House battery grounding question

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rgnprof

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My son has been adding a Li house battery and a 400 watt solar system to his old (1968) Airstream. Most of the install is done but have a question - the old house battery was not directly connected to a ground - the negative battery terminal had wires extending to the converter and to the 12v system. He has those wires reconnected and everything seems to be working properly - all the 12v stuff is running etc.

Question is does the new Li house battery need to be separately grounded to the frame? I am doubting it - assuming that the system is currently grounded since it is working. But just wanting to be safe...and sure?? The other reason for my question is that my house battery in my Class C - which is under the hood - is directly grounded to the frame with a pretty heavy gauge wire.

Thanks, ryan
 

A Traveler

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Where does the positive lead from the battery go? Is there a chance that this thing is a positive ground setup?
 

DonTom

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Question is does the new Li house battery need to be separately grounded to the frame?
No need to change anything to deal with the ground just because you're changing to lith.

In most cases, a ground is only to reduce the number of wires necessary. Have a single wire for one side of all circuits instead of two wires to every circuit. It would be very unusual for the builders of the RV to NOT use a ground. I have never heard of such, but it certainly is possible and could be done. But I seriously doubt that is such in your case.

Positive ground is also very unlikely, but still possible.

I will assume your negative battery lead is grounded somewhere. Or else you will have a lot of wires going to your negative post. I assume your negative cable is going to the frame somewhere.

-Don- Auburn, CA
 

Kirk

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Is there a chance that this thing is a positive ground setup?
No.
Question is does the new Li house battery need to be separately grounded to the frame? I am doubting it - assuming that the system is currently grounded since it is working.
I'm not sure what you mean by separately grounded but there must be an electrical connection between the negative post of the battery and ground but it could be shared by sharing the connection with the converter or something else as long as it is a large wire. Electrically everything that is connected by the same cable is a common point. The reason that your class C battery has a cable to connect it to ground where it is located is mostly because of the ease of doing that when compared to connecting it to the converter that is then connected to chassis ground. If your son has the new battery connected the same way that the previous one was connected, it is fine. If there were no connection between the negative post of the battery and a ground connection somewhere, then the battery could never supply power to anything.
 

rgnprof

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I love this place! Thank you for the confirmation and thanks from my son.
 

Ex-Calif

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In a way the ground wire is like playing tag when you were a kid.

Let's say the swing set is "safe" - Doesn't matter what part of the swing set you are touching it's all safe.

If the trailer frame is used as a ground, it's the same thing. Connect to the frame anywhere and everything is not only grounded but connected "to" each other via the frame.

There are reasons not to use a vehicles metal body as a ground circuit, primarily when the construction of the vehicle is predominantly a non-conductor like plastic or fiberglass. i.e. its super rare to see boats using anything but a ground bus where the wires run back to a terminal strip that is then connected to negative battery terminal.
 

Mark_K5LXP

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Grounding or in this context, bonding to chassis serves a purpose beyond just having a convenient conductor back to the power source. It minimizes the issue of connected devices passing current between themselves due to voltage gradients through inadvertent common connections. In a system where there is no ground/chassis reference then every single thing being powered must be isolated or these currents can occur, and could cause unintended operation at times. Ensuring isolation is not as easy as it seems, many 12V devices in vehicles and RV's will have it's housing or bracket one of the power conductors, and if it's mounted to a conductive cabinet or panel along with some other device then there's a current path there. Might sound like picking nits but I've troubleshot my share of chassis-ground path problems in vehicles and I wouldn't specifically invite that issue if it wasn't necessary for some other reason. So my vote would be to put the ground in at the battery and whether any particular device connects to battery negative through a wire or chassis then becomes an installation choice, and it may obviate potential problems going forward.

Mark B.
Albuquerque, NM
 

Ex-Calif

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In a system where there is no ground/chassis reference then every single thing being powered must be isolated or these currents can occur, and could cause unintended operation at times.

How true - On the MG when things get corroded it is absolutely possible to get reverse current through a ground loop. Especially in the "first generation" of modern vehicles when they were trying to build integrated circuit type controls. The first seat belt module is a bit a a rube goldberg machine and 50 years on they are all basically failed - LOL...


Might sound like picking nits but I've troubleshot my share of chassis-ground path problems in vehicles and I wouldn't specifically invite that issue if it wasn't necessary for some other reason. So my vote would be to put the ground in at the battery and whether any particular device connects to battery negative through a wire or chassis then becomes an installation choice, and it may obviate potential problems going forward.

Mark B.
Albuquerque, NM
Given a choice I agree that a ground bus is the easiest to maintain.
 

Ray-IN

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The elephant in the room,IMO, regardless of where the battery is finding ground/earth, is the quality and capability of that ground. It must be capable of safely carrying the entire battery load fuse amperage. Otherwise the risk of fire is present.
 

rgnprof

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The elephant in the room,IMO, regardless of where the battery is finding ground/earth, is the quality and capability of that ground. It must be capable of safely carrying the entire battery load fuse amperage. Otherwise the risk of fire is present.

Ok - this has me somewhat concerned. If the system was working before and he has replaced the old deep cycle with a Li, is there now an increased concern about the "quality and capability of that ground"?
 

DonTom

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If the system was working before and he has replaced the old deep cycle with a Li, is there now an increased concern about the "quality and capability of that ground"?
You're very unlikely to have such an issue with grounds, especially if there was no such issue with your L-A batteries. A vehicle ground can normally handle a lot of current, usually a lot more than all your circuits combined.

While that Lith battery is almost a volt more, the current draw of your 12V stuff will not change much because of it.

You will mainly notice better and longer performance with the lith battery. The voltage stays higher longer, doesn't drop as fast as a L-A battery--but that can also mean less warning when you are down to the last 1 or 2 percent of your SOC.

-Don- Auburn, CA
 

imnprsd

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If this was a 48V LiFeP04 battery, how would you ground it?

...I would think 12/24/48V it's all the same. I.e., wouldn't you just run the positive and negative battery cables direct to the inverter and leave it connected that way.

As for 120Vac, it's my understanding that current wants to return to it's source or if given the change it will take the least path of resistance.

So now that we are taking about AC current, we have to ask, how is the RV bonded to ground?

It's my understanding that when you are on generator power, the ATS switches and bonds the neutral with the ground so that both the neutral return current, and the ground wires connected together... and thus return current and current-ground-shorts (if any) currents are returned to the stator/rotor (it's source). This way you don't get the so-called "hot skin" problem since the AC 120V ground lead is NOT connected to the chassis which is connected to your aluminum walls here and there. So any short to an AC ground, with the generator running, is isolated from the chassis ground.

And when you are plugged into shore power, (white) neutral wire, current is return to the transformer from which it came. And the AC-ground wire is connected to the shore power post ground wire. This all is handled by the brilliant yet simple 8-pin relays inside your ATS.

FYI, the shore power ground utilizes some copper rod stuck 6+ feet in the ground somewhere, and if if a short occurs, current will take the least path of resistance and dissipate into the soil (aka ground). (I think this is all correct, but if not let me know.)

This is why I typically unhook my AC shore power during freak lightning storms -- so that a nearby lightning strike to the ground does not deliver 10's of thousands of Volts (mega-joules of energy) back-feeding current though that grounding rod and into my RV -- which could blowing up all/some of my AC devices.

Consequently, if I need power during a heavy lightning storm, I unplug my shore power and run my generator. (Note: The rubber tires offer good insulation from ground-spikes!)
 
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Ray-IN

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Ok - this has me somewhat concerned. If the system was working before and he has replaced the old deep cycle with a Li, is there now an increased concern about the "quality and capability of that ground"?
I was referring to an unknown ground source, which is what I understood from your initial post.
 

JayArr

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He has those wires reconnected and everything seems to be working properly - all the 12v stuff is running etc.

There is no definitive need to have the battery negative connected to the frame of the trailer.

There are a number of reasons to use the frame as ground but the biggest one for the manufacturer is cost savings, when you use a frame ground you only need half as much wire.

If Airstream went to the trouble of running return wires from every 12V device back to the fuse panel then why would you want to undo that?

I wonder if there is some specific airstream - aluminum chassis - reason not to use the frame as ground? Something to do with current in aluminum and corrosion?

If everything works I'd stop looking for a reason to rewire it.
 

imnprsd

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JayArr: If you repair these inverters I yield to your experience(s), but I'm not sure I can go along with your last statement... in all cases.

You said: "If everything works I'd stop looking for a reason to rewire it."

In this case, okay. But I have to say, I would not apply that reasoning to high-frequency inverters. ...Not that I know how to explain why, but I would like to ask you more about this.

For example: If I want to run an residential refrigerator, on a 1200W HF-Inverter, like one of those China models you see for sale on Amazon, can I run it with concern of a fire?

Note: 99.5% these inverters are made in China, so that's not what I'm taking about. That said, I would bet all the RV brand names we are familiar with use China manufactures too. So when you buy a brand name, in any market, basically you are buying their Q&A testing and reliability. Do you agree?

What I want to know is why a cheap LF-Inverter might be preferred over an expensive HF-Inverter when you have loads that run continuously for long periods of time?

I know this is a rather subjective question, since you don't know anything about the components inside, but based on design alone, is a LF-Inverter always a preferred design over a HF-Inverter?
 

JayArr

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We repair both, The Xantrex Prosine series are HF inverters and the main advantage is weight. The High frequency allows them to run without the big iron transformer that a LF unit like the Freedom 458 or the Combi use. A lot of really large bus/coaches need 6000 to 9000W of power and to try to get that from LF inverters would require massive iron core transformers. Using two or three Prosine 3.0K units does the trick.

I would think that either LF or HF would run a fridge without fear of a fire but that depends on the quality of the inverter more than the type.

I don't think I would ever prefer a LF over a HF inverter. I wouldn't spend money to replace a LF with a HF but if I didn't own anything I'd buy the HF.

The best advice I can give is buy an inverter twice as big as you need. That way the transistors inside aren't running anywhere near their maximum rating and they don't build up a lot of heat. Heat is the killer.
 

imnprsd

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Thanks. I like learning. Can you expand a bit on what you said... "...I don't think I would ever prefer a LF over a HF inverter."

I was under the impression the cost of building a LF-Inverter was higher vs. the same wattage for a HF-Inverter; and that the stability of running high loads (10A+) for long periods of time was safer using a LF-I vs a HF-I, but I don't know if this is true or not?

In addition when it comes to starting heavy duty motors, like table saws or other 1000W+ loads, I don't know if the LF-I will work better than a HH-I if you just have 2x the rated wattage for each inverter being compared.

Concerning my RV needs, here's the fundamental question I need help with. Example, if I had 300aH of LiFeP04 batteries to power a 8A portable AC and residential refrigerator, all day long in my RV, would you buy a high quality $1,500 PSW, 1500W, LF-I ...or would you buy a $600 PSW, 2000W or 3000W HF-I?
 
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JayArr

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OK, that's a bunch of questions so let's unpack them one at a time.

I prefer HF because it weighs less and runs cooler. That's just a preference. Both work.

cost of building a LF-Inverter was higher vs. the same wattage for a HF-Inverter;

I don't know who told you that but I doubt it's true. The LF one needs a huge transformer but that's just a chunk of iron with some copper wrapped around it. The HF needs a ton of engineering to get the sine wave right so it has double or triple the amount of logic circuits. It's like trying to figure out if it costs more for GM to make a front wheel car or a rear wheel car. They're just different.

the stability of running high loads (10A+) for long periods of time was safer using a LF-I vs a HF-I

Safer? That's a funny word because it can mean a lot of different things. If we consider the inverter to be well designed and built - a good industry name with some reputation like Aims, Magnum, Trace or Xantrex - then both LF and HF can run 10A+ all day long if the batteries hold out.

when it comes to starting heavy duty motors, like table saws or other 1000W+ loads, I don't know if the LF-I will work better than a HH-I if you just have 2x the rated wattage for each inverter being compared.

If they are well built both should work. Table saws start up at no-load so they are easy, they don't load up until the wood hits the already running saw blade. AC compressors are probably the hardest to start, at rest the pressure in an AC system equalizes itself so that the initial push onto a compressor can be substantial.

if I had 300aH of LiFeP04 batteries to power a 8A portable AC and residential refrigerator, all day long in my RV, would you buy a high quality $1,500 PSW, 1500W, LF-I ...or would you buy a $600 PSW, 2000W or 3000W HF-I?

Well can we assume the residential fridge is a newer type and it's current draw is 5A or below? That would make your power requirement 13A = 1500W. A 1500W inverter trying to run both of your units would be running at 100% capacity all day long, that's not good no matter if it's HF or LF. So with the double the power rule I would recommend a 3000W inverter. Since 3k inverters are harder to find I could also recommend a 2500W as being sufficient, you would be running it around 60% load and it should be fine to do that all day long.

If you can afford high quality buy this:


Inverter 101:

Remember it's the heat that destroys them not the surge. They are well designed to handle the surge and there are dedicated circuits to cut them out or clamp them down if it's too large. The power transistors that are used typically have a surge current that is four times the regular operating maximum so a transistor that can safely run 50A continuous can handle a pulse of 200A.

The big difference between HF and LF is where the energy used to make the sine wave is stored. In LF units a large transformer is used and very few capacitors so the storage bank is inductive and you're drawing from the magnetic flux. In HF inverters there is no large transformer but there are huge banks of capacitors and you draw from a capacitive energy pool.

HTH
 

imnprsd

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JayArr: Thank you. Your answers are very clear and logical. Plus I appreciate your other threads.

Please indulge me further by answering a few more questions, if you don't mind, because you have me really thinking that a HF inverter is actually more suitable for an RV than a LF unless vibration and heat with a HF inverter is a concern?

Question: For the same 1000W load, running all the time, would a 2500W HF inverter last longer than and 2500W LF inverter? i.e., what is more robust: Drawing current from flux off iron coils or drawing current out of a HF capacitance pool?

Does the LF run hotter than the HF inverter? I assume it does. And when it comes to selecting the right loads, I think I read the optimal load is ~65% of the rated inverter wattage. Is that correct?

...And are you "in the danger zone" with a HF inverter running 80+% of the rated power for long periods of time... due to heat?

What I'm kicking around is how to run best power a 8A portable AC with a soft start, in my main cabin; and a 2A residential refrigerator that spikes to 5A when it starts.... off 200A or 300A LiFeP04 batteries and 300W of solar. And what's attractive are those hybrid HF Inverter/Chargers since all the necessary functions (and internal relay power switching) is inside. Or do you thing buying separates are more reliable and easier to diagnose if any subsystem fails?

Note: I looked at the inverter you suggested, but it's a MSW inverter; and since there are so many PSW inverters on the market under $1,000 I'm more likely willing to sacrifice brand vs. Amazon sourced China made PSW inverter models.
 

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