The Battery Protect 100A disconnects the battery from non essential loads before it is completely discharged (which would damage the battery) or before it has insufficient power left to crank the engine.
The Battery Protect is a uni-directional device. It can only deal with current in one direction, so either current to a load, or current from a charger, but not both currents at the same time. In addition to this, current only can only flow from the Battery terminal to the Load terminal. In case of use with a load, the battery connects to the Battery terminal, and in case of use with a charger, the charger connects to the Battery terminal.