I believe a solar charge controller is designed to take a higher voltage like 18-24 volts from the solar panel and step it down to the 12-14 volts for battery charging. A DC to DC charger has the ability to take a lower voltage such as the 12.5 volts many vehicles step down to when the vehicles battery is fully charged and step it up to a higher voltage for charging the aux or trailer battery (or battery bank).
-Curtiss
Yeah I thought maybe that was the case but on this example spec sheet for a PWM charge controller it has an input voltage of 12v and a boost voltage of 14.4v.
Specifications:
Nominal Voltage: 12/24V auto recognition
Rated Charge Current: 30A
Max. PV Input Power: 12V/450W, 24V/900W
Max. PV Input Voltage: 55 VDC
Self-Consumption: < 22mA/12V; < 16mA/24V
Equalization Voltage: Sealed: 14.6V; Flooded: 14.8V; Gel: N/A; Lithium LI: N/A
Float Voltage: Sealed: 13.8V; Flooded: 13.8V; Gel: 13.8V; Lithium LI: N/A
Boost Voltage: Sealed: 14.4V; Flooded: 14.6V; Gel: 14.2V ; Lithium LI: 14.4V
Operating Temperature: -13°F - 131°F
Temperature Compensation: -3mV/°C/2V
Dimensions: 6.54 x 4.66 x 2.1 inches
Weight: 1.55 lbs
Enclosure: IP30
Terminals: Up to 8 AWG
I believe the benefit of an MPPT controller, and I could be wrong, is that it can accept lower voltages and boost them to a higher voltage for charging at the expense of amperage. Of course after paying for an MPPT controller you might as well just get National Luna kit.