DC-DC Charger (Battery to Battery Charger)
Units that raise and regulate the voltage taken from the vehicle's starter battery or alternator and deliver it to the service battery with the correct charge profile. They compensate for the voltage drop in the cable and, thanks to their current limit, provide controlled charging without overloading the alternator.
With a direct connection, the service battery is supplied from the alternator voltage minus the cable and relay losses. That voltage often fails to reach the absorption threshold and the battery is chronically undercharged. A DC-DC unit steps in and fixes its output: even with an input of 12.8 V it can produce 14.4 V at the output as the chemistry requires.
The units are typically offered in 20–60 A output classes and are selected according to the capacity of the service battery. Roughly, a charge current of the order of 10–20% of the capacity is considered reasonable for lead-acid; for a 200 Ah bank a device in the 20–40 A class is a good fit.
In an RV energy plan the second benefit of a DC-DC unit is chemistry independence: whatever the vehicle alternator does, your service bank charges to its own profile. This is an almost mandatory component in systems that move to LiFePO4. If you want to recover the levelling, refrigerator and inverter loads along the way, this is the element that sets the charging rate.
Context in which this term is used: RV automatic levelling guide