An additional note for
@Trouble. If rapid charging isn't necessary but you want to make sure that the batteries are able to receive a maximum charge when connected to the truck, then the DC-DC converter can be effective. This device should be installed on the trailer and programmed for your specific battery chemistry.
In my post above I alluded to some "very technical" reasons for my preference for a DC-AC inverter; what follows is a description of those technical issues.
Some DC-DC chargers will attempt to maintain a constant output wattage; for example, a unit labeled "30A" may actually be programmed to maintain a constant 360 watt output (30A * 12V == 360W). This is where the problems start. While the output of the DC-DC charger may in fact be, say, 14.4V and 25A, the voltage on the input side will be much lower because of the voltage drop between the truck's charging system and the DC-DC charger. For example, if the alternator is putting out 14.4V and there's 50 feet of 10 AWG wire between the alternator and the DC-DC charger (in terms of resistance, this is probably about right due to the truck's wiring plus the wire between the trailer's power plug and the batteries, and also remember there is added resistance from all of the terminals and the plug/receptacle connection), you'd be down to 11.9V on the input side of the charger given 25A output.
![1083609]()
So while the alternator is putting out 360W, we're already down to 11.9A * 25A == 298W at the input of the charger (and this doesn't even account for losses within the charger itself!). If the charger is programmed to maintain a constant 360W output, it will compensate by trying to draw more current, but the truck's supply voltage can't increase. If we jump up to 30A the result looks like this:
Now we are at 11.4V * 30A == 342W, which is still less than the 360W the charger is programmed to supply. It will compensate by increasing the current demand even further, and since we're already at 30A, what happens next is it blows the fuse on the truck side (12V AUX is generally fused at 30A to protect the 10AWG wire).
I used a discrete jump from 25A to 30A for illustration purposes; of course the charger is probably incrementing more gradually but the point still stands. For you nerds, your homework is to formulate and solve the first-order differential equation whose solution as time -> infinity gives the steady-state DC-DC charger input voltage for this scenario. If you divide 360W by that number you get the amperage required to sustain 360W on the output side; I suspect the answer is in the ~40A range. Put simply, it's not going to happen given 10 AWG wire and a 30A fuse.
While solving ODEs may seem too theoretical, the result itself is not. The very expensive
Victron Orion DC-DC chargers behave in exactly this fashion. I learned this the hard way when I installed an Orion 12/12-30 charger in my fifth wheel camper; there is no way to program an input current limit. I wound up returning it and using a 12/12-18 charger instead.
Just keep this in mind if you decide to use a DC-DC charger.
