How do you charge the scooter on night watch?
you get the quick charger
You only have the scooter when you are chasing the security cart around the mall. You use the password (frost) to get it from the Manager's Office, and charge it with the charger device from MacGuffin's. After you chase down the cart, Eddie the snake has the scooter (and may still have it when you leave).
When you are chasing the robber in his stolen security cart, Gus suggests you get the scooter that is in the office. Use the password (frost) to get it. Go to MacGuffin's to get one of the quick chargers to charge the scooter's battery. Leave the office with the scooter, and go to the escalator on the far side of the atrium.
The security cart is still driving around the atrium while you get the charger from MacGuffin's but may not be there when you exit on the scooter. Try driving to the left side escalator, where you will eventually go to trap him. If he doesn't appear, go left into the mall, then come back and the chase may refresh.