# What is the best way to turn 6 volts ac from bike hub dynamo into 12 volts dc battery?

I have a bike hub generator and normally you use them to run 6 volt lights but I have 12 volt lights and a 12 volt battery.  I need to change the ac from the dynamo into dc and change the voltage from 6 volts to 12 volts.  I hope in this way to have a bike which will never need to be recharged from mains, as long as I use less electricity than I make from the dynamo.

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7 years ago
Put the 6VAC through a transformer to step it up to 12V, put the output through a full-wave rectifier, use that to charge the battery.

Of course you will lose half the current when you double the voltage. Depending on how much power your dynamo produces and how much the bulb draws, you may have trouble keeping up with it. (At least you _are_ planning on using a battery, so you can store power for use later; that may be what makes this practical.)

And, of course, the energy has to come from somewhere. In the case of a bicycle, it comes from your muscles. Rolling with power being drawn from the generator will experience more resistance than with that circuit open. You will have to pedal more and/or harder than if you were not running the generator.
7 years ago
+3. Three good points.

Last option, outside the scope of the question: Get 6v lights and 6v batteries...?
Noblenutria (author)  frollard7 years ago
That would work better for the generator but 12 volts has many overriding advantages. Anything that would work for a car or motorcycle also works for my bike. I have a radio, lights, and a usb adapter that run off 12 volts. You know what I have thought would be fun to try but would cause a huge amount of resistance is to have two hub generators wired in parallel.
7 years ago
Again, anything you want on a bicycle at 12 volts could be made/adapted/built to run on 6 volts. Many circuits take the 12 volts and burn most of them off to run the (usually digital/silicon parts) guts at 5 or less volts.
Noblenutria (author)  frollard7 years ago
Do you think it would be more efficient to have a 6 volt battery? Is the amount of power needed to change voltage directly proportional to the voltage difference. I mean, Does it take less energy to change 6 volts to 5 volts than 12 volts to five volts? Maybe the most efficient battery would be the lowest common denominator between the voltage requirements of each device.
7 years ago
Depends on how you convert it -- most commonly you burn off the excess voltage in a regulator (glorified smart resistor) as heat - so going from 12 to 5 volts produces 7 'volts' of heat at the same current as your 5 volt load. I know I don't like blowing away over half my available power.
7 years ago
The magnetics of bike dynamos are usually crap. I'd experiment with a BLDC motor, and some rectifiers.
7 years ago
That would be my solution. Or go with high-intensity LEDs since in my area bike lights are mostly to help drivers see you rather than to help you see the road.
7 years ago
I suppose you ride around all day, and charge the batteries for ~5 hours say, but you may only use the light for an hour or so at night. The load is 20% lower then.

Steve
7 years ago
That's sorta the point I was (badly) trying to make. If you expect to do equal amounts of daytime and nighttime riding, you may need a generator which produces an annoying amount of resistance to rolling. If most of your riding is daytime, this is more likely to work.

Can't hurt to try it, anyway...
7 years ago
I've often wondered how much the load of a bike dynamo is down to crappy magnetics, and how much to the real load ?

Steve
seandogue7 years ago
Rectifying the 6VAC will not result in very much (if any) usable power for charging a 12V battery.

your best bet is to rectify the 6VAC, then use that (~8-10VDC) to power a buckboost converter to deliver the required ~13VDC for charging the battery.
Noblenutria (author)  seandogue7 years ago
Are you saying rectifying 6 volts ac turns it into 8 to 10 volts dc; rectifying automatically steps up the voltage a little?
7 years ago
Rectifying 6VAC and then adding a cap will result in a voltage of somewhere around 8-10V usable. I do not believe that simply rectifying the AC will result in a usable charging voltage. To create that, some form of step-up DC DC converter would be used following the initial rectifier. For a trickle charger, it's pretty simple, since you need about 13.2V for that purpose, so it just needs a 13.2V output DC DC converter with a current limiter. For anything more, you'll need a true battery charging circuit.

You may be able to buy a 5V input, 12V battery charger or charging circuit to reduce your effort, in which case, you'd rectify the 6VAC, p8sh it through a 5V regulator, then into the 5V to 12V charging circuit.