DC 2 DC Chargers what are they?

Just like a normal mains powered charger which is AC input but a DC charger is DC input voltage.

Some of the considerations are when the charger should turn on, so ignition inputs or input charge voltages in excess of OCV are used to initiate the charging process. Additional programming or hardware design has been made in some cases for regenerative braking systems and the fast surge input power generated. In some cases, the input voltages on a start battery with regen braking are in excess of 17 volts DC, which is very high on a 12v system.

Some manufacturers include multiple charge profiles or relay control systems or low voltage disconnect features to isolate discharging equipment.