If you are renting a non-Disney
scooter then you must be diligent with recharging it every day. Make sure you did it right by checking the scooter for a full charge each time. If it's a Disney scooter they recharge; it's one of the reasons you pay more for a Disney scooter. It's a do nothing approach with them. If the Disney scooter died for battery reasons you simply get a replacement.
The most important factor for battery life each day is shutting off your scooter when you are not moving in the parks like with shows, live street entertainment, parades, fireworks, eating, etc. We go as far as shutting it off in long line queues like Toy Story Mania. This will preserve your charge the best.
Your weight and the terrain plays into the life of a charge. If you are maximum weight on any scooter it has to work harder. This includes items you place in the basket, etc. There are weight limits to each type of scooter. I believe the Disney scooters are rated for 350 pounds. The smaller scooters go 250 to 300 pounds weight limit. If you are going up a lot of hills especially higher ones that uses more power and therefore uses the battery up faster. You're pretty safe at Disney, but there are places in Busch Gardens, Tampa where it's not recommended to go with a scooter or even a wheelchair.
Obviously, the longer you stay in the parks the more your battery charge gets used up. If we took a break back at the resort with our NON-Disney scooter rental we gave it a boost recharge before returning to the park. Also, pay attention to the battery usage gage. If you are getting low you will see it.
By all means, if you rent a non-Disney scooter keep the battery charger with you in the parks. You can get a boost in the parks with Disney outlets. They are hard to find sometimes, but in an emergency a CM should be able to help you in this regard. There's a maual override with the scooter that allows you to push it if you completely run out of a battery charge.
Also, the bigger the scooter the longer the charge lasts in most cases. Some scooters come with two batteries and those scooters can go even 25 miles with one charge. Other scooters have a heavy duty single battery that can make to about 10 miles per charge. A single lighter duty battery will go usually 6 to 8 miles on one, full charge.