The automatic charging relays sense the voltage on the starting battery and when it is charged up (above 13.7VDC for example) connects the house battery and charges both. Shut the engine off, the start battery voltage falls towards its resting voltage, the relay opens at about 13.2VDC and the house battery can run down and not prevent you from starting your truck. Same goal as an isolator, but relays have little voltage drop and can run higher current for the dollar. Run a large size wire to minimize the voltage drop and relays are a good solution for recharging house batteries running down the road.
The DC-DC or AC-DC chargers can do more for you with the multiple voltage/current levels they can provide, or pulse waveforms. They boost the voltage when needed above the DC supply voltage which might be from a DC power supply at 13.2VDC fixed output, or a small size long wire from your truck system.
So I think the DC-DC solutions are of limited value when your supply voltage is very close to the alternator output voltage, for example in high current capable low voltage drop wire system. The alternator wil lprobably get you well into the 14VDC range. If you are getting your power off a #14GA battery charge wire at your bumper 7-circuit RV connector, then you have lots of voltage drop and low current. The boost charger will be very helpful to more completely, if not somewhat slowly, charge your house battery.
There are DC-DC boost units rated at 20-40amps for ham radio use that coudl be applied here. They are used to boost the battery voltage as it drops slowly under load. Radios do nto like low voltage. A battery may start out 12.6VDC and drop to 10.6VDC at full discharge. The booster keeps a regulated 13.8VDC. This allows the radio to run more efficiently and run longer as they may cease working at 11Volts or so, and draw more current as the voltage falls. They also likely have a low voltage warning and disconnect. MFJ is one manufacturer.
Another solution I use for my home radio installation (and on my motorcycle radio install at times) is a 12VDC power supply set to 14.7VDC connected to a West Mountain Radio Super PowerGate40S isolator and charger unit. It does not boost, but most AC to DC fixed output power supplies are adjustable. You set the voltage a bit higher to compensate for the diode isolator voltage drop (about .3-.6VDC depending on current) and also to give the builtin 4-stage charger the higher voltasge it needs at times.
The key to selection of the right solution is what your 12V source will be. Is it fixed output from an AC to DC power supply? Adjustable output voltage? The truck system? What voltage drop you will get.
Some alternate ideas for you.
- Mike