-After being on the island for a year, some of the GPS units are starting to lose their charge and to keep demand for them high, the traders and tamdals have decided to keep the chargers to themselves.
-Fortunately you can now turn off spare GPS units (just click on the GPS unit in the inventory) and they'll keep their charge until you turn them back on (click on the switched off GPS unit).
-A fully charged GPS unit should last about 50 days but if you keep turning it on and off, the battery will last a lot less.
I've got a loads on one alt, something like 14, I keep meaning to get to the trader, but I find myself wandering the jungle instead. I've turned all but one off, and they can stay off for the next few days whilst I kill big game.