I was up on North Ambergris a few days ago in a Moncho's cart. Battery ran down before I got back to town. I was near Journey's End then. I called on my cell phone, and they told me to go to the Caye Mart at Essene Way (an agent for Moncho's) and trade the got there. I did make it to the driveway to Esssene Way before the cart completely died. It took just a couple of minutes to switch carts, and I was on my way again.
But the lesson is: To go as far as you can north (these days, you can easily get past Portofino), you'll need a full charge.