Dynamo hubs

Those commercial usb-thingies seem damn expensive. Do they have some super speshul component wizardy inside or am i missing something here?

Yes, if you just provide 5V on the power lines of a USB cable (without making it a "charging cable") then the device will probably only draw a maximum of 100mA. It may also trigger the device to turn off so that it cannot be used whilst plugged into a power source.

To draw a higher power (500mA) most devices will need to negotiate (via the USB data pins) with the power supply, or you need to use a "charging" cable (see below). This is the extra super speshul stuff that exists in some of the commercial usb-thingies.

Also, the negotiation for higher power may also mean that the device can charge whilst still being available for use.

Some of this behaviour (device switching off when being charged) is also seen with different USB cables, one being a full on USB "data" cable and others being "charging" cables (or "dumb" cable) which have one of the data pins grounded - this tells the device to just pull as much power as it likes without negotiation.

Regardless of cable, my Edge 705 will go into charging mode (and end the current track recording) when I plug it in to the power source (a PortaPow USB battery pack), but I'm able to turn it back on once this happens so that it can be used whilst charging.

If the Edge 705 is plugged in to the power source when first turned on then it only whinges about "External Power Lost" if the cable wiggles loose, it doesn't end the track when the connector wiggles again and power is restored.

For a ride >10 hours I start with the GPS plugged in to the battery pack and continue this way until the power pack is exhausted. It means I can use the backlight on the GPS (I tend to ride with the map page showing and navigate against a track displayed on the map as a light blue line, I stopped using prompted navigation ages ago) and never worry about power. For a prolonged stop I'll try and top up the PortaPow pack from the mains.