Can I run the refrig on AC from an inverter in the tow vehicle?

[Bill]

Jim -

Originally Posted by Freedom

I had never heard of the shunt. If I understand you correctly, you're basically measuring the voltage drop from one side of the resistor to the other with your small wires going to each side of the resistor.

You got it exactly right!

I did like Wayne's idea of a switch to use one battery and then the other - I just figured I would move the wires from one battery to the other. It would be pretty handy to have a switch to do that!

Wayne's solution is really nice.

I wish I had bought two six volt batteries like a lot of you guys since I now understand that they are a lot better as far as deep cycling. I just bought the two batteries on the TM before I found this site. They’re Interstate’s best group 24 “deep cycle” or so they say. When they wear out, I'll figure on putting in a couple of golf cart batteries, and maybe solar panels so we can camp on the beach in Mexico with my brother!

Don't worry, you 'll have a chance to buy new batteries sooner than you really want! Say hi to your brother from all of us.
And post pictures!

Bill

Bill.

I love the idea of the shunt. I had read the web page about the digital display for both voltage and current and thought it was a rather "in your face" display. However, after further consideration it seems to me that it would be quite feasible to construct a small display box, showing both voltage and current, and that this box could be plugged into anywhere that a suitable plug had been installed. I'm thinking one on the tongue, next to the batteries on my 2720, and another near the other equipment inside. Kills two birds with one stone. It would be awkward to have an ammeter in both the sink area and also near the battery. However, by using the shunt, I could easily run three small wires (two plus ground) to any number of places and then plug the display into the plug.

BTW, I ran the suggested voltage test and have concluded that with every 12-volt appliance in the trailer running, my TV can still put out enough energy to charge the TM battery. This is based on voltage across the battery. I'd feel better if I had an ammeter on it, but I feel pretty good about it for the moment.

While pulling my 2720 out of the garage this morning, my neighbor observed that the closest I came to dragging bottom wasn’t the rear bumber, but the battery tray. That pretty much eliminates my ability to add T105 batteries in the future. The T105’s look like they are too tall to fit in the current tray, which is pretty far back because of the swing-away hitch. I thought I would be able to lower the battery tray, but not if it is already close to dragging on the ground. I added a second group 24 battery today, since I already had one. I will likely replace them both with group 27’s some day, when they die.

I had been planning on installing two group 24 batteries into a single long battery box. However, while talking to my TV mechanic, he suggested that the entire point of a battery box was to protect against battery failures. Two singles would be better than one double. So I now have two battery boxes, each with an SRM-24 battery in it.

The real test of the double SRM-24 batteries will be this coming Wednesday through Sunday. This will be our first trip out. If the batteries don’t last the entire time, I now have a Honda EU200I generator to recharge them.

I still plan on adding a battery switch, but I am running out of time before my first trip. Maybe next month.

[Caver]

It's very useful to know the battery charge/discharge current as well as voltage. At the same time it's difficult to determine how many Amp-Hours (AH) remain since the capacity is a function of type of battery, age, voltage, temperature, and current. Having a small generator to use is certainly a good option if you are going to boondock and be in places where you can run the generator to charge the battery and run some appliances.

I use the Bogart Trimetric meter. It keeps track of the net charge/discharge AHs. It's sort of like the fuel gauge on your car but instead monitors the net battery charge state. These use a shunt located near the battery. Unless you put a meter right at the battery I would recommend using a shunt to measure current. Search on the word Trimetric and you'll see some of my earlier posts and photos of these meters. I purchased mine on e-bay. New prices are about $135 and $20 for a 100 amp shunt.

You suggestion that you could put in multiple connection points would work for measuring voltage and current but to keep track of battery capacity the meter needs to be connected continuously.

You also want to avoid discharging your batteries beyond 40 to 50 percent of their capacity to maximize lifetime so it's helpful to know how you're doing so you can adjust your usage or run the generator in your case.

Let us know how you make out. Ray

Now I'm confused! Doesn't take much for that to happen! Didn't Bill say that the shunt is a resistor? Don't resistors by their very name mean that they take power out of the battery - or make it more difficult for the power to flow? Now "Caver" is suggesting a 100 amp shunt. Won't that in itself cause a battery draw and we're trying to "save" as much power as we can, aren't we? Sounds to me like maybe I'll follow Bill's first advice to check it all out with a digital voltmeter which I already have and if the TV is charging the battery while running the refer maybe leave it at that. The portable generator sounds like the best solution other than solar panels! Confused in Washington.

[Caver]

Jim,

Almost all ammeters us a shunt which is a very small value of resistance so it takes very little power. The voltage across the resistance can then be read by a meter. The difference is that the external shunt or resistor allows the meter to be remoted. If you ran wire the same distance to the ammeter there would be far greater power loss. The shunt is also a precision value of resistance.

I hope that helps a little. Unfortunately if you are going to camp with only battery power you are going to learn a lot more about volts, amps, and resistance. Ray

[Bill]

Jim - Mostly its the terminology that is confusing. The term "shunt" suggests that something is put across something else, or provides an alternative path for something. Originally, a shunt resistor was electrically connected in parallel with (or "across") an ammeter, to shunt some of the current around the ammeter. Thus the ammeter was made less sensitive. For example, an ammeter with a 1-amp scale (sensitivity) could be used in a 100-amp circuit if 99% of the current was shunted around the ammeter. Of course, as Ray says, the value of the shunt had to be quite precise to ensure that exactly 99% (and not 98%, for example) of the current was diverted.

In the case I just described, it is the shunted ammeter that tells you how much current was flowing in the circuit. But when current flows through the shunt resistor, it also happens that a voltage is developed across that resistor. That voltage is directly proportional to the amount of current flowing, and if we have a sensitive voltmeter, we can read that voltage and calculate how much current is flowing. Practically no current flows through the voltmeter, by the way, it all flows through the shunt. As a result, we can remote the voltmeter, locating it anywhere we want.

You commented that a resistor consumes power, and you are correct. Since we don't want to waste power, we make the resistance of the shunt resistor very small. But then, the voltage across it is very small, and we must have a very sensitive voltmeter in order to read it. Fortunately, sensitive voltmeters are available today for only a few dollars - that wasn't true 20 years ago.

The term "shunt" has persisted, even though it is not used in the RV world to shunt current around an ammeter. Today we put the shunt resistor in series with the battery lead, and ALL of the battery current passes through it - not just 99%. Then we measure the voltage across it with a sensitive voltmeter, and infer the amount of current that is flowing.

In my example, I used a 10 milliohm (0.01 ohm) shunt resistor. Why? In my TM, 20 amps is about the max amount of current that ever flows in or out of the battery. When 20 amps flows through the shunt resistor, it creates a voltage of 200 millivolts (200 mV, or 0.2 volts). Then I use an inexpensive 200 millivolt meter to read the voltage.

Caver uses what's called a "100 amp shunt". Though this term is common, it isn't fully descriptive. What it means is "when 100 amps flows through this shunt, it generates 100 mV of voltage." When 20 amps flows, it generates 20 mV. This is too small for my cheap $8 meter to read accurately, but Caver has a much nicer (and more expensive) system than mine.

The resistance of a "100-amp shunt" resistor is (100 millivolts / 100 amps) = 1 milliohm - a VERY tiny value. Thus his shunt consumes practically no power at all. Incidentally, 1 milliohm is the resistance of about 1 foot of #10 copper wire. As Caver points out, if you remoted the ammeter itself, you would need a lot more than 1 foot of wire, and hence would waste more power than his shunt does. You could make your own shunt out of a foot of #10 wire - but again as Caver points out, you need the resulting resistance to be fairly precise, and that is hard to do.

Hope this helps.

Bill

OK, I think I understand. Basically I could put the shunt in the line and then use my Fluke DVOM to measure the millivolts whenever I want to and not necessarily leave it hooked up all the time. It is one of those auto ranging things, so I would probably have to keep the conversion from millivolts to amps in mind when reading it. Thanks again!

[Bill]

You've got it. And Wayne's idea of using small wires to bring the measuring point to some readily accessible place is a great one.

If you choose the value of your shunt properly, the conversion from millivolts to amps is trivial. I use a 10 milliohm shunt (10 millivolts per amp), so that a reading of 200 millivolts amps implies 20 amps. Just drop a decimal place. Caver/Ray uses a 1 milliohm shunt (1 millivolt per amp) so that 20 millivolts implies 20 amps. Of course, if you use an odd value of shunt, the conversion is a bit more complex - but not much.

Perhaps the most important thing is to keep the value of the shunt as small as possible, consistent with an accurate reading on the lowest scale of your DVM/DVOM.

Bill

Gentlemen/Ladies
I just bought my TM. It is a new (unsold) 2004. I have towed it about 2400 miles on two trips. I had the refrigerator on 12 volts and the fan on and so far have had no trouble with the tow battery. Can I expect it later on?
Thanks
Bill

[Bill]

Bill -

Only a few TM'ers seem to experience the dreaded problem - "TM battery discharges while towing if you run the refrig on 12 VDC". Let me assure you, the problem is not in the TM, it is in the tow vehicle. The TM is properly wired in all respects.

I am one of the afflicted ones. I plan to run a few tests this summer and find a good simple solution that will be useful for most of us who experience the problem.

In the meantime, to answer your question. If you have not experienced the problem yet, then I am quite certain that you will not experience it later. Like most diseases, either you've got it or you don't. If you don't, you are fine.

Bill (another one)