I build the neck wall out on the inside with a firring strip so I can insulate it a little bit. That also gives room for the screws in the front.

So be sure to use either a gasket between steel screws and alum wall or get alum screws to avoid any cross-metal corrosion.

I wouldn't use aluminum screws to hold two 60lb batteries. I have never had a corrosion problem in using plated steel self tapping screws.

If you use stainless fasteners commonly found at the big box stores, you will have few problems with compatibility,

Gard

Here's some food for thought:

http://www.sae.org/technical/papers/932357

Gard

Again , No, Never, Nodda, Didn't happen, Not even maybe, Not just a little, No Ifs, No buts, No Nothing, Not, Bla, Bla, Bla,.....................Bla, Bla, Bla,

So,

Away.

 If you want to securely fasten a box, that will hold a 60# weight for the rest of the trailer's life span, I would seriously recommend the usage of bolts, nuts and fender washers.

If you look at the excellent photo HW has provided, you will notice the threads and self drilling tip of the screws. It is only these threads, about half the diameter of the screw, a fraction of an inch, that are being used to attach this weight. Also, not all of those threads are being used, only those that are in actual contact with the thickness of your aluminum bulkhead. Maybe two at the most? Granted, the static weight will be supported in shear by the screws, but what happens during the trailer's travels?

The weight of your battery is not static. It is moving vertically, horizontally, fore and aft, with every movement of the trailer. The total force can and will exceed the battery's actual weight.

If you were to install 10-32 or 1/4-28 bolts and self locking nuts, with fender washers on the box's flange and behind the bulkhead, the surface area of your support would greatly increase, from a small fraction of an inch to aprox 1 1/2" square for each fastener. Each bolt/washer combination would have more of a bearing surface, than that of all of the combined screws. Fender washers are available in two sizes per center hole size. The larger of the two will ensure the largest surface area.

Self taping screws are not a good choice for structural aluminum attachments. The threads are designed for thin sheets of steel. When the screw is drawn tight, the metal us pulled and slightly twisted at the joint. The aluminum is usually more rigid than the ferrous sheet metal, and because it cannot flex as easily, a strong pressure point will develop at the junction. This can be the source of a developing crack radiating from the screw.

A bolt will hold much better in tension than a screw, and with the addition of the washers, the factor is substantial. The screws are quick, easy to install and cheap. That is why they are used. Instead of a dozen steel screws for a couple of dollars, eight stainless bolts and hardware might cost double that amount. It might take an additional five minutes to install the bolts. With a few extra minutes and dollars, you won't have to worry about corrosion, or long term longevity.

Another thing to consider is the placement of the battery box. If it's possible, place it nearby any bulkhead bracing that might be present. The weight of the battery will cause an unsupported panel to slightly flex. Having the battery box better supported will lessen the movement of the battery, and reduce the possibility of the aluminum developing cracks.

Gard

HW - you implied that you have installed many, many boxes. Were these installed on your own personal trailers? If not, how do you know that they never failed a few years later?

I don't build that many, 5-6 year, for the last 7-8 years so there could be a possibility that one or more that have gotten out of touch and of those maybe all the battery boxes have fell off and I just wasn't made aware of it.

 The original poster acted as if they were done with the inside of the trailer so I thought maybe their only option was to use screws from the outside towards the inside. Maybe I read into wrong. But I have used a #14 self-tapping screws in those cases and have had good luck, I have since researched and determined that a #14 Zinc plated screw has a tensile strength of apprx 300lb. so as I suggested 12 of them should hold a battery box on a trailer just fine. Maybe not????????????

I guess you could say the best way would be to use 50 1/2" bolts with a 4"x4" backing plate behind them. I was trying to solve the problem that I thought had been presented, but the almighty blatherer seems to want to chop my legs out from under me all the time, thats fine, I can defend myself, and my answers on this forum. I would be surprised if the master blatherer could pick out a wrench from a screwdriver, but he seems to have alot of time to pick apart the ideas that I and others suggest. So be it, I can play games with the best of them.

  I have since researched and determined that a #14 Zinc plated screw has a tensile strength of apprx 300lb. so as I suggested 12 of them should hold a battery box on a trailer just fine. Maybe not????????????

This is getting really scary.

You, I and every mechanic in the world, know that the tensile strength of a piece of hardware, is NOT a measurement of its attaching strength. That force will vary greatly, depending on the length and number of the threads that are engaged, and by the type and thickness of the material in which it is installed. In this case the soft aluminum will fail long before the screw, and with only a few threads that are being used, any tension produced will be very small before it rips out of the hole.

It appears that you did not read my thread, or at least you didn't understand it. If you had, you would have known that my concern was not for the strength of the screws. It was for their material incompatibility when used with aluminum, and for the very weak holding strength, they will have while installed into a soft, thin piece of aluminum.

I hope your statement was a smoke screen, only uttered in an effort to discredit my ideas. If, however, you actually believe what you have printed, and are constructing products accordingly, then the integrity of the structures you are building can be questionable.

Gard

Forgot to mention LOC-TITE. 

I am continually amazed at the constant jiggling and rattling that my rig goes through everytime I take it out and find something on the floor in the DR.  I only have the ring of flat steel and a bolt and wing nut holding my LP tank... I'm always checking it so that I don't lose it sometime.  Can't use loc-tite there...

Forgot to mention LOC-TITE. 

I am continually amazed at the constant jiggling and rattling that my rig goes through everytime I take it out and find something on the floor in the DR.  I only have the ring of flat steel and a bolt and wing nut holding my LP tank... I'm always checking it so that I don't lose it sometime.  Can't use loc-tite there...