Freewheeling diode is used to protect the semiconductor components used in the charger from the back emf, which is induced in the inductive coils of relays when the supply to the relays is cut ***. The magnetic field in the relays collapses and induces high voltage in reverse direction. This emf is shunted by the freewheeling diode, which is connected in reverse bios with the output.
DC filter is used to smoothen the output, which has ripple. Ripple frequency is same as system frequency for half wave rectifier and 2 times of system frequency for full wave rectifier. The filter, which is a capacitor, will oppose any change in voltage. Thus the ripple will not be allowed to come to zero.
