My confusion concerning DC/DC charger is that I believe if I got the factory 12V system from the trailer builder that they would have beein charging the battery from the 7-pin and no addtional Dc/DC charger. As such, is there a particular reason why some people have one and some dont?
North53: When a normal 12v trailer is put under load (e.g., running a fridge on '12v' electric for safety, while towing), it will typically pull its own Trailer Battery down to about 60% State-Of-Charge, before power pulled from the towing Vehicle (through the 7-pin "Bargman" connectors and cable begins to dominate the power consumption. And you don't really get much actual CHARGING through that cable, unless you reconfigure (to either a DC-->DC Boosting Battery charger, or a much larger cable).
What happens without a DC-->DC boosting battery charger, and without a separate large cable: Right at engine startup, your Tow Vehicle Alternator adjusts under-the-hood voltage (under the control of the "engine control module" computer which monitors and corrects for Voltage issues) quite high, up to about 14.2 Volts. That's to recharge the Starter Battery within the Tow Vehicle (hereafter "TV"). But shortly after the Starter Battery is "recovered" to being nearly full (as measured by ECM), the ECM will direct the Alternator to maintain only a much lower "Float Voltage" Voltage value under the hood - typically in the range of only 13.4 - 13.6 Volts. When such a low "charging Voltage" is applied, Lead-Acid batteries will only charge very slowly.
Three other things happen through the very thin "Trailer Battery Charge" wire within the Bargman cable. #1, because it is relatively thin, it has "Voltage Drop" power loss - which further reduces the amount of Voltage seen at the teardrop Trailer battery(s), when you are powering loads or trying to recharge the Trailer batteries. (They will accept even less input charging power, because the Voltage falls even lower than the TV "float Voltage" when current begins to flow at a significant rate of Amps.) #2, the TV's "Engine Control Module" doesn't see that Voltage drop - because it mostly occurs within the wire. And #3, because the batteries are not sucking in much power - the ECM again sees no excessive "accessory loads" pulling down the Voltage under the hood.
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With a DC-->DC boosting battery charger (normally installed within the Trailer where it can monitor the Trailer Battery "State-Of-Charge" a lot better), the DC-->DC Booster pulls HIGHER current along the Bargman, ignoring the "Voltage Drop" and raising the Voltage back up to a value which the Trailer batteries "want" to see. (Typically 14.2V, for lead-acid batteries which need considerably charging). Only when Voltage Drop falls much too low, typically around 11.2V, will the DC-->DC charger "give up" on sucking power in from the Bargman.
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With a much bigger add-in cable, you can also avoid the 3 problems of running "charge current" through the Bargman Cable. With the much larger cable, the Voltage at the Trailer Batteries and the TV Starter battery is kept nearly identical, with almost no Voltage Drop and with High Current capability. If the Trailer batteries are low, they begin to pull substantial balancing current from the TV Starter Battery and Alternator (through the big cable, with almost no Voltage Drop along the cable). The ECM directly sees low voltage in the engine compartment, and will boost Alternator output "to bring the Starter battery back up to 100%" - with BOTH batteries TV and Trailer, being charged together, by the higher Voltage, until they are BOTH near 100%.