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Randonneuring requires a navigation device (smartphone or GPS) which will be needed for much longer than their battery will last and a dyno-powered USB charger is a great solution.
Here are some comments from the Randon list:
Tim Feldman asked:
David Dean, owner and electrical engineer, Sinewave Cycles said:
The charger output can turn on any almost any speed (with no load you can measure 5V at working speed), but the question is really
what is the minimum current that your device can draw, and at what speed can the Revolution provide that much current,
which is really a question about how much power the dynamo can provide at any speed, which is also affected by different wheel sizes, etc, etc.
The drag is a question of the efficiency of the dynamo, which is not so clearly advertised, but I do know of some people who have done third-party testing.
The effect of the charger and headlight on each other is really difficult to predict.
It includes all of the above variables, and also the device being charged and a time-based variable
(if the light is able to pull away more power it may further limit the phone's ability to draw current,
if the phone IS able to draw more current it may keep the light at a lower level.
Then you hit some speed threshold or the device changes the power it wants or the standlight capacitor finishes charging and then everything changes...
It's been frustratingly difficult to declare which devices can charge directly and which should use a buffer battery.
Two quick stories to illustrate the difficulty:
I had an iPhone 6s that charged very well at low speeds.
At some point I had the battery replaced and it no longer changed at low speeds.
So I could not say whether an iPhone 6s can charge directly or not, it depends on other factors that will vary from phone to phone.
You are probably familiar with how some Garmin units will try to auto-shutdown when you come to a stop.
I've heard from at least one customer where the behavior only started after they performed a firmware update.
So, while it can seem like a cop-out, that's why I only recommend that customers test their setup and determine if they want to use a buffer battery or not.
Even by knowing the phone or GPS make and model I can't say for sure how it will behave.
'Jacques Bilinski' commented:
If you're tinkering with this stuff one or more 'USB power meters' are useful devices.
They are available cheaply on places like ebay and amazon.
You plug them in series between between a USB powered device and a USB power source and they measure various things including the total Energy used in a time period.
In addition to the really cheap ones there are more expensive ones like the Klein Tools USB Digital Meter, USB-A (Type A) which is a little more solid and has the ability to store data.