Another option would be a dedicated battery for the GPS such as one of the small sealed gel cells. Use a plug-in charger at home for recharging at night, and you'll have several days' worth of operating capability between charges. Back in the dark ages a guy in my USAF barracks ran an 8 track tape player in a 6V Volkswagen that way, using a full sized 12V battery as a power source. He recharged the battery about once a month.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!