Join Date: Oct 2009
Location: Austin TX USA
Bikes: Bob Jackson 853 Arrowhead w/ Chorus (road); Swobo Del Norte (street), Catrike Speed (recumbent trike)
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I've been using a cheap 2 AA battery booster with USB-out with my iPhone when I go riding. The phone's internal battery will suffice for almost all the riding I do, so this is a belts-and-suspenders approach.
You can get a idea of how many batteries you'll need by looking at the voltage and amp-hours of the GPS' internal battery, and its run time. If (for example) your GPS has a 3.7 V battery with 1500 mAh, that means it's producing 5.55 watt-hours of power. And let's suppose it gives you 6 hours of run time on that. So you know you need about 1.1 WH per hour of run time. You'll need to recalculate these figures with your actual device, of course.
NiMH batteries are typically about 2200 mAh at 1.2 V, or 2.64 WH, so each one in theory will add about 2.5 hours of run time. Obviously you'll need to use them in pairs or fours, depending on the charger you use, and there will probably be some loss of efficiency using an external power source.