6V 3W bicycle hub alternator to Phone's 5V charging port

It helps if you can provide links of specs. to your intended generator with expected drag vs efficiency tolerance for overcoming this generator's inefficiency. An MPPT circuit is essential which must match the performance of the generator and rider's preferences for low speed cut-out or manual over-ride.

Rather than reinvent the wheel (pun intended) list the most popular commercial products. Maybe technology has improved a lot (e.g. LEDs) but when I was a kid, I just thought these generators were "a drag".

The output of a permanent magnet alternator varies with both speed and load - at any given speed, the voltage will drop as the load increases.

That makes it seem to be a good use of "MPPT" based regulation, as used with solar panels.
It's a system that constantly adjusts the load current to optimise power output.

I'd start with one of these little MPPT modules; it looks pretty good, from the specs provided:

Feed that via a "voltage doubler" rectifier circuit from the alternator, as the MPPT need 8V minimum to work.
See the "Delon circuit" in this article:

Use Schottky diodes rated several times the alternator maximum current, and eg. 4700uF 25V caps??

Tony Stewart said:

Rather than reinvent the wheel (pun intended) list the most popular commercial products. Maybe technology has improved a lot (e.g. LEDs) but when I was a kid, I just thought these generators were "a drag".

Click to expand...

The hub dynamo on my Brompton does not have drag that is noticable when riding. I can detect some drag when spinning the wheel when it is off the ground.

I had an incandescent front lamp originally. I changed to LED to get one that stayed on for a bit when I stopped, not to reduce the drag.

Tony Stewart said:

It helps if you can provide links of specs. to your intended generator with expected drag vs efficiency tolerance for overcoming this generator's inefficiency. An MPPT circuit is essential which must match the performance of the generator and rider's preferences for low speed cut-out or manual over-ride.

Click to expand...

I really don't see what MPPT would gain in practice.

Bike generators, like car alternators, produce a fixed current that drops off at low speeds, but is reasonably constant from moderate speeds up to the maximum speed. For a bike generator, that is road speed, and for a car alternator that is engine speed.

An MPPT circuit would need a voltage converter that adds a whole load of complication. It could increase the power available for charging a bit, but it won't be much unless the generator voltage is allowed to go very high at high speeds, which has its own problems, and that may be limited by the generator anyhow.

If most of the current from the alternator can go to charging the battery without an MPPT, and the battery is at 4 V, and the generator can go to 6 V, then the best an MPPT circuit could do is to increase the rate of charge to 1.5 times what a simple circuit could do. So the battery would be charged in 2/3rds the time. However, the OP wants to tour with his bike and will probably have far more time on the bike than is needed to charge the battery, so there's no real gain.

The only exception might be if you could have some way of charging a lot faster at high speeds, and use the power that is otherwise lost to air resistance on downhill sections.

Interesting article here with some decent test results, of various bike "dynamos":

FW bridge & Cap are well-known to be very low power factor with power transfer with a very low duty cycle before peak voltage. In order to apply maximum power transfer (MPT) conjugate impedances must be matched to source. This means active PFC before rectification after a stepup on Vac. I wonder if anyone has seen this.