In preparation for a 2 week crossing of the Pyrenees where we won't be able to recharge batteries, I've built a front wheel with Alfine dynamo hub which generates 6V, this is married up to a Kemo switch device that has a mini USB output of 5.7V DC. I've plugged this into my Garmin Oregon 450 and when I spin the wheel, the unit bursts into life but enters mass storage mode, taking about 30 seconds to get going. When the wheel slows to less than 2-3 mph the unit loses the external power and then when it speeds up it all starts again. This will drive me potty, I'm looking to smooth out the power supply or configure the Garmin to prevent this, any ideas on how I can do this? I've already changed the unit's profile to Spanner but am at a loss how to store the power when going slow or stopping for a bit. Also I've heard that over about 18 mph, the unit cuts out due to too much juice being generated.
Thanks in anticipation.

Use the dynamo to charge an external battery pack? I have a battery with USB output I use for my iPhone and Garmin edge 200 on multi-day trips.

Sounds interesting, I ideally wanted to run the Oregon directly from the dynamo as it is backlit when powered via a mini USB which makes it much easier to read. What make is your battery pack, is it rechargable?

Could you use something like this?

http://www.ultralightoutdoorgear.co.uk/ ... _pack.html

That looks as if it will do the trick, I was hoping that I could power the Oregon directly from the dynamo like a light unit. It feels slightly inefficient to use the dynamo to charge batteries then discharge the batteries into the GPS but if that's the only way then so be it.

pistonbroke wrote:...the unit bursts into life but enters mass storage mode...

I don't know if this will solve your problem, but there are four wires in a USB lead - 2 for data (usually white and green), 1 for power (red) and 1 ground (black). You might try cutting and re-soldering a USB cable to remove/disconnect the data wires, which I think might be causing it to go into mass storage mode. I made a shortened lead for my GPS charger (Minty Boost DIY kit I got from the US), and just soldered up the power wires and not the data ones. Works just fine.

Mine is branded "New Trent". Put New Trent battery pack into Amazon's search and all sort of options pop up. Here's an example similar to mine.
http://www.amazon.co.uk/gp/aw/d/B000NDQ ... -8&pi=SL75

Probably worth giving bigrobracing a shout. Seems to be his area of specialism.

Interested to know how you get on as i've been looking at this option as well.

My friend had the same problem. The data pin trick often works for fussy devices, but it didn't for him, and we ended up using the smallest, lightest USB buffer battery to prevent this happening. Something like this should work just fine:

http://www.ebay.co.uk/itm/Silver-Lipsti ... 56508393fe

It's an annoying solution, but these things draw so little power that, although inefficient, the power loss is miniscule.

It's probably worth caveating that last post by adding that this is NOT a good solution for a dynamo light, where the power loss will matter a lot more. It's also not clear what the multiple small cycles will do for your battery life, if you use it with a light, but it certainly won't help it! Oh, and of course, the Exposure Revo or similar modern high end dynamo light, doesn't need a cache battery as it stores and then dispenses is power cleverly in capacitors very efficiently ;-)