Hi, Jen! You must be so excited about your upcoming trip!
Electric Conveyance Vehicles (or scooters as some Guests call them) are a very common way for those with limited mobility issues to maneuver the Parks. I've been known to log in over five miles of walking per day on my trips, which can be very tiring!
I spoke with representatives from both Buena Vista Scooter Rentals and Best Price Scooters and Strollers, and while they give a battery life estimate of 6 hours, it is common for Guests to get a full day of theme park touring on a single charge. The scooters are electric and come with a large battery and charger. They recommend charging it nightly in your resort room. Battery life will all depend on the weight of the Guest, and how much the scooter is being ridden. Also, hills and inclines will put more stress on the battery life. I'd suggest if you take a break for lunch or go to a show, just plug it in then and you'll be ready to go. Many counter service restaurants in the Parks have electrical outlets along the wall and in the floors, and you'll also often find them outside restrooms and at Guest Services locations- just ask a Cast Member for help. There is also a gauge on the scooter that tells you how much charge you have left. On the off chance your battery dies, you can always exchange your ECV for a new one if renting through Walt Disney World. If you are using a Disney Featured Provider, just give them a call, and they are usually able to show up within an hour to provide you with a new vehicle (they will even meet you inside the Parks). Another quick tip I learned is that the scooters cannot get wet, so if you go inside an attraction or restaurant, cover the steering handles with a poncho.
I hope you and your family have a great trip to the Most Magical Place on Earth, Jen! Come back and see us again soon, if we can provide you with any more planning advice!
Eean