Have you already tried and ruled out OSMAND with your current phone? It has a few quirks, but if you get the hang of it, you can use it with no network access and, hence, reduce battery needs.

I keep my screen off most of the time and turn it on just when I have a lot of closely-spaced turns to make, and that doesn't seem to drain the battery too quickly.

If you definitely need the screen on a lot, you could consider connecting it to an external battery -- large-capacity units can be purchased fairly cheaply through Monoprice.com. Of course that add to the bulk, but if your alternative is to carry both your current phone and a separate GPS, I suspect this would be a cheaper option.