Koho'olawe is mostly barren and is in the midst of a massive reclamation project to restore the vegetation lost by, among other things, 50 years of being used as a bombing range. The scene shot in heavy vegetation were NOT shot on The Sacred Island.
When Tani tries calling Junior, she gets a no signal message. Grover asks if his battery could be run down and Tani says something like, Junior knows better. But Junior, who fell down a ravine and is injured and can't get out, is seen using up his battery by playing games on the phone. So, Junior doesn't know better, and his battery dies.