I have SpinPOWER USB charger kit, and I am using a bottle dynamo with it. I have heard that the power output from a hub dynamo is higher than from a bottle dynamo. Does anyone have any information about the merits of using hub dynamo vs. bottle dynamo? Also, what is a good hub dynamo, and what is a price range for the installation?
I have not used a bottle dynamo for a very long time, but have a hub dynamo. Broadly, the hub dynamos are more expensive but more efficient - you get less drag for the same power output. Most systems put out 3W peak power for historical reasons, although a lot of new lights use less than this. Bottle dynamos are noisier and wear on your tyre, which may be an issue if your tyres otherwise last a long time. A good bottle dyno will be nearly as efficient as a hub dyno and cost about half as much, plus you don't have to build it into the wheel. A cheap bottle dyno should be removed and thrown away.
Some dynamos will put out more than 6V, which is how you can get more power out of them (the Schmidt ones do - I used to run a 5W white LED straight off one). Often at decent speed they will go to 12V at the same 500mA, so you can get 6W. There are also dynamos specifically designed to do this, and they usually come with a regulator and various output options for charging things like phones.
Hub dynamo systems cost at least a couple of hundred dollars because you need to build the dynamo into a wheel. But they last a long time (I don't know how long as I've only had mine for 10 years and the ones I know of that have failed have done so when the bike was destroyed around them, or the owner pulled them apart). Any decent LBS should be able to get them if they don't stock them, or refer you to one who does. Peter White Cycles is one of the better dealers in the US with a wide range of options, likewise St Kilda Cycles in Australia.
I have a hub dynamo, albeit from the 1950s, but still going strong. Allegedly hub dynamos have improved since then, there are some expensive European made models that people swear by, there are also some affordable Shimano units that work rather well and can be bought built into a wheel for around the £100 mark.
I would be tempted by the Shimano units because Shimano have put some effort into this product line in recent times and they have tried to widen the market with better prices than their rivals have been charging.
Your local LBS will be able to build up a wheel for you, you can also get something mail-order.
The USB charge kit that you already have is quite innovative in that it has an extra box to smooth the power out into 5V DC (the dynamo typically gives out anything between 0 and >6V AC).
Therefore, you must plug the new dynamo into the USB doo-dad that you already have (or put together a smoothing capacitor with voltage regulator circuit).
Failure to smooth and cap at 5V could be damaging to your iPod although I expect that it probably could charge fine on 6V with no problems. (6V is the standard voltage for bicycle dynamos).
As well as the hub dynamo I have had the bottom-bracket mount dynamo and plenty of the sidewall 'bottle' dynamos. The 'bottle' dynamos are noisy, but at night, for lights I think that is a fair payoff. For what you are trying to do the hub dynamo is definitely the way to go - completely silent, exceptionally efficient and useful for lights as well as iPod gadget.
My 1950's hub dynamo is going strong - an investment of £100 - £200 will be forgotten in time and you will have much enjoyment from having electrical power on your bicycle.
For anyone wanting to put together a charging circuit of their own, here is an 'instructables circuit':
The parts should cost < £5
I can attest that the bottle type eventually (actually, relatively soon) wears out. The sideways force on the top bearing causes it to wear quite rapidly.
I once had unit that fit in the "kickstand mount" area behind the bottom bracket, and it had a wide wheel (the dynamo was inside the wheel) that bore square on the tread of the tire. That one lasted a long time (in dynamo years) -- probably in the neighborhood of 2000 miles operating (headlight back and forth to work). I've never found a decent replacement, so I went to battery units.
The main advantage of the bottle dynamo is that it introduces zero drag when not engaged. Bicycle Quarterly did an analysis of the drag from dynamos and a bottle dynamo back in 2005. Note that the bottle dynamo was internally capped at 6.2V, but otherwise is only slightly less efficient (in terms of drag) than the hub dynamos up to around 30kmh.
Of course, modern dynamo hubs have only improved, with the SON20 now branded as the SONdelux and the newer Shimano DH-3N81 having even less drag than the 3N71.