(Linked)
Here is how I've charged my n-gage and friends 6600 for two weeks. the cost of charger is under 40$ , handmade. It survived a 2 weeks long mountainbike trip, and charged even when riding. Currently thinking of designing a charger from a bicycle dynamo. That would charge even faster.
A panel costs about 15$. Three panels would provide around 500ma after the charging circuit. I hope that would be sufficent to charge 770.