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Discussion Starter · #1 · (Edited)
I'm preparing to make a box that is going to hold a battery pack. I'll be using 3/4" Birch plywood (Edited). My biggest concern is joining the sides of the box to the bottom as the battery pack will weigh about 200lbs. I want to ensure that whatever method I use will support the weigh when the box is lifted. Lifting will not occur frequently, but when it does, I don't need the battery falling out of the bottom. Am I crazy to think that using 3/8" hardwood dowels through the side panels into the bottom panel and wood glue wood be sufficient?

Thanks all for any advice!
 

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The Nut in the Cellar
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That should work. If it were my project, I'd probably do a dado and rabbit for the bottom with a dado run around the sides and ends with a rabbit around the bottom piece to fit into the dado. I make drawers this way. The bottom comes out flush with the sides and ends on the bottom of the box/drawer.

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where's my table saw?
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You didn't give enough information for a comprehensive answer.
How will the box be lifted? are there handles? who can lift 200 lbs from a recess?
What material is the box made from? plywood? laminated hardwood?
Can you have angle iron braces at the bottom all around?
Can you wrap steel strapping around the sides and bottom with lifting holes?
Is this for a golf cart or electric vehicle? Will battery acid spill out and rot the wood?
Is there a plastic liner to catch it?
Will it be open or ventilated to allow the vapors to escape? Or are the batteries sealed like gel types?
 

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I'd be wary of using such large dowels (compared to the material thickness) because of the reduction in the amount of material around them.I'd be happier with 1/4". On the other hand,I do have a dovetail jig for my router and would dovetail the sides and ends together with a groove all round the inside for a rebated bottom to locate in.
 

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Build the strength of your box in the direction of the load. I like the notched design fareastern showed because the floor of the box is notched into the side boards giving some vertical support. I also like the idea of metal strapping or bracing. Also, agree with others' comments need more information on the load. Good luck!
 

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3/4" birch of 3/4" birch plywood?

Whatever you do, make sure you leave enough material beneath the bottom to give strength. Whether you use dowels or a dado bottom, leave at least an inch under the dado or dowel so there is more wood to support the bottom.

Is this outside?
 
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Discussion Starter · #8 ·
Is this outside?
Perhaps you could consider it outside. It will be located in the interior of a sailboat. There's no climate control so it will be subjected to heat/cold and humidity, but not sunlight.

The material is 3/4" birch plywood.
Current lifting plans are rope handles on the sides.
Batteries are lithium, so no toxic gases, battery acid, or physical orientation to be concerned with (as long as it isn't upside down)

Reviewing all of the responses, I'm thinking I should take several pieces of advice. A dado around the bottom makes sense to me (although difficult with my current tool inventory), but I think in addition to the dado I could still use the dowels? In terms of reinforcement, I could use fiberglass and epoxy rather than angle iron or metal straps, or something that would require screws to attach them. I'm trying to avoid using screws or anything that could easily puncture the lithium battery cells if something were to go wrong. Don't need a fire on the boat.
 

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where's my table saw?
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Perhaps you could consider it outside. It will be located in the interior of a sailboat. There's no climate control so it will be subjected to heat/cold and humidity, but not sunlight.

The material is 3/4" birch plywood.
Current lifting plans are rope handles on the sides.
Batteries are lithium, so no toxic gases, battery acid, or physical orientation to be concerned with (as long as it isn't upside down)

Reviewing all of the responses, I'm thinking I should take several pieces of advice. A dado around the bottom makes sense to me (although difficult with my current tool inventory), but I think in addition to the dado I could still use the dowels? In terms of reinforcement, I could use fiberglass and epoxy rather than angle iron or metal straps, or something that would require screws to attach them. I'm trying to avoid using screws or anything that could easily puncture the lithium battery cells if something were to go wrong. Don't need a fire on the boat.
What if the lifting straps went under the batteries rather than under the box?
After all you are wanting to remove the batteries for replacement probably?
They can come out in sections rather than one 200 lb massive load that way.
Easy enough to use ready made sling straps or have some sewn up and folded flat with loops for your hands.
Lifting straps or slings:

Or make your own:
 

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3/4" birch plywood will be plenty strong. If you don't need to have the bottom flush, I'd make dadoes, 3/4" up from the bottom, and 1/4" deep. I don't think dowels to help strengthen the base is needed, but if you want to avoid screws, use dowels to strengthen the corners of the sides. Rope slings aren't necessary as long as the box bottom is dadoed into the sides.
 

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Just thinking out loud here about the possibility of moisture accumulating around the bottom of the box . . .
I would use 1x3 pressure treated wood to support the bottom of the box and a strip for the box to rest on to prevent water wicking into the plywood. There is a special construction adhesive specifically for P/T wood projects in the Box Stores. Stainless or Brass screws with the adhesive should make it totally secure for your needs. (that is IF you don't have really tight space & size constraints.
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For the intended purpose,a simple box,nailed together with almost any glue will work.Just as long as you cover it with a bit of glass.No need for epoxy,just basic polyester and glass mat will work adequately.Round the edges to make it easier for the glass to conform.Unless there is a really compelling reason,I'd go low tech with the batteries too since thermal runaway is just about unknown with lead/acid.
 

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where's my table saw?
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There is a 200 load in the box. That's more than one man should lift, JMO. There needs to be a simple means to lift the box OR it's batteries separately. This needs to be addressed before we go designing a box that the bottom won't fall out. Straps, ropes or secure handles are part of the solution, especially if this is below the deck where bending over would make it extremely difficult for one person.
The OP needs to come back with more information.
 

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Discussion Starter · #18 ·
For anyone interested, this is the battery. It is approximately 13.75" wide, 23.125" long and 9" high. 16 x 12lb cells connected in series. The picture does not indicate the orientation of everything in the box as the BMS (red) will be located on a shelf above the cells.. The box is intended to have multiple compartments, one to house the cells and another to house a few electrical components (fuse and shunt; possibly some power distribution bars).

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where's my table saw?
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If I'm anywhere near correct, those wires will nut carry sufficient current for the output of the batteries. They are too small in gauge based on my experience with DC batteries. For safety sake and better performance I would use a larger wire, a numerically smaller gauge like 14 GA or 16 GA.
 

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If I'm anywhere near correct, those wires will nut carry sufficient current for the output of the batteries. They are too small in gauge based on my experience with DC batteries. For safety sake and better performance I would use a larger wire, a numerically smaller gauge like 14 GA or 16 GA.
I'm assuming you are talking about the wires running to each cell? If so, then I'm pretty sure you aren't anywhere near correct. Those wires are for the BMS, not the load. The single blue and black cable coming off the BMS (not connected to the cells, a lug on the end of each) are for the load. They look more like 6 gauge. I'll defer to @wpduhe though, as I have limited experience with a BMS.
 
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