the EN standards as without the potentially third party add on it could be a totally standard e bike.

Of course it would be just as easy to have a second battery sitting on the carrier with the small generator.

 

[flecc]

... Still carrying on my argument, there is no particular requirement that the duty cycle be 50% the charging voltage could be much higher as it is average charge to which the battery responds. The motor must be constrained to the 48v to meet the EN standard.

True, but the drive would still be detached for a part of the duty cycle, reducing the power available.
.