If you add a small car battery to the moped's existing battery
pack to increase the Voltage by 12 Volts that might increase the
maximum speed of the moped by overvolting the motor, or it might burn
out the controller and or motor from overvolting them.
If you
decide to try this then a small car battery is not the right battery
type to use because it is a starter battery and not a deep cycle
battery. A starter battery would work for initial testing though, and
could then be replaced with a deep cycle battery later on.
Craig Jerke