Sorry to resurrect an old thread, but I thought I'd add some info for other's use. The most pertinent to the question above is that on my truck, after turning off the truck ignition, power continues to be supplied to the 7-way plug for only about an hour. Then, power is shut off by the truck computer, presumably to prevent draining the truck batteries. (I am eager for any suggestions on how to modify this setting such that it will charge the trailer for a longer period of time. If I screw up and drain the starter battery too low, I do have jump starters available)
To re initiate the ~ hour of power supply, it is not as simple as turning the ignition to "on". Instead, I must actually start the engine and then shut down.
On my 2020 RAM 3500, here are some data: My truck has two fairly new AGMs in excellent condition, the trailer has 3 older flooded lead acid batteries that tested "decent" with a "toaster style" load test 2 months ago. All connections are clean and tight, and trailer battery cells are topped off with distilled water. Factory wiring on the truck and on the 5th wheel trailer harness. Prior to doing this test, the resting voltage at the truck batteries was 12.4 volts, and the trailer batteries were discharged too far at 11.8 volts (trailer batteries measured with a bit of draw). Without starting the truck, the trailer will receive about 3 amps through the 7 way plug in this situation.
With the truck running, as the alternators kick in, I measure 14.3 volts at the battery, and also at the 7-way plug when the trailer is not connected. After I plug in the trailer with discharged batteries, the voltage at the truck side of the 7-way plug instantaneously drops to about 13.4 volts. This 0.9 volt drop in voltage is apparently reasonable given the what I believe is 10 gauge wire along the truck chassis (and in the trailer wiring harness). With the truck running, it pushes about 10.1 amps to the batteries (instantaneous) in this situation- and then it declines to 7 or 8 amps after 5 minutes, apparently due to a "surface charge" measured at 12.3 volts, but then dropping lower to around 12.0 volts if I disconnnect any charging source and the voltage "settles."
I recognize I probably should replace the old trailer batteries, but will instead try to desulfate them after this trip and try to limp along with the setup. My boondocking duration is often only a few days between access to 110V to recharge the batteries fully with on-trailer charger. I do have an awesome EcoFlow power station that supplies my 110V when boondocking, and I could also use it to recharge the house batteries if needed. If I could instead get my truck 7-way to provide 3 amps for 4 to 8 hours, I would be self-sufficient even with the older batteries. And with 160 amp-hours of capacity under the hood, I'm confident I could afford to drain 20 amp hours without a concern in warm weather starting. Certainly, the "poor boy" solution is simply to turn the truck around and hook up some jumper cables for 15 minutes. But, I was instead hoping for a more elegant solution.
My main purpose to post was to inform others of the "time" at which the 7 way plug shuts off on my truck. It was driving me crazy until I learned that it automatically shut off. My question to the RAM braintrust is whether this is an adjustible parameter? If it is not adjustible, perhaps a DC-to-DC setup, or a direct auxilliary line with larger gauge wire is in my future. Suggestions or links appreciated!
