DC/AC Inverter Installation

Our new Brookside fifth wheel RV is approximately 8 feet wide by 30 feet long and 12 feet high. The storage area of this unit is the bottom, approximately 8'x30'x3'. This area is not air tight. I had planned to install a DC/AC inverter and extra two deep cycle batteries (total of 4 batteries - 360 amp hours) in the front of this area; in Tempo Marine battery boxes. The instructions that came with the inverter caution against placing the inverter and batteries together.
The two batteries that came with the RV are in a separate, sealed, vented compartment. If I were to drill holes in the Marine boxes and run a sealed hose from each box to that vented battery compartment, does anyone think it would be adequate? Any suggestions would be greatly appreciated; Thank you in advance.

I don't think Tempo marine battery boxes are airtight.  The idea probably would work OK if you came up with an alternate sealable container.
Art

If you use AGM batteries, they don't need to be vented and could be put in the same compartment as the inverter.  However, the inverter will need sufficient air around it for proper cooling.  If you're planning on putting all 4 batteries in one bank then they should be mounted close together and should all be of the same type, size and age.

The [legal] concern is that batteries can gas and the inverter could conceivably spark. If the battery gassed enough (it is an explosive mixture if in the right quantities) and the spark occurred at just the wrong time...  Wouldn't blow the roof off the RV, but could make a mess and even start a fire. I've seen batteries under the hood of a car "explode" and it ain't pretty!

Here is a source for a Sealed & Vented Battery Box  .

I don't think the battery boxes have to be air tight to vent them adequately - it doesn't take much.  However, I think you would want to arrange for some airflow (an intake and an outlet), rather than just a tube to another compartment.

Using sealed batteries - gel or AGM is the simplest solution, though not cheap. But you save the cost of the battery boxes (just need tie downs) and venting, so it may wash out. I went the AGM route when I did something similar in a previous rig.

BATTERY BOX VENT

Thank you for your input. If you look at a typical motorcycle battery vent, it's top is sealed as far a gases are concerned, all it has is a small vinyl tube that vents it. I believe I can "Rube Goldberg" something similar for my project. I am seeking the expert technical advice of three large lead-acid battery manufactures. When I have my design, I will post it here.

BATTERY BOX VENT continued

I have consulted three engineers at Excide, Trojan and Interstate Battery companies, regarding my design for venting Hydrogen gas from the Tempo Marine Battery Boxes. All were in unison it would work as designed, that part of the project will come later.
For now, I have started this reply for members who expressed an interest in my project. The first picture is of all the parts before the installation. The one part that is not shown, is a modern, electronic relay. I will show it in the future and explain it's operation.

The Tempo battery boxes I have seen (and googled) are NOT air tight enough to prevent hydrogen gas from escaping into your compartment.  You would need to somehow seal all around the cover, especially where the cables come out. (Unless you have found a model designed for that purpose)
Art  (electrical engineer and long time power boater)

King said:

The Tempo battery boxes I have seen (and googled) are NOT air tight enough to prevent hydrogen gas from escaping into your compartment.  You would need to somehow seal all around the cover, especially where the cables come out. (Unless you have found a model designed for that purpose)
Art  (electrical engineer and long time power boater)

Click to expand...

Thank you for your input.
I don't need an airtight box. The cover of this box is approximately 2 inches deep. At the high point of that cover I will have a short length of 1/4" ID tubing routed to a vented factory battery box with a louvered door. Hydrogen gas is lighter than air and seeks to move upward. This gas will travel thought the short length of tubing, long before it can fill the 2 inch deep cover. The open area, approximately two inches down, will allow fresh air in, to help facilitate the gas's escape.
Bruce (Retired Detective)

It doesn't work that way.  The hydrogen will disperse into the air due to molecular activity long before it rises.  You will develop an explosive mixture within the battery box which will disperse in all directions.
Art

MODERN ELECTRONIC RELAY

In the left side of the picture is the Grey plastic relay, it is attached to the circuit breaker box. It is a SP/DT, 20A, 24/120 VAC relay. The SP stands for single pole (output), it is ONE wire; DT stands for double throw (2 inputs) connects to TWO other wires; it is rated to carry 20 amps; the coil numbers 24/120 VAC means it can be switched by either 24 volts or 120 volts AC. The "single pole" output connects to the air conditioner, the "normally closed" input connects to the DC/AC Inverter-Battery bank, the "normally open" input and the coil wires connects to shore power.
When the trailer is NOT connected to shore power, the "normally closed" battery/inverter input is connected to the air conditioner. When the shore power IS connected, it energizes the coil and the relay switches to the 120 VAC shore power input.

Bruce,
Let me make a few suggestions:

1) reverse your thinking about having the normally closed contact connected to the inverter output. Here's the reason: If you have the a/c running on shore power while you are away and the power goes out, the relay will drop out and switch to the inverter power. Chances are good that you'll return to totally discharged batteries. You would also want to install a switch to operate the coil to switch to inverter power, as there may be times when you don't want the a/c to have power even though you're not on shore power and the inverter is turned on and running other things.

2) ...and you've probably thought about this already, make absolutely sure the relay switches the hot side of the inverter output and shore power; not the ground, which would pose a shock hazard. 

3) You may want to consider using a 30A relay; the 20A relay is being asked to operate near its' maximum rating.

Hope this helps.

Karl,

You are absolutely correct in suggesting that the Inverter NOT be wired to the normally closed contacts of the transfer relay.

I wired my first inverter installation that way and it got fried when I plugged into shore power, some where in Wyoming, while the inverter was still on.

I'm not sure if it was due to contact "bounce" or a "make before break" condition, but it cooked it beyond repair.

By wiring the inverter to the normally open contacts I now prevent the shore or (gen-set) power from superseding the inverter power, or, as you stated, inadvertently switching the inverter back in due to a power failure.   

It is still an automatic application of ac power (through the TX switch) when the inverter is turned on, but it now requires an overt action to turn the inverter it's self on. 

I also like your idea of switching out (opening) the hot leg, prior to turning the inverter off, as added insurance against the possibility of the above described condition happening in reverse.  With probably the same results.....

Another switch and possibly a relay would be cheaper than a third inverter.  ;D

lou 

King said:

It doesn't work that way.  The hydrogen will disperse into the air due to molecular activity long before it rises.  You will develop an explosive mixture within the battery box which will disperse in all directions.
Art

Click to expand...

The following statement is a FACT that CAN NOT be disputed: "Hydrogen Gas is the LIGHTEST of ALL gases." Below is a copy of an email correspondence I had with an ENGINEER at Trojan Battery.

Bruce,
Your venting is adequate enough to keep the Hydrogen level below the 4% LFL level of ignition.
thanks,
Craig Quentin                                     
Technical Support Engineer
Trojan Battery Company
12380 Clark Street, Santa Fe Springs CA 90670
Phone 562.946.8381 -- 800.423.6569 x3045
Direct Dial: 562.236.3045
Fax:562.236.3145
--------------------------------------------------------------------------------
From: [email protected] [mailto:[email protected]]
Sent: Thursday, December 21, 2006 5:53 AM
To: Technical
Subject: Question for technical support
Importance: High

Name : Bruce N. Torsney
Application : Recreational Vehicle
Phone : UNANSWERED
Email : [email protected]
Comment : A group 24 Tempo marine battery box has a cover that is approximately 2 inches deep. I have sealed the two existing vent holes in the cover. At the high point of the cover, I have attached a short length of 1/4" ID vinyl hose and routed it to a vented compartment. Will the Hydrogen gas from the battery exit out, via the tubing, before it can fill the cover or escape down the 2 inches to where the battery cables exit? Thanks, Bruce

SUNNYBROOK STORAGE BAY INTERIOR WALLS

From the relay, I have run 12/2 w/gnd Romex wire to a tie box in the ceiling area, where I want to mount the DC/AC Inverter.
In this photo I have removed the storage bay interior wall, right side of photo. It exposes the DC converter wall, 1/2 inch plywood that goes from floor to ceiling. It contains the 40 amp DC converter, two Buzzman "Shortstop" circuit breakers and associated wiring. The 30 amp Buzzman is for the front landing gear/truck 12 VDC line and the trailer's DC distribution panel. This line allows the landing gear to operate, either on the house batteries or truck batteries/alternator. The 20 amp Shortstop is for the single slideout. If you look above the square cutout, where the red and white wires exit from the plywood, you will see the top edge of the silver metal vented house battery box. I want to mount the two extra battery boxes in this area, so I can run venting hoses, to vent hydrogen gas from the new battery boxes to the factory battery box. I would like to be able to remount the interior wall back to it's original location when finished.

Bruce,

Your venting is adequate enough to keep the Hydrogen level below the 4% LFL level of ignition.

Click to expand...

I suggest you find another "ENGINEER". No real engineer would make a statement like that based on the slim amount of information given him/her. Hydrogen doesn't need oxygen to burn, and it takes only a little to make it explode. You might remember the Hindenburg disaster in Lakehurst, NJ. While not without merit (at least in theory), your assumption that a lighter gas (hydrogen) will displace a heavier gas (air) won't hold true in the confined space of 1/4" tubing; at least not in the short term, which is what we are talking about. The mass(weight) of the air, mostly nitrogen, would prevent this. If you choose to use an enclosure to prevent the corrosive action of battery acid on surrounding electronics, I would suggest you install a small aquarium air compressor to provide positive ventilation through the tubing. They don't cost much, use very little power, and don't produce sparks which could ignite the gas.   

King said:

It doesn't work that way.  The hydrogen will disperse into the air due to molecular activity long before it rises.  You will develop an explosive mixture within the battery box which will disperse in all directions.
Art

Click to expand...

When hydrogen escapes into the atmosphere, it is so light that it scatters immediately upward in the air (it?s 14 times lighter than air).

If you need more proof, check out this link: http://www.hydrogennow.org/Facts/whatishydrogen.htm

BNTorsney said:

When hydrogen escapes into the atmosphere, it is so light that it scatters immediately upward in the air (it?s 14 times lighter than air).

If you need more proof, check out this link: http://www.hydrogennow.org/Facts/whatishydrogen.htm

Click to expand...

Not true.  Hydrogen does not escape into the atmosphere easily in your application. Please heed what Karl is telling you for your own safety.  Now this is from somebody that has witnessed a hydrogen explosion and it isn't a pretty sight.  The individual involved is lucky to be alive even with the scars.

BNTorsney said:

When hydrogen escapes into the atmosphere, it is so light that it scatters immediately upward in the air (it?s 14 times lighter than air).

Click to expand...

I am curious as to what entices the hydrogen to go to the 1/4" opening in the tubing and then exit the battery case, even if it is at the top of the battery case. It seems to me that hydrogen from six sources (two 6-v batteries or one 12-v battery (or twice that number of sources if you have four 6-v or two 12-v batteries) will get pretty well mixed with the air above the battery(ies) well before it can find a 1/4" opening in a piece of tubing to make it's rapid exit from the battery case. Just curious!

Richard

The key word is "SCATTERS" in the atmosphere.  The first thing that happens is the hydrogen disperses (scatters) uniformly within the air into which it is being introduced.  It becomes an explosive mixture when the hydrogen reaches 4% of the air.  An air mass containing 4% hydrogen will not be enough lighter than the air surrounding the case to cause it to rush through a quarter inch tube.  It will continue to disperse (scatter) out the openings in the box until all of the air in the storage area becomes contaminated.  You do not want to take the risk.
Art

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