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In other words, you can try search for " GPS solar charger" If there is none, Chances are that that word will get around in the web until an interested manufacturer get wind of it and make one specific for that GPS purpose that demand live time charging that can keep your battery up all the times.
If your smartphone has GPS feature ,this is great and it consumes a lot of electricity. What you will need is either a huge rechargable battery that hold thousands of millamper-hour or a decent sized solar charger at 5 volt that can flow a minimum of 5 watts.. Typical smartphones carry own 1500 milliamp-hour rechargable battery at 3.7 volt . If you want to use your GPS app in your smartphone, it will last minutes before running out of juice. Any charger has to be a bit higher in voltage in order to charge a lower voltage like 3.7 volt. So it is important ot watch volts, amps, and watts. Most smartphones do not require more than 5 watts even if you use GPS straight time. I would favor a solar charger over a rechargable backup battery because of weight. Each has its own advantages.There is so many different products out there that you need to do adequate research and comparing prices which vary widely despite same product. Just dont linger in the shadows long enough to run out of juice so that is why I favor a solar panel charger. There is certain solar chargers that is too small to charge fast enough in live time because of its small sized panel. For a 5 watt solar panel that can deliver 5 volts, you will expect something like a foot square panel or less similar to the ones used for solar attic fans , for instance. I never tried it but I just know where to look for the right solar charger that can keep the GPS in your smartphone going full time. Stay out of shades as much as you can.