The standard charger that comes with the NST2018 is a 9 hour charger. It puts out a constant current to the battery and that is why the charger should not be left on for long periods. The constant charging will cause the battery to become warm after it reaches full charge.
The FSMVC charger (or fast charger) is a computerized fast charger that monitors the battery while it is in the charger and terminates the fast charging when the battery reaches a full charge. It then switches to a maintenance mode to keep the battery charged. The voltage is monitored and more charge will be applied if the voltage drops in the battery.