Security of Floor Under Battery Box

Our TM (2720SL) has the battery box located in the back, accessible from the outside above the bumper. A portion of the floor core is removed so the battery is countersunk a bit in the floor. This leaves the heavy battery resting on what appears to be only the outer skin of the flooring. Has anyone ever had or heard of any issues with the battery breaking thru the floor? I was thinking of having a small metal brace welded under the floor to the frame as a safety device.

Wade

Wade,
I am not familiar with where the structural members (rectangular steel tubing) are located under the floor with respect to the battery box in your 2720SL. I assume that your battery sits in a tall plastic box - like mine did in our 3124SL. In our 3124SL the plastic box sat at the bottom of a cabinet. The upper aluminum sheet and foam were cut out in a crude rectangle, in which the box sat. My original battery was a "Marine-RV" battery that weighed about 40 pounds. When it died, I replaced it with an absorbed glass mat one (AGM) which weighs 65 pounds. Although there is a steel tube cross member about 1/8" under the bottom sheet in the area of the cut-out, I still did not feel comfortable with this heavy battery bouncing up and down on the lower aluminum sheet. Therefore I built a new battery box, made of stainless steel sheet. This new box was shallow (about 1-3/4" deep) and flanged at the top so that the flanges could rest on top of the floor. With this arrangement the total weight of box and battery are now carried by the entire thickness of the floor.

It may be wiser for you to do something similar to what I did rather than attempt to weld a steel brace under the cut-out area. Welding under the floor can be messy - your welding arc will be very close to the bottom aluminum sheet. If you don't melt the aluminum, at the very least you may melt the floor's foam core. If you can't make a steel box, you might consider making some bent bar supports for your battery box. These could be made out of 1/8" thick X 1" wide steel bar. Bend the bar into a flat dish shape (the inside should be wide enough for your battery box or battery) with a horizontal flange at each end. If the height of your dish shape is just less than 2" (thickness of floor), the bars will rest on top of the floor. Three of these bars might be ample support for a heavy battery. You could weld cross members to these three parallel bars and create a mesh-type of box. If you weld up a new battery box of mild steel, I suggest painting the inside of it with an epoxy paint to resist the corrosive effects of spilled battery acid.   :  :

Larry, your metal battery tray suggestion to accomodate the added weight of an alternate battery sounds good. I will likely take this route as it would seem to provide good support with the flanges resting on the total 3" thickness of the floor. I will need somthing like this if I choose the heavier T-105 6 volt batteries, of which I may put another on the left side of the compartment for a set of 2 in order to achieve 12 volts. They are ~63 lbs each. Thanks for your input.

Wade

When this question came up, I was looking at a 3124KS nearby and observed exactly what was described above.

My "planned" solution, if I became an owner, was to install some plywood shims under the exsisting lip of the plastic box.  Longterm the metal box solutions, designed to fit the existing hole, would be better.

I would have to find someone to fabricate something like that and it would most likely be aluminum in my case...

Either way, the idea is to transfer the load to the top of the floor instead of the bottom.

I checked the battery box on our 2720sl this weekend and it is about ready to come thru the floor.

I plan to use wood blocks to transfer the load to the upper floor as well as re-enforce the aluminum skin with some 1/8-in aluminum plate.  I will run a bolt all the way through the wood, floor, and the aluminum plate.

Let me know if think of anything else we might be able to try... :-/

Oilspot,
If I understand your plan, you intend to attach wood blocks underneath the flange of your plastic battery box so that the weight of the battery will be transferred through this built-up wooden flange to the top of your trailer's floor. You also plan to place a rectangular, 1/8" sheet - with a rectangular cut-out area into which the battery box will fit - on top of the floor so that the battery's weight will be distributed over a larger area than that of your wooden flange. Is this correct? If this is what you intend to do, then it should work well. I would not, however, recommend that you attach the 1/8" sheet and wooden flange with bolts penetrating through the entire floor. Not only would these through-bolts increase the likelihood of road moisture seeping through the floor but also you increase the risk of electrolytic corrosion of the bottom aluminum sheet when moisture gets between the bolts and aluminum. A more practical solution might be to make the rectangular cut-out in your 1/8" sheet sufficiently snug so that the battery box will not have much wiggle room inside the cut-out. Then glue the 1/8" sheet down to the floor with a coating of construction adhesive. This is the all-purpose adhesive (available at all hardware stores) that comes in tubes and bonds masonry, wood, metal and most plastics. After hardening it will be more than strong enough to hold your 1/8" sheet from moving around while your TM is bouncing around on the roads. You can apply small dabs of this adhesive or a continuous bead of silicone caulking between the bottom of your wooden flange and the 1/8" sheet to assure that your battery box will not move. If, for some reason, you later need to remove the battery box, you can slip a knife's edge underneath the wooden flange to pry it up off the 1/8" sheet.
;D ;D

Built a wood support for the battery box last week. Essentially a wood frame that sits on the floor between the plastic battery box and the floor so that the battery weight is on the floor rather than outer skin. I fastened mine to the lower tray of the battery box so it lifts out as one piece. The top of the plastic battery box simply screws down into the frame I built. Result is no more weight on the outer skin of the TM. The original engineering design of the battery box cut-out really leaves a lot to be desired and is kind of surprising to me. I would think a larger battery would begin to push right through the floor skin after riding over a few bumpy roads.

Wade

I have done the same thing until I can construct something more substantial...

I may have a metal shop bend up a box to my specs, which slips into the hole and "hangs" on the top side of the floor.

Still haven't decided the best design yet.

I agree a shallow formed metal box with a top flange bent at 90 deg from the sides to form a lip to rest on would be ideal. I doubt you would even have to bolt it down as it would likely fit nice and snug in the cutout. Maybe some construction glue around the lip and two drain holes in the bottom to match the ones already cut and screened. This would be a very sturdy setup and all you would have to do is find a shop to cut a single piece of metal (maybe heavy aluminum?) and bend it to shape. Snip the top 4 corners so you could bend them down to form a lip and drill two holes in the bottom. I'll take one!

Wade  

Aluminum is not the best material to use for a battery box since the battery's electrolyte, dilute sulfuric acid, readily attacks aluminum. Mild steel would be a more suitable material. Most auto battery compartments are fashioned from mild steel - and still corrode, as you all have noticed, after the paint wears off! If you're going to use AGM batteries, then aluminum should be perfectly fine since there will be no battery acid leakage from these sealed batteries.

If you can't or don't want to fabricate a steel (or stainless steel) battery box, constructing a fiberglass one would be an excellent alternative. This method would require the building of a simple, wood male mold (much easier to lay up cloth around a male one than inside a female one) that is coated with wax before you apply the first coat of resin and cloth. Suitable epoxy resins and glass cloth are available in most hardware stores and, if carefully made, the box can be extremely strong and impervious to battery acid. Also, it may be feasible to cut down your present plastic box, add an aluminum or wood flange to it and cover the entire thing with layers of glass cloth and resin. Be sure to wear disposable gloves to protect your hands from the epoxy resin. Some individuals can develop dermatitis from exposure to the epoxy resins. : :