Currently I only use the dynamo for the headlight. It draws a lot more power than a Garmin or a smartphone and it's safety critical at night, so not having to worry about batteries is more important than with a GPS logger, where you may just end up losing data for Strava.
In 2012 I tested three dynamo USB adapters from two different suppliers in Germany, but had problems with both on high speed descents. Not only did this kill all the adapters, but it also damaged the devices connected at the time (a USB battery, a smartphone), which must never happen.
Basically the adapters weren't adequately protected against high voltages from the dynamo at high wheel RPMs with the headlight off. In my case this was probably compounded by the fact that my wheels are 25% smaller than 700C. Still, the adapter I burnt out on the Sagamiko side of Otarumi toge at about 55 km/h with 20" wheels probably would have been fried on a 73 km/h descent with 700C wheels.
I don't know how speed-proof other adapters by other suppliers are. Switching the light on for descents should protect the adapter in most cases, as it draws enough power to prevent voltage spikes. Nevertheless, if the adapter doesn't cope with high speeds and you have an expensive iPhone or Android hooked up, it could be rather costly forgetting to switch the light on for the descent...
As for the start/stop issue: yes, a buffer battery is a must, at least around populated areas. Without it the device keeps waking up every time you stop at a traffic light and a second time when you start moving again. After each change of power event the screen tends to stay lit for 30 seconds to 1 minute. Unless a buffer battery covers for those traffic light stops, you may burn through the device's internal battery charge quicker than you replenish it in between traffic lights, simply because the screen ends up being lit too much of the time.
I have not tested any of the following products yet:
- The PedalPower+ adapter comes standard with a user replaceable 18650 buffer battery. That's a great idea because 18650 cells are generic and relatively easy to find.
- The Busch & Mueller "USB-Werk" (EUR 99) also comes with a small built-in buffer battery.
- For the Busch & Mueller "E-Werk" (EUR 147) a buffer battery comes as an option (EUR 74). I find the E-Werk / buffer combo a little pricey for what it does.
- The Busch & Mueller Lumotec IQ2 Luxos U (EUR 150 at Bike24.de) is an interesting combination of a dynamo headlight with a USB adapter with builtin buffer battery. If I were to equip my bike from scratch right now, I would give this one a try.
The simplest and safest solution of course is a USB battery. It will never generate over-voltages, is relatively cheap to replace if it dies and not that much heavier (if at all) than a USB adapter. I ran both my Samsung Galaxy S3 LTE and my Garmin Edge 500 on a single USB battery during the 25 hour, 360 km Fuji brevet weekend. On its own the Galaxy S3 LTE lasts 6 hours, the Garmin 16 hours while GPS logging.
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