Doesn't the smartphone have a 120v charger? Just plug it in whenever you are near 120v. If the GPS in the smartphone runs the battery down there are 2x to 3x oversize batteries with modified back cover available for most smartphones: http://www.mugen-power-batteries.com...FWmhtgoddE0DnA My smartphone now goes 2-3 days even with frequent use of the GPS.

Don in Austin