Originally Posted by Inland RV Center, In
All modern tow vehicles, can easily charge the trailer batteries. Thousands of owners, do it everyday, without any problems.
However, there are some "must haves" in order to do so.
The charge line wiring within the TV, must be 10 gauge or larger.
The trailer/TV connectors must be in excellent condition.
If charging from 12 volts is inadequate, wow, we then have hundreds of thousand of RV owners that have dead batteries. NOT.
There is a general sentiment in the broader RVing community that the power from the TV cannot be relied upon to provide any meaningful amount of charging. Let's take a closer look. As you note, a typical tow vehicle has 10 gauge wire to the trailer, typically protected by a 30 to 40 amp fuse or circuit breaker, depending on the TV. Often this circuit also feeds the clearance lights on the trailer, through a relay.
Under optimal conditions -- clean, tight connections and relatively new batteries in the Airstream -- the math in my initial post tells us that we're going to max out somewhere below 30 amps, because at 30 amps the voltage at the batteries in the Airstream is going to be 12.7, and at 12.7 volts the batteries won't take 30 amps. What will happen, more typically, is that the batteries will charge at around 15 amps, and if there's a 12 volt
fridge or another electrical load in the trailer, the charging rate will be reduced by something like half the amount of the load. As the batteries in the Airstream get older and their internal resistance increases, this rate will drop. It will also drop as the batteries approach full charge.
At this rate, with two group 31 deep cycle batteries, it will take around 8 hours of towing to get from 40% to 90% charge. So yes, the batteries will charge, after a fashion. But they will not charge enough
that in a typical move from one boondocking site to another any substantial charging will take place, and they will not charge enough that it makes sense to idle the tow vehicle engine for a period of time just to charge the batteries. Hence, there are people who carry a generator, for no other reason than to charge the batteries.
Originally Posted by ROBERT CROSS
Hassle to you....convenience to me. [...] idling at the campsite, waste of time & gas. A modern genset is much quicker and more efficient.
Gensets have their advantages, among them the possibility of running air conditioning. They also have cost and weight associated with them and in many setups require their own fuel source and require that the generator be physically moved from a storage to operating location. While there exist well-integrated generator installations in Airstreams, that draw fuel from the propane tanks on the tongue and deliver power through a transfer switch, they are rare due to the various difficulties they pose. While it is true that there are efficiency tradeoffs, they are not as great as one might think, because modern pickup truck engines are more efficient at idle than the small gasoline engines in gensets. If we were willing to pay for a water-cooled genset engine with sequential multipoint fuel injection, oxygen sensor, computer, etc., the tradeoffs would be more considerable.
A well designed dual alternator setup on a tow vehicle could easily deliver 75 A to the batteries, at idle, more than any but the more sophisticated generator/converter combinations.
To go through the trouble of supplying 120 AC to the converter that's in the trailer, is to me, a waste of time trying, and a waste of money.
First off all, the TV alternator would "NOT" run at constant speed, therefore it cannot continuously provide 120 volts AC, at 60 cycles, which is required. Any frequency for the 120 volts AC much different than that, will more than likely, burn up the 12 volt DC converter.
Or, did I miss something in the original post?
Andy, I think we're in vehement agreement on that. ;-) That wasn't the suggestion in my initial post. I believe you're replying to a comment by barts.