Fabricating a curved side battery box?

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Tony01   10 W

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Fabricating a curved side battery box?

Post by Tony01 » Oct 15 2018 2:31pm

The time has come in my build to have to figure it out, and soon. I’m building a full suspension Schwinn cruiser and need a box that can stick to the curved contours of the frame.

My battery will be about 17kg, plus controller and onboard charger probably another 3kg, bus bars etc so let’s say it needs to hold about 20kg.

It needs to be seven inches wide, and follow the contour closely because I do not have much space. My lipos are very big for the ah.

What material to use? Obviously by the pic I am proficient in general fabrication but have little experience with sheet of any kind. Sheet steel I dunno how to do the curved corners. If I did it I’d probably use rivets. I looked at aluminum sheet but it is expensive and difficult to work with (mapp brazing). I have a welder but I don’t think I can do anything thinner than about .050” steel without blowing thru all the time.

I am thinking of doing ABS and welding it either with soldering iron, friction, or plain oil glue, but will it be strong enough to hold 20+kg of contents? How thick of abs sheet is needed?
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X2flier   10 W

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Re: Fabricating a curved side battery box?

Post by X2flier » Oct 15 2018 3:43pm

Tony,

FWIW, I would build that battery box with flat sheets and extruded angle stock, riveted/welded as needed. You can drill and tap the angle stock, for removable panels to provide future battery access. When you have the box fitted and mounted in the frame, then just fill in any minor gaps with filler materials, which could be Bondo, a fiberglass slurry, or anything that works for you. You can fill most of the spaces between the frame and box with strips of corrugated cardboard for lightness, laminated together, and then just smoothed out with a coating of your filler material and sanded smooth. Your battery box will look and feel custom-fit, and the batteries will fit inside in straight rows. I would use flat pre-made fiberglass (or other non-conducting) sheets for the large flat sides, to avoid any future short-circuits, as in an accident. Carbon fiber should be considered as a conductor (and a bad choice) here.

My $.02 worth.
Cheers,
X2flier

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Icewrench   10 kW

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Re: Fabricating a curved side battery box?

Post by Icewrench » Oct 15 2018 11:37pm

viewtopic.php?t=12847

Might find something useful in the link.
Good luck

Philaphlous   1 kW

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Re: Fabricating a curved side battery box?

Post by Philaphlous » Oct 16 2018 7:03am

Few suggestions. Fiberglass is a good option because essentially you could make the inside of the box area out of foam in any shape you want. Cover the foam in alu foil and fiberglass the entire thing then just use something to melt the foam or cut it up and out.

Abs plastic is a bad idea or acrylic because it's too brittle. Especially when it gets colder it'll snap before it flexes. A softer more maluable plastic like ldpe, hdpe, or polypropylene will likely be your best bet. Pp would be a good option because you can easily bend it to shape. Those plastics will be better for flexing then holding a tight ridged shape and then snap from some bumps.

I wanted to do a box like you're describing but came to the quick realization that it would take way more time then I have and be way more expensive. Plus the weight of the. Ox would be a big deterrent. I ended up buying a bag and it works maricles.

Tony01   10 W

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Re: Fabricating a curved side battery box?

Post by Tony01 » Oct 16 2018 12:25pm

Ahhhh crap... fiberglass sounds like the way to go. I’ve just been trying to avoid it because it is another area of fabrication I know nothing about... I already feel as if I’m way in over my head on this build and haven’t even finished the swingarm yet!!!

Making it non removable sounds easier than not. I’ll be using those large 8ah pouch cells with custom made bus bars from 6101 aluminum, in a 20s3p configuration. To fit the pack with no padding the box needs to be about 7” wide on the inside. I would like to make a box that is part of the frame, but if not then that is ok too.

As you can see, I’m gonna need all the space I can get. Only 3 cells are pictured, but they will be 21 thick for the top two and 18 thick for the bottom one. 2x 7s3p and 6s3p. A 3p set of them measures about .970” thick.

I guess if I go with glassing, I can always make a faux fuel tank to hide the controller in with a large heatsink as its base.
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X2flier   10 W

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Re: Fabricating a curved side battery box?

Post by X2flier » Oct 16 2018 12:43pm

Tony,

For those flat battery packs, it may be better if you make two panniers, one for each side. They would attach to maybe the seat post and/or the main frame down tube, straight below the seat, not "hiding" the rear wheel (as with most panniers). Being very slim in profile, they would not interfere with pedaling. Just a thought.
Cheers,
X2flier

donn   10 kW

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Re: Fabricating a curved side battery box?

Post by donn » Oct 16 2018 2:39pm

Tony01 wrote:
Oct 16 2018 12:25pm
fiberglass sounds like the way to go
How will you deal with access? In my small experience with fiberglass, the edges are what makes or breaks it. Philaphlous' approach sounds like laying it up as one big envelope around the form, so you'd saw out an access panel, and figure out how to work the edges when it goes back in. And, at least the way it's commonly done, it isn't really super light - not like plastic. Light fiberglass things like kayaks sandwich a layer of foam, or sometimes wood, between the glass layers.

I thought X2flier's suggestion, to join the panels with angle stock, sounded promising. I don't know, it might not bend real gracefully to take all those curves, but I bet you could make it work if you can weld a clean joint in the middle of the down tube side. Then the whole thing, paneled with light gauge sheet (on the outside of the angle frame, where it will hide the places you cut to help it bend), will be fairly light and plenty strong, at least in terms of supporting weight on the inside. It will dent easier than fiberglass.

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X2flier   10 W

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Re: Fabricating a curved side battery box?

Post by X2flier » Oct 17 2018 12:36pm

donn wrote:
Oct 16 2018 2:39pm
I thought X2flier's suggestion, to join the panels with angle stock, sounded promising. I don't know, it might not bend real gracefully to take all those curves, but I bet you could make it work
Tony,

Ummm . . . There is more than one way to build a box. This is a cross-section view of the edge of a battery box that fits like a saddle over the frame tube(s).
The Purple represents the frame tube.
The Black represents the extruded angle stock.
The Green represents the (large) box side panels, and the inside width panel.
The Red represents the fasteners, either rivets or screws.

For "visual" perfection, you can use strip stock as thick as the angle stock, to fill any gaps left by the swoopy curved frame tubes. One edge of the strip will be flat, of course, and the other edge would be curved to match the frame tube, as needed.
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Tony01   10 W

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Re: Fabricating a curved side battery box?

Post by Tony01 » Oct 18 2018 11:02am

Thanks for the replies. However curving angle extrusion and bonding it to steel is not gonna fly. Been there done that bracing the rear end of a Schwinn with 1” c-channel. Strong but time consuming.

At the moment I’m doing a cost analysis. I was thinking at first to weld a continuous tab to each side of the frame to provide a flat mounting surface for the fiberglass, which would resemble a cup on its side for each half. Then I could seal it with a single continuous strip of rubber. Some of the components could mount direct to frame. However getting the metal tab to follow the frame would be difficult, and welding it at several points all around could weaken the frame some.

I think I might just do the box inside the triangle and cut out a window for access. Lot less work. Can make curved brackets for frame mounting or weld some tabs around.

Outside layers of carbon fiber are lookin real good... too bad it’s so damn expensive... around $25 per yard of 6oz cloth! Edit, looks like it’s a bad idea to mix cloths. Maybe black fg.

Also thinking of just doing a 3/16” rod wireframe and welding sheet metal to it. not anywhere near as messy as dealing with resins. Wouldn’t be as aerodynamic as fg but much easier to set frame sliders in different places. So I got some stuff to think about.
Last edited by Tony01 on Oct 18 2018 1:54pm, edited 1 time in total.
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teklektik   10 GW

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Re: Fabricating a curved side battery box?

Post by teklektik » Oct 18 2018 11:54am

See: 18650 battery pack build Kepler style

Order 5052 aluminum strips cut to width at OnlineMetals.com.
Hand-form using available pipe or round objects as jigs.
Use epoxy or pop rivets to join strip splice(s) with small extra piece as lap joint.
Fab sides using material of your choice.
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Tony01   10 W

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Re: Fabricating a curved side battery box?

Post by Tony01 » Oct 22 2018 1:20pm

Thanks for all the effort you guys put into this post. I’ve researched the snot out of each method and decided I’m gonna be going with fiberglass. It seems like the smart choice. For the same price I could have got welding gas and some sheet metal, but I think I’ll get a better looking product like this with less work. Along with the ability to make other parts I could never do without expensive tools in steel.

I’ll be starting a build thread and will link to it soon as I finish a couple more items on the build to-do list.
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