Cries on hills
Join Date: Jun 2007
Location: Central NH
Bikes: 2007 Trek Pilot 1.2, 1995 Gary Fisher Avantage, 1985 Panasonic Sport 1000, 1969 Raleigh Sprite 5
Anything is possible--with enough work that is. I have not taken apart any hubs, but my reading into most generators is that they limit overvoltage so as to protect bulbs; and odds are, they are going to do that via one of two methods: 1), they sense the excess voltage and somehow cut back field, so as not to waste energy--the hub draws only 3W of power from the wheel and no more (this would be a high dollar solution). 2), they waste excess voltage in a regulator circuit--but this means that, if it generates 3W at 6mp, the hup will draw 6W at 12mph, but dump 3W into the air doing nothing (cheap and easy). Having not played with these things (yet), I don't know the answer--but I suspect, if you were to ride with the lights on, at high speed and for a while, you'd feel the unit warm up a bit.
Anyhow, the point of that is to say, odds are, there is plenty of power getting wasted in these generators. If you could remove any such protection, that would be great. With the cheap regulator in place, you're just not getting as much out as you might like to, either in terms of power or in terms of efficency.
Either case, what you would want is to follow the generator with essentially an SLA float-capable charger. "Good" ones allow you to put a load onto the battery; and if the load is heavy enough, the charger will just limit the charging current to the battery to its set current limit (usually C/10). But, in the absence of a load, the charger will kick out and go into float mode if the battery is actually charged up.
The issue would then be, if you are stuck with a 6V output, either you go with a 6V battery system, or you first boost the 6V output up to 12V. With an unregulated hub, you would have to follow with again some sort of regulator--but this regulator by design would be some sort of low loss switched mode supply, meant to generate probably around 15-16V to properly run the charger.