DC charging systems
Direct Current (DC) That's what a battery provides - DC Power. DC chargers use battery power to recharge other batteries. Examples of this would be using a start battery to charge a house/service battery. Or more specifically the vehicle's alternator which charges the start battery, but then the DC charger amplifies the voltage to provide the correct voltage to the house battery which might also be a different type of battery to the start battery. For instance, a start battery that a wet cell can happily charge at 13.5 volts but a much larger house battery that's an AGM technology might require 14.7v for a much longer period than what an alternator will maintain a steady 13.5 volts for.
That is why we need DC charging systems or DC to DC chargers these days with modern vehicles because a vehicle's alternator doesn't operate as it used to 20 years ago and it's even more important with Euro 5 & Euro 6 vehicles.