3D Printed Bike Light Battery Holder

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Finished box clipped to Topeak bike rack on my main commuter.

I've been interested in 3D solid design and 3D printing for a long time, but didn't have much call for it in my professional work. I've done a lot of technical drawing, but mostly for fairly simple parts that were easily described in 2D drawings. We didn't have engineering requirements that called for the use of dedicated CAD software aside from the old and relatively clunky geometry editors in the version of ALGOR I used for design tasks. I developed an expert ability with Adobe Illustrator, which was useful for plots and sketches for papers, talks, and posters, and was fine for the technical drawings we needed to do.

But recently, I figured I'd take some time to learn something new, so I learned how to use Blender to design a part to be 3D printed. 

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I designed the box using Blender and exported to STL format.

I wanted something that I could have printed and actually use. I commute by bike almost all of the time and do a lot of riding at night. When I got a new bike, I machined myself a bike headlight and bought a big Lithium Polymer battery pack from batteryspace.com to power it (and whatever other electronics I decide to add to my bike in the future). The headlight, shown below, has a machined aluminum housing holding a 12V AC/DC LED tracklight bulb. The housing has a glass lens with o-ring seal.

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Bike headlight, a LED tracklight bulb in machined aluminum housing.

Originally, I had the pack strapped with Velcro to the bottom side of the cargo rack on the back of my bike, but I'd been meaning to put something a little more permanent together. So I decided to make a battery housing.

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Render of model back side with snap clips.

The battery is nice but quite expensive, as is a 3D printed product this size (34x70x180mm approx box external dimension) from Shapeways. I decided on a clip-on design that I could stash in my bag and bring inside for charging. I made measurements of my rear rack and settled on the three-point clip system above. The wedge shaped openings were intentionally designed with a little more interference than what I thought would allow a safe snap action without overstressing the clips.  I wanted a strong hold, and at the time, I hadn't identified a stress/elasticity package for solids that I could justify just for an educational and personal project. So I used an experimental approach with this prototype and used a file to enlarge the wedge-shaped openings to give a strong grip without overstressing the clips.

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Front side.  Decided to add my stamp.

My first model had 4mm thick walls. To strike a better balance between strength and printed plastic volume (which is directly tied to cost at Shapeways) I inset the walls of most of the box, leaving the thicker plastic as a reinforcing lip around the edges and in the clamp locations. The thinner wall sections are 2mm thick. The top and the thick sections underneath the clips are still 4mm. I left the top of the box without an inset so that water doesn't pool on top in the rain. I intend to paint the box soon, because the sintered materal isn't actually waterproof.

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Back view of the real thing, "White, Strong, and Flexible" sintered Polyamide from Shapeways.

The bottom of the box is just open. I considered some kind of hatch arrangement but needed to set that aside because of time constraints. I am going to use a strap or rod across the bottom between two small holes to keep the battery in. In the photos it's just held by friction, but that doesn't work too well on bumps. I wanted a bottom design that didn't allow water to pool inside, and a completely open bottom works for that.

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Box with battery clipped to rack.

The last couple of photos here show the clip arrangement in more detail. It's pretty easy to clip and unclip but it requires an intentional effort and is an excellent rock-solid and rattle-free fit when it's clipped into place. The round part of the clips was designed with no intentional clearance, which generally means some interference.

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This angle clearly shows the three point clip design.

The battery is shown below partially slipped into the case. I've since added a tab of tape to allow the battery to be easily pulled out of the case. It's intentionally a tight fit. I've made some modifications to the original battery. For now I've removed the little LED "fuel gauge" circuit board until I decide on a better and weatherproof way to mount it. I had some trouble with charging this particular battery pack and I've now added a balancing connector to allow a fancy LiPo charger to charge the individal cells properly.

batt_view_sm.jpgThe battery is a PL-7548168-3S-FG 11.1V 6.4Ah LiPo from batteryspace.com.

To hold the battery in, I used a couple of #4-40 bolts and some aluminum standoffs (screwed together with a short stud made from a cut bolt).  I used some low strength purple Loctite to help keep them from vibrating loose. This could be more elegant in a revised design; I think at the very least I'd design in holes and recessed counterbores to make it look cool and I'd make the bottom opening edges thicker to make it stiffer and stronger.  

boxend_sm.jpgBolts and standoffs to hold the battery in.

This project was a nice first 3D printing project. I thoroughly read the Shapeways documentation and was well prepared to avoid problems in my design, though I did need to let Netfabb fix some slight mesh problems as described in this tutorial. The problem was something that arose when I inset the faces of the clips and could have been avoided in a fresh re-design.

I intend to do a project suitable for an extruder-style machine for my next 3D printing project. We'll see if that happens before or after I actually buy myself a 3D printer... stay tuned.