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Old 08-31-2011, 05:21 PM   #1
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Did I Dodge a Bullet?

Last weekend we were camping at a nearby urban campground and the second evening I decided to get some extra time on our batteries by reconnecting the umbilical to our Suburban. We had been away during the day and we were into the campground's quiet hours so I didn't want to fire up our Yamaha generator.

Next morning I noticed the battery status on our panel was red ... oops! I'd forgotten to turn off the exterior light when we turned in for the night .... Sure enough the car wouldn't start either. All I got was solenoid chatter. So after 9:00 I fired up the generator with the umbilical still connected. I had forgotten to bring the 12 volt cables supplied with the Yamaha.

After about an hour, we heard the generator go into "eco mode," so I assumed the batteries were charged--the indicator was green--and shut off the gen set. I went to our Suburban and it started right up also. That was a week ago, everything seems to be working fine, no smoke from under the Suburban hood, don't see any melted wires, and all things electrical/electronic work just as before.

My question: Is it OK to leave the my Suburban connected to the umbilical when charging the house batteries on the Airstream, or was I lucky and simply dodged an expensive bullet this time?

Thanks for the help,

Paul
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Old 08-31-2011, 05:50 PM   #2
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There's no reason you can't. The same limiting factor to charging the trailer batteries while driving applies, though.
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Old 08-31-2011, 06:08 PM   #3
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No harm in it. I've done it that way for years. In most setups the tow vehicle battery and coach battery are simply connected in parallel through the trailer cable.

In some tow vehicles the trailer battery charge line is connected to the tow vehicle electrical system through a relay that only connects when the engine of the tow vehicle is running. In those the tow vehicle battery won't charge through the trailer cable, but on the other hand, it won't discharge, either!
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Old 08-31-2011, 06:34 PM   #4
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Probably the Suburban has a fuse/breaker to protect against grievous errors. Can't see what you did harmed anything. You simply charged your tow vehicle battery enough to start it.

OTOH I just heard from a friend who received a trailer with the two trailer batteries wired in series (24 volt) and "very bright lights and blown radio." Wonder what else he's in for....told him to get it back to 12 volt and check fridge, power converter and water heater. He said water pump worked VERY powerfully.

Moral of story, you haven't hurt any thing at all.
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Old 08-31-2011, 09:08 PM   #5
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Thumbs down Bad idea, in my opinion

This is a good reason for tow vehicles being wired to break the charge line to the trailer connector when the ignition switch is off.

In my opinion "stealing" power from the tow vehicle battery is a bad idea.
Two reasons pop into mind immediately;

1. Shortly after being plugged together, the TV batteries will be pulled down as both battery systems equalize.

2. You should never intentionally jeopardize your starting batteries, because you can't predict what emergencies might occur during the night that would require the use of the TV.

Just a thought.

Ken
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Old 08-31-2011, 09:45 PM   #6
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Originally Posted by w7ts View Post
This is a good reason for tow vehicles being wired to break the charge line to the trailer connector when the ignition switch is off.

In my opinion "stealing" power from the tow vehicle battery is a bad idea.
Two reasons pop into mind immediately;

1. Shortly after being plugged together, the TV batteries will be pulled down as both battery systems equalize.

2. You should never intentionally jeopardize your starting batteries, because you can't predict what emergencies might occur during the night that would require the use of the TV.

Just a thought.

Ken
But his question was if it would hurt to CHARGE the tow vehicle battery in this manner. In that case, it's hooked up to some kind of charging device, not discharging the tow vehicle's battery.
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Old 08-31-2011, 10:29 PM   #7
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Originally Posted by overlander63 View Post
But his question was if it would hurt to CHARGE the tow vehicle battery in this manner. In that case, it's hooked up to some kind of charging device, not discharging the tow vehicle's battery.
The fact that it wasn't his question, doesn't make what was done to run the batteries down in the first place good practice.

I considered his original question answered by the other responses.

Perhaps, I should have minded my own business and ignored what I feel is not a prudent thing to do. However that is not my style. It was not my intent to insult the OP, but rather to hopefully cause others to think about the ramifications of doing something that could potentially cause unforeseen problems.

It was also not my intention to offend you. I am sorry if you were.

Perhaps, I should have titled the post differently.

Ken
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Old 09-01-2011, 06:35 AM   #8
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Quote:
Originally Posted by w7ts View Post
The fact that it wasn't his question, doesn't make what was done to run the batteries down in the first place good practice.

I considered his original question answered by the other responses.

Perhaps, I should have minded my own business and ignored what I feel is not a prudent thing to do. However that is not my style. It was not my intent to insult the OP, but rather to hopefully cause others to think about the ramifications of doing something that could potentially cause unforeseen problems.

It was also not my intention to offend you. I am sorry if you were.

Perhaps, I should have titled the post differently.

Ken
Unplugging the tow vehicle when boondocking or not connected to shore power is a good idea. I thought you thought he thought that was what he asked.
And not that anybody asked, but modern auto and light truck alternators are not really designed to charge the battery(s), only keep them charged.

Also, I realized after the "edit time" was up, maybe what I posted could have been taken as a written wrist-slap. I decided not to make a correcting post, and hope for the best...
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Old 09-01-2011, 09:27 AM   #9
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Unplugging the tow vehicle when boondocking or not connected to shore power is a good idea. I thought you thought he thought that was what he asked.
And not that anybody asked, but modern auto and light truck alternators are not really designed to charge the battery(s), only keep them charged.

Also, I realized after the "edit time" was up, maybe what I posted could have been taken as a written wrist-slap. I decided not to make a correcting post, and hope for the best...
Terry,

Here is what I was thinking. I was left with the impression that the OP felt that dead TV battery was due mainly to the porch light left on.

It is my belief that the TV battery would have been very weak even without the porch light.

Since the trailer batteries were nearly dead, there was a significant difference in voltage between the TT and TV.

As soon as the TV and trailer batteries are connected, the difference in voltage between them is going to cause current (only limited by the resistance of the wires between them and the batteries' internal resistance) to flow. This current is going to continue until the voltage in both battery systems are equal.

Since it is likely that the travel trailer batteries have much more energy storing capacity than the TV battery, the whole system will come to equilibrium much nearer to discharged than to charged.

If two batteries in two different vehicles are connected together to jump start a car, the magnitude of the current is seen by the spark that occurs when they are connected. The current in the TV-TT system will be much less due to the diameter of the wires, but still significant.

So I believe that had the porch light and everything else in the trailer been off, the TV battery would still have been near dead in the morning.

My whole point in posting was to cause readers to realize that connecting the two systems together when the trailer batteries are low, does not simply allow you to tap into the TV battery for a few amps to power the trailer. It also significantly discharges the TV battery because it, in effect, is expending energy trying to charge the trailer batteries.

I hope this is clearer. I am open to learning about any faults in my reasoning.

I should have amde my intent and explanation clearer in the original post.

But then it would have been this long.
Ken
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Old 09-01-2011, 10:13 AM   #10
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Fellow Airstreamers,

Thank you all for your comments and help. Simply put, I think the consensus of this thread is that while probably not a fatal mistake, it was not a smart thing to do. Short term, I'll simply use my gen set more often to keep the house batteries fully charged. Long term, I'll look for ways to increase amp-hour battery capacity.
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Old 09-01-2011, 10:21 AM   #11
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Fellow Airstreamers,

Thank you all for your comments and help. Simply put, I think the consensus of this thread is that while probably not a fatal mistake, it was not a smart thing to do. Short term, I'll simply use my gen set more often to keep the house batteries fully charged. Long term, I'll look for ways to increase amp-hour battery capacity.
In good weather, even a small solar system can go a long way to keep the batteries topped off during the day. Now the problem, try to find a decent price on them.

I have a couple on the roof of the trailer, but would really like to get a portable setup. so I can locate them where they would do the most good.

It makes more sense to have the trailer in shade and the panels in the sun.



Ken
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