You might remember that I have a Traxxas Stampede VXL 4×4 RC truck.  (Then again, you might not.) It’s a pretty fun toy I bought a couple of years ago, and the other day I decided on impulse that it was time for a more powerful battery.  I was a little too impulsive as it turns out.  The additional batteries I bought were WAY to big.  Like ~17mm longer than the battery space.  Rather than do the smart thing — return them for the right size — I decided to take this as a challenge: could I 3D print an adapter that would let me use these too large batteries in my truck?

Well, it turns out the answer is mostly yes, but getting from never having done an original 3D design to the point where something worked was a interesting process, which is why I’m blogging about it.

I decided I would use Tinkercad.  It’s an easy online CAD program that lets you do some pretty sophisticated designs.  You start off with a blank slate, add shapes and letters which you can then resize and move around in 3D space, and then you can download it.  It turns out your can download for 3D printing or for Minecraft, and this Minecraft capability has created a whole new series of questions for my son and I to answer at a later date.

Since I was new to the software and to 3D design, I decided to start by answering a very simple question: could I build an adapter that would fit over the too large battery and stock battery holder at the same time.  I used the published measurements, and built the simplest thing I thought might possibly work:

Which when wrapped around a battery and connected to the stock battery holder looks like this:

So on the plus side it fit around the battery and my idea of connecting the battery holder through a narrow slot worked as I suspected.  On the downside, the battery still slid around, so this wouldn’t work for a car in motion.

Attempt 2 was really just a longer version of this sleeve, but with a solid end.  This also worked, since it’s basically just a stretched out version of Attempt 1, but in the end the battery would still slide around.

I started from scratch for Attempt 3 and did two designs, one for each end of the battery box.  I reused the public measurements, and then created a 5mm overlap designed to close the box up.  The walls are 3mm thick, so one side of the overlap was 2mm and the other was 1mm. A couple of things I learned here. First, apparently I made a math mistake when I created Attempt 1, because the published depth of this battery — 25mm — isn’t quite right.  It’s more like 27.5mm.  So my battery didn’t fit in the new box. Second, in order for the stock battery holder to work, I needed to leave more space to account for the larger battery’s need to hang over the place where the battery holder clips in to the truck body. Third, the 2mm + 1mm registration on the 5mm overlap was too tight for a 3d printed box.  It took quite a bit of force to get the two pieces together.  Finally, I need to be more careful about printing straight lines that span 45mm and start 20mm in the air.  The printer did remarkably well, but that’s not a recipe for success.

Attempt 4 addressed all of these issues and I’m pleased to say it worked fairly well.  The lipo battery pushed the truck into high gear and my in-the-dark truck vs. fire hydrant accident wasn’t fatal (although that’s probably not because of the battery box).  The 3D model for the back half of the box now looks like this:

 

This version also wasn’t perfect, for example the sides of the battery holder sleeve cracked at some point. I’m guessing that the strain of a relatively heavy battery on top and crashing at speed was a bit too much. My next iteration will have two main design changes. First, the battery holder sleeve will need to be on both sides of the box. This will help to keep it together. Second, the 5mm overlap will be changed to 1.5mm and 1mm to eliminate some of the slack where the box comes together.

Overall, I’d call my first 3D Design Experiment a success.  The design process doesn’t take that long, and printing it out gave me a huge amount of feedback very quickly.  Once I’ve had a chance to do these, I’ll share the designs.  As a side note, the adapter does raise the center of gravity for the truck, which means my already dangerous driving is now even more subject to flips, but that’s a subject for another post.