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
