Has anyone any recommendations for a battery pack to be used for extending the use time of my GPS (Garmin 510)?

At the moment it would just need to supply power but in the future I'll be looking at having a dynamo setup so being able to charge it from such (through a regulator) would be an advantage. No great need for it to be able to act as source and sink at the same time. Something that would extend the Garmin's 16hrs to a weekend or maybe three days would be ideal.

Usual requirements of cheap, lightweight, robust, waterproof/resistant

I've been using one of these & it's spot-on for a 2-3 day trip-

http://uk.maxell.eu/en/products/5200mah-silver-692.aspx

Not waterproof, but light for what they are. You can pick them up on Amazon or even in Cosco.

Whichever one you get, i'd recommend one with at least 2 ouputs..

Edit:
£11 Yay!!
http://www.amazon.co.uk/Maxell-790403-B ... ll+5200mAh
£4.59 Shipping!! Boo!!!