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I'm about to build some battery boxes for my Leaf batteries.

Is there a reason I should build them in steel, or can I use hard plastics, if that's easier?

I have someone who makes marine tanks that might be able to make something for me that's weatherproof.
 

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Aluminum as a box material does not do well in a cell fire.

Tesla uses mica sheets in its aluminum battery box to protect the car floor & occupants from burnthrough.

Looking at my burnt Model X, it buys time and doesn't make it "fireproof". Each cell, oriented vertically, is a little blowtorch/roadflare when they light off - usually out the vent side of the cell. When the floor eventually gets breached you get little rockets flying around in the cabin (the cells that were down-oriented in the pack), igniting all that fake cow interior.

So, part of your box strategy is whether you want it to simply buy you time to GTFO, or whether you want to contain/compartmentalize fire damage to the vehicle.

Fun times when the cells light off, lol
 

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Looking at my burnt Model X, it buys time and doesn't make it "fireproof". Each cell, oriented vertically, is a little blowtorch/roadflare when they light off - usually out the vent side of the cell. When the floor eventually gets breached you get little rockets flying around in the cabin (the cells that were down-oriented in the pack), igniting all that fake cow interior.
All good information, but of course only applicable to metal-cased cylindrical cells, such as those used by Tesla. Most EVs have pouch cells, and a few have prismatic - they still burn dramatically, but don't become rockets.
 

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I think fire safety shouldn't be the primary factor in designing the battery box. Instead, the structural integrity and heat exchange should be the priority. Certain non-metallic materials can be plenty strong for structural integrity, but they're typically good thermal insulators as well, making them less than ideal, especially in a weatherproof setup. Aluminum is a great thermal conductor, allowing for much better passive cooling of the battery pack through its outer shell. My second choice would be stainless steel, and then mild carbon steel. Structurally it may also make sense to have a combination of rigid members for bolting and sheet metal for the actual shell of the pack. So with that last point I mean something like angle iron where the bolts go through the modules, and the actual shell made out of thinner sheet metal.

Now the reason fire safety should not be a concern... is any DIYer seriously expecting to build a pack that will contain the fire in case of a thermal runaway ?
 

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Car crash, dude. Always nice to live through those. Immolation is a horrible way to die, right up there with cancer.

Every build is a set of tradeoffs, and shows the builder's decisions. Awareness of what the elements being traded are is key.

Example: stainless is HEAVY, has a high melting point, and is a crap thermal conductor. Aluminum's the opposite. Both don't work (as in bend) well and are somewhat exotic to weld. Carbon steel is a Goldilocks metal....for me.
 

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Again, it is comical to expect that an average DIY build will take a crash with some tolerable outcome.

Stainless is heavy, but... how thick are we talking ? 1.5-2mm is all we need to give a reasonable protection to the battery pack while the rigid member take care of the structure and mounting. High melting point ? Why does that matter ? Crap thermal conductor ? Better than plastic! The advantage of stainless is in the name - it doesn't need any treatment for corrosion resistance. Which means any components with heat dissipation attached to it will be able to transfer the heat way more efficiently to this poor conductor than say to a painted/powder coated carbon steel. Galvanized may be an alternative, but there are some caveats to it too.

Don't weld if you don't feel comfortable with that. Use rivnuts or plain rivets for assembling the box out of the panels. Bending is not a problem, and is an advantage for designs with bends! Like I said, average pack isn't surviving a crash. The practical concerns are reasonable weatherproofing, and reasonable mechanical protection from adjacent elements especially when servicing the vehicle.
 

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It may be comical, but some jurisdictions take crash safety seriously before they'll issue a roadworthiness certification. I think it was New Zealand that spec'd mounting bolt sizes for battery boxes, but my memory's not that great these days. Brian's the Human Encyclopedia here.

As I said, it's the builder's set of choices based on many factors. There is no right way to do it, no better way, because everyone's criteria, skills, and resources are different.

A Californian could care less about corrosion, or even rainproofing for that matter, as an example. Makes you wonder why roadsalt-liberal Minnesotans didn't goe (sic) all-in on stainless-skinned DeLoreans, lol
 

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A Californian could care less about corrosion, or even rainproofing for that matter, as an example.
More comedy, ok. I lived in LA/OC for 6 years continuously, riding a motorcycle. GOT PLENTY WET. When it starts going there, it really dumps. Even when it doesn't rain though, corrosion is often caused by indirect water presence from condensation. Again, temps swings even in Southern Cali are pretty great in the Winter time, with droplets easily forming on all exposed metal surfaces.
 

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My destined project vehicles are over a decade old, from there, and are pristine underneath, in the rockers and in blind corners like rear feanders and truck cab corners.

My tongue in cheek posting was perspective from a kid that grew up in Canuckistan and got plenty of rust flakes in the eyes wrenching on his car builds and who has bought California cars the past few years (shipping hurts, but zero rust is such a joy to work on vs trying to patch something that disintegrates between your fingers).

I guess you never noticed a lot of T-buckets in Ca. have no roof. Try a Woody in Michigan and you'll wind up with barn boards.
 

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I plan to build a battery box structure from steel angle extrusions. I will weld them together and use the chassis' engine mounts to hold the battery box. After I have the framework and floor made from steel I will rivet aluminum panels on the sides with silicone sealant in between. The LEAF modules I will maintain as much of the original mounting plates and structure as possible and bolt them solidly to the box floor.
 

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More comedy, ok. I lived in LA/OC for 6 years continuously, riding a motorcycle. GOT PLENTY WET. When it starts going there, it really dumps. Even when it doesn't rain though, corrosion is often caused by indirect water presence from condensation. Again, temps swings even in Southern Cali are pretty great in the Winter time, with droplets easily forming on all exposed metal surfaces.
I've worked as a mechanic in LA & Santa Monica for 38 years, believe me, rust is a problem in the beach communities. The nightly temp swings cause the salt air to condense and it can be very bad. Get a couple of miles inland and not so much.
 

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You could use wool but it retains water between the fibers. 1/2" or 1" building styrofoam is like $10 usd a sheet

I would like to take this time to point out that gen 1 Volt batteries have a composite battery cover and only catch fire in puncture accidents.
 

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You could use wool but it retains water between the fibers. 1/2" or 1" building styrofoam is like $10 usd a sheet

I would like to take this time to point out that gen 1 Volt batteries have a composite battery cover and only catch fire in puncture accidents.
There are multiple kinds of foam insulation used for buildings. EPS and XPS probably closer qualify as "styrofoam." I believe they're as flammable as the regular packaging styrofoam. Polyiso is non-styrene based kind, supposed to have better flame retarding properties.
 
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