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Old 10-07-2015, 01:55 PM   #61
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Originally Posted by Carol H View Post
If I am not mistaken the battery should also be left for a number of hours disconnected as well before you get a accurate reading of the batteries state.
That's another, and slightly more accurate way, but may be overkill for an RV battery.

What I actually do after charging is, and while watching my VoltMinder, I turn on one load and watch the battery voltage drop for a few minutes. It will soon stabilize and I consider the surface charge to be burned off by then. Then I turn off the load and call the no load reading my battery charge level. It's a bit less accurate but, if you do it the same way every time the numbers will have useful value.

But Ya, the battery Guru's and engineering types like the prolonged rest method.



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Old 10-07-2015, 05:20 PM   #62
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Lot of reports about the fridge on 12 volt drawing down the battery even while driving down the road. Too little alternator output, drop in voltage from long run of thin wire back from tow vehicle engine are the common culprits.
My Dometic RM8551 4.3 CF refrigerator pulls 130 watts on battery, or 11 amps. My Jeep's alternator puts out 160 amps or 15 times the draw of the refrigerator. The resistance of the wiring to the trailer is the limiter for the amount of charging. But 11 amps isn't too much for use while towing. I ran 12 v refrigerators for years on several different tow vehicles and always seemed to have enough current to keep the refrigerator running OK. However, if I were to run the refrigerator on 12v for hours, such as through the night, it could discharge both my trailer and TV battery.

This year, I drove about 400 miles with the TV charging the battery in the trailer and powering the refrigerator. After arriving, I then dry camped for 4 nights without a problem. If you have one, I would first check the size of the wire through the TV and trailer from alternator to trailer battery. Size 10 wire should be sufficient to make it work.
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Old 10-07-2015, 07:35 PM   #63
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When one is trying to measure battery voltage you could be fooling yourself. I've seen all kinds numbers thrown around.
A bit about meters. If you meter 3 digit meter that would read something like 12.8 Volts the actual voltage could be quite a lot different. Because it's a digital meter the last digit, in our example the .8 could be anywhere between .7 to .9. That doesn't take into account the variation listed in the specifications which could as much as 10% of the reading. You would need to read and understand the meter's specifications to know what the accuracy is. That said a fully charged battery can vary quite a bit also. All you can hope for is something close to fully charged.
You typical motor vehicle electrical system is designed to be 13.8 Volts, as measured with very accurate meter.
Another example is the charge controller for my solar panel, it's set to stop charging at 13.9 for flooded cell batteries and gel cell batteries at 13.8.
One of the converters used by Scamp, American stops at 12.2 according to their sales engineer.
This is one of those thing one can obsess over, but to what gain?
If you run on battery while towing and get your desired boondocking camp site with enough battery power, what more do you want. If you don't have enough battery power and you worry about stuff in fridge getting too warm pack some ice in fridge.

Now enjoy camping instead obsessing so much.
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Old 10-07-2015, 09:58 PM   #64
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All the variations in voltage & ability to misread things there is why you should check out https://handybobsolar.wordpress.com/. He's a bit ranty & angry, but knows his stuff.

In line with Bob's sutff, I've got a Bogart Engineering | manufacturer of the TriMetric battery monitor which measures volts, amps and amp-hours for battery systems TriMetric TM-2030 & 100@ shunt (ends up running almost $200). With that I can tell excactly how many amps/watts are put into the battery & hence tell what percentage charge the battery is at all the time. No guessing or fiddling with voltage or float charge. Not necessarily the path for everybody, but I'm a nerd & like knowing exactly what's going on. I'll be getting a solar panel & Bogart's SC-2030 solar charge controller here at some point.

We tend to boondock & I rarely expect us to be anywhere there are hookups, so being self sufficient is more important to us. I'm not a fan or noisy generators & really the only thing we'd need them for is A/C, everything else can be easily taken care of by a battery in good condition & a solar panel with a good charge controller. Different people have different needs though.
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Old 10-08-2015, 10:13 AM   #65
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Car charging systems are a finicky bunch of parts. They have to see the need to charge the system based on the amount of draw. With the battery charged in the trailer hooked up to a fully charged car battery the alternator may not be seeing the extra charge needed. Check your cars voltage. Check your voltage in your trailer battery. Now hook up your car to the trailer and start your car let it run enough to charge the starters draw. The starters draw will excite the alternator to replace the electrons used. During this period of recharging the alternator should be putting high 13 volts. Once the electrons have been replaced the alternator should drop 1/2 to 1/4 of a volt.

Now turn on your 12 volt fridge. The alternator should see the need to replace electrons and step up the voltage again. you should see this step up with a voltage check at the tugs battery and the trailers battery. If you don't see a rise in charge voltage at the battery's then try revving the motor up to 2000 rpm and while at that speed the alternator should be putting out close to max power and the voltage should rise, if it isn't rising. Try turning on the headlights, that is a direct draw on the auto side of the system and should trigger the alternator to produce to maintain voltage and charge the battery.

Most cars have larger alternators these days and should produce the charge needed. Providing the wiring between the tug and the trailer batteries are capable of not only providing the current but also allowing the the cars charging system to see the demand. You might want to check your alternators specifications and see what amperage it is capable of producing. Remembering that a cars alternator only produces maximum amperage at full speed. Some don't produce much at idle speeds.
If your alternator is less than 100 amps then your going to have trouble charging a dead battery or replacing the current draw of a fridge while in tow. Especially once you get off highway speeds.
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Old 10-08-2015, 11:49 AM   #66
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There is more than the car's alternator output in the equation here.
The current for the refrigerator will be drawn from the source that is the lowest impedance and highest voltage AT THE SOURCE OF CONSUMPTION.
This means that with the long wire to the alternator/battery of the TV the resistance is higher than that to the trailer battery. Under load the current may still be drawn from the trailer battery instead of the car battery until it's voltage (and therefore internal resistance increased) drops lower than the loaded voltage from the car including the resistance of the wiring and especially the connector plug as well.
The TV electrical system should be easily capable of supplying the 11 - 12 amps required if it is actually being used by the reefer.
If the resistance of the wire and connectors (ground connections too!) were only .25 ohms at 10 amp draw the drop would be E=I X R = .25 X 10 = 2.5 volts.
As you can see it does not take much resistance to drop a lot of voltage.
Also when checking the ground resistance to the trailer is just as important as the + wire!
A very small voltage difference will cause the current to be drawn from the highest voltage source after all of the resistances are taken into account including the internal resistance of the batteries.
Big wire (for the + and - ) and good low resistance connections.
Often lots of effort is expended in tracing the positive leads and none in the negative (ground) leads and this is very often where the problems are. Corroded connections etc.
Load down the system and measure the voltage from the frame of the trailer to the - side of the battery with a digital meter. This will tell the tale.
Many trailer have the ball as the only ground connection between the frame (ground) and the TV.
The 13 pin euro connector has three ground connectors to help overcome this problem. The critical systems are individually ground returned to this for this as well.
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Old 10-08-2015, 12:56 PM   #67
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Folks, I'm an Electrical Engineer. Despite the best intentions, there is a lot of misinformation here. Anyone who thinks the charge line resistance can cause a voltage drop and stop the trailer battery from charging, does not understand the circuit. Please read my post #50. This is basic DC circuits. I will be happy to answer any questions but will not argue with folks. No offense intended. Take care, Raz
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Old 10-08-2015, 01:58 PM   #68
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I think you need to rethink the IR drop thing.
No argument intended, just basic DC theory.
It depends on the relative voltages from the sources, the total impeadances and of course the currents in both circuits, the load to the TV charge circuit and the trailer battery.
I have not looked at all of the posts, but in working with aircraft, industrial circuits etc. I have a small amount of knowledge of these things as well.
as far as I know E still equals IR and if current flows through a wire the voltage drop will still occur.
10 meters of #10 wire (33 ft') has a resistance of .0239 ohms X 10 amps =.238 volt drop.
10 meters of # 14 wire has a resistance of .0829 ohms 10 amps = .829 volt drop.

Does this small voltage drop matter? The answer is that it depends.

Mostly I was trying to point out that the total resistance including the ground return counts, not just the positive side. That and the resistance of any poor connections in the system.

Personally I have Norcold 12/120 swing compressor system and it draws ~3.8 amps on 12 volts (when it is actually running to cool) and it is not a problem to run, make ice and provide power to charge the battery. I also have a #10 wire from the battery to the rear plug and from the plug to the 12 volt system. A big healthy ground as well.

My goal is to have a nice healthy solar panel on the roof so that I won'r really care either way!
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Old 10-08-2015, 02:08 PM   #69
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I don't think that the issue is about just being able to charge the battery. I don't think that many are having that problem. It's about being able to charge the battery with the 10-11 amp load of the refrigerator running at the same time, and dealing with any voltage drops than may occur due to the resistance of the wiring/connections, between the source and the RV, dropping voltage.


As I am sure you know, the refrigerator will continue to draw current as the voltage drops, but the battery will stop charging as soon as the voltage drops below about 12.5 volts, and there is no difference in potential between the source and the battery. In short, the refrigerator wins in the electron lottery.


Besides wire size, I think that the #2 culprits for voltage drop in these RV's are the 7 blade connector with nice dirty and oxidized surfaces and inadequate grounds. As a precaution I often use the unused center pin as a redundant ground at both the TV and trailer side.


With a 10 amp load, any voltage drop will decrease the current flow towards the battery.


I have dealt with this problem both on trailers as well as in motorhomes, and the answer has always been in having adequate wire size and good connections.



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Old 10-08-2015, 02:53 PM   #70
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Dang Bob, I have to agree with you!
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Old 10-16-2015, 05:51 AM   #71
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[snip]
As a precaution I often use the unused center pin as a redundant ground at both the TV and trailer side.
Bob

Both my factory stock 2007 Silverado and 1999 Casita use that "unused center pin" for the backup lights

If I or another were to use it for ground I'm afraid the blue smoke would come out somewhere

Joe
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Old 10-16-2015, 06:20 AM   #72
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The backup lights on the center pis is pretty much the defacto standard now.
I wired my trailer and added backup light to aid in parking at night.
This is where the Euro standard 13 pin might address this problem with three separate grounds and the left side etc are grounded separately so that the likelihood of losing all lights and brakes (in N.A.) is lessened.
However the NA 7 pin plug/socket is a little more robust having looked at both side by side.
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Old 10-16-2015, 07:26 AM   #73
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Here's the actual pin-out on the plug of my 2010 Scamp. I agree. It appears that the center pin standard is pretty much the "backup" lights.
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Old 10-16-2015, 07:44 AM   #74
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Bob

Both my factory stock 2007 Silverado and 1999 Casita use that "unused center pin" for the backup lights

If I or another were to use it for ground I'm afraid the blue smoke would come out somewhere

Joe
I made the decision that having a good ground for lights, battery charging and (someday) brakes, was important 100% of the time, and the need for back-up lights was less than .1% of the time, and took the back-up light lead off of the plug. I then made that pin a redundant ground on the TV side and in the trailers harness.

My back-up light is the LED porch light on the back of my, rear entrance, Hunter, but anyone could simply add another porch light back there for the rare occasions it is needed.

And, if I plug another trailers 7 blade plug in that has back up lights connected, no problem, they just won't work. In the world of electrical common sense, it seems a little odd that the big pin in the middle is used for the smallest load and not for the ground, so I changed that and have two grounds.

And, if for some far fetched reason, my trailer should get connected to another TV, all it will do is blow the b/u light fuse in the TV, no smoke, no mirrors. And, I believe your 2007 Silverado has 4 separate fuses for the trailers lights, 1 for brakes and a 50 amp guy for the charging line, all courtesy of GM. (Small plug for GM's)


Forgot to mention, my Hunter didn't have back-up lights anyway so to moi, it was an unused pin.



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Old 10-16-2015, 09:08 AM   #75
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Here's the actual pin-out on the plug of my 2010 Scamp. I agree. It appears that the center pin standard is pretty much the "backup" lights.
The center pin on old Scamps such as my 1992 is actually the blue line. That was the standard that Scamp used for many years.

There are actually 3 Standard ways for 7 pin trailers to be wired.
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Old 10-16-2015, 09:20 AM   #76
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I don't think that there are any states or provinces that require back-up lights....


On mine the center pin was "Unused" because I didn't connect the TV's back-up light wire to it during installation and the trailer never had back-up lights.


I really don't see any real arguments against improving the ground circuit in this manner.



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Old 10-16-2015, 04:03 PM   #77
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There is one additional ground between the TV and Egg and that is the Ball and Hitch although sometimes that is intermittent.
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Old 10-16-2015, 04:48 PM   #78
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There is one additional ground between the TV and Egg and that is the Ball and Hitch although sometimes that is intermittent.
Very intermittent if your hitch is properly lubricated to prevent metal to metal contact, and a resistance connection if you are trying to charge a battery. A hapenstance ground path.



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Old 10-16-2015, 06:59 PM   #79
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Very intermittent if your hitch is properly lubricated to prevent metal to metal contact, and a resistance connection if you are trying to charge a battery. A hapenstance ground path.
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There is one additional ground between the TV and Egg and that is the Ball and Hitch although sometimes that is intermittent.
Yea... I though the guy in front of me was trying to send me a message in Morse Code using his trailer's tail lights, so I watched the dimly lit dots and dashes intently, and recalling my ham radio experience, was able to decipher the message he was sending...
It read, ""I have lost my ground wire connection to the trailer"
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