Mystery Volts

Good afternoon again.

After reading the original post it looks there are 2 issues here
.
1 - R/V Batteries not charging from the Trucks alternator

2 - Voltage Meter countdown in the R/V when no battery present in the R/V and the electrical pig tail is disconnected from the truck to the trailer

So take the easiest one first - #2

I fully agree that somewhere in the R/V there are capacitors in a device that is causing your volt meter to "countdown" Its the only answer as all other forms of 12v power source have been removed.
Also there is no way this device or devices with capacitors inside them would have any effect on the charging circuit from your Truck to the trailer. Now as soon as 12 vdc is applied, you have in effect "re-charged" the capacitors in what ever device they are in. Hence another count down begins

Hidden power draw in R/vs

Every R/V is going to have some hidden draw when the R/V is not in use. Your electronic boards/hardwire propane/carbon-monoxide detectors and even some radios will draw current (Amps)when not in use. The accepted rule of thumb is 3/4 to 1 amp - per hour.

So lets tackle the issue of the truck not charging the R/V batteries.
You need to start at the source and that would be the truck rear pigtail. You will need to look in your Trucks manual or sometime the cover of that pigtail there is a diagram of the wiring. You need to confirm that there is power present constantly - while the truck is running. If not - I would suggest you trailer tow fuse is blown in the truck fuse block or in rare instances the trailer/tow relay that opens and closes to protect your Truck battery's from discharging is bad.
So for the sake of argument - you have 12dc present at the bargman plug (7 pin connector) in the rear of your truck.

You now need to check if you have 12dc power coming into the R/V itself. I would do this with/out the R/V battery's in place, so that you do not get a false reading from the R/V battery's. You should be reading above 14vdc. I recently came across a situation with my friends Truck Camper battery stop receiving a charge and after an hour of trouble shooting, found both side of the charge wire(Truck side & R/V side)were fine - BUT the copper terminal inside the plugs was crappy looking.A dab of die-electric grease (from NAPA) and working the male plugs in and out several times, voila - charging was restored.

Mikeeeeeeeeeeeeeeeee
P/S - My vote is going to be a bad grounding wire someplace!!!!

If there is a radio installed or something similar, you could see voltage coming from the capacitors that are inside. The countdown is the capacitor that is slowly discharging.

No- he had the batteries out, umbilical pulled and was seeing measurable voltage with no 'power' source on camper side.

Hello Everyone

Hi Clearsky

There is really no mystery here. What you are getting on your multi meter is a "Surface Charge Reading" in volts from the battery. Remember you have a 12 volt battery - not a 14.5 volt battery.
The decrease you are seeing in your volt meter is the dissipation of that surface charge. Come back in an hour and your norminal battery charge should be around 12.5 volts.
Todays modern Alternators have a voltage output of 14.5 volts - plus/minus a bit.

Mike Tassinari

Interesting, though never had a reason to look before. Metered batteries@14.6 (solar cycle), turned off solar, ran fan to get down to about 13.3. Let set then checked at master disconnect. Turn off batteries, meter at camper side of disconnect, yes shows voltage but drop almost immediately to 4-3 volts then slows. Counting down same as OP. Turn back on then turn off noting second hand on clock-as volts drop so does speed, at minuet 3 it was down 0.20 volts @ 4min 0.10v at 5 min was at 0.05 volts-got tried of holding leads. Only thing that was on was lp detector-possibly radio standby. I would have assumed longer bleed down.

Yes battery draining due to parasitic load, and/or not receiving adequate charge - related in the fact weak batteries but different unrelated issues. Is truck only source of charge outside of shore power? You mentioned wire to alt- if on the small side you will see full voltage but its the amps you need and thats thru largest clean wire you can install. Don't forget you need good equal size ground wire.

I took the time to check our parasitic loads-just cause.
Turned batteries back on, meter @10 amp across disconnect terminals, turn off batteries so load now thru meter-drawing 0.05 amps, turn on light meter goes to 0.23 (8" florescent). Surprised me-parasitic loads only at 0.05a or 50 milliamp. Was always curious & assumed more.

Thank you all so much for your answers. I will look into all your suggestions, but I think it is leading me back to not great power coming from the truck. I went and followed the line from the alternator in the truck and found different sized wire and a couple of crimp connections. Going to measure for parasitic draw, as well, but big wire just makes sense. Thanks you all again!

Though Ive never checked parasitic loads on the camper I imagine higher than truck.
A 3amp draw is a lot, would pull batteries down pretty quick, days , but I don't believe what your seeing is parasitic load. What your measuring "mystery volts" is residual power in system if battery(s) disconnected. Wont help you identify, well much of anything.
Mysterious maybe but why you don't start touching stuff on any electrical thats just been unplugged-you can still get shocked. Until that 'stored' power bleeds off. Though you may indeed have power 'leaking' thru converter- tested as I understand, isn't verifying that.

Curious Im going to go meter our camper-turn off batts & solar see how long it takes to bleed system down and numbers I might see. I'll also check that turning light on will bleed it off. Reconnect power see what the draw is to reinitialize system, (though our converter is newer I assume it will still 'power up'), and then turn off solar and check parasitic load between camper and batteries just to see.

I'm not assuming anything here but to measure parasitic draw of camper, meter between source and load, batteries need to be connected, unless your meter can provide that? With everything turned off pull one cable, I use neg. though on our camper I think i can check at master disconnect-hmm, meter (@ max amps)between battery cable & battery post.

The rectifier in the charger is pointed towards the 12V side, so the capacitors discharge due to leakage of the rectifier diodes or other parasitic loads. This is pretty normal and I doubt it has anything to do with the batteries not getting charged from the truck.

You need to check the voltage at the battery terminals with the truck running when the batteries are discharged. When they are fully charged, they will only draw a couple of amps. Spiral AGMs will draw a lot of amps if partially or fully discharged and the terminals are held at 14.5. My guess is that the charge circuit from the truck has a lot of resistance, as many do. When the batteries are discharged, the current goes up, voltage goes way down, and charge rate slows. Most stock truck and camper wiring harnesses have maybe a #10 wire and several light weight connectors between alternator and battery. Many of us have installed a dedicated charge circuit of #6 or even #4 wire, with appropriately rated connectors, to get good charging of the house batteries.

You could try turning everything on in the camper to simulate a low battery, but you then have the camper battery contributing (unless it is disconnected), and that does not check the final few connections to the house battery.

Clearsky,

The wire from the truck 12v circuit is tied to the output of the converter/charger which has capacitors on the output. They act as power filters, smoothing out the DC voltage to a consistant value. They charge when power is applied and if your 12V circuit has a high resistance to ground, will slowly discharge.

I would think that it would discharge fairly quickly, back through the rectifier circuit in the charger.

Does everything work with it plugged into 120v AC power? It sounds like the charger has an open rectifier.

You said " I measured the power coming off of the truck and I am getting about 14.5V at the battery terminals with the truck running. So it seems that I have good batteries and good power coming from the truck."

If this was measured with no batteries or other load connected,it does not necessarily mean you have a good connection. With no current flow through the wires any voltage drop due to excessive resistance or bad connection would not be evident. You should remeasure with some electrical load on the connection.