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Wall-Powering Devices With 3D-Printed AAA Batteries
Published by KermMartian on September 16, 2015 at 3:59:13 PM CST | Discuss this article (24)

Every year at World Maker Faire, we face the challenge of powering a dozen graphing calculators for two long days. While the newer calculators with rechargeable batteries can be powered over USB, the older calculators can only be powered with AAA batteries. Since 2012, our solution has been to use dead AAA batteries to wedge wires against the positive and negative terminals of our calculators' battery compartments, then connect those to a 6V cell phone charger. However, as the Faire progresses and young visitors try to pull our displays closer to them, these wires would sometimes get pulled out. This year, with two of our members in possession of 3D printers, we decided to try something more polished and more reliable.

The Idea
To power our graphing calculators more reliably at World Maker Faire, we decided we wanted to be able to attach the wires from our external power source to some kind of battery-shaped insert. Once I convinced my co-conspirator Tim "geekboy1011 Keller that soldering wires to dead AAA batteries was a bad idea, we began to explore how we could 3D print battery forms into which wires could be added. 3D printing a cylinder is not too challenging, but we needed to figure out how we could also add reliable electrical contact to the device's battery holder. We came up with the idea of using screws and washers: we would use a very small screw sticking out of the positive end for the positive terminal, and print a "pocket" of sorts for a washer for the negative terminal. Our first step to do this was to determine the dimensions of a AAA battery, build a model, and select screws and washers. We are using the following:
  • Positive terminal: 0.5" long pan-head slotted machine screws, size 0-80 (2.94mm head diameter)
  • Negative terminal: number 4 washers, 3.18mm inside diameter, 7.92mm outside diameter
  • Negative terminal: 4-40 Phillips machine screws (4.76mm head diameter, to retain washers)
By attaching wires to the two screws, we could make solid electrical connections to the devices' battery terminals. With these in mind, we set out to design a 3D-printable design for a battery that could hold these screws and washers.

McMaster-Carr parts for 3D printed AAA batteries (left); finished 3D model (right). Click to enlarge.

The Execution
Although we use other 3D modeling software for more complicated projects, we used TinkerCAD (from Autodesk) to model our AAA batteries. It offers easy-to-user Boolean modeling operations, so it took us adding and subtracting a number of cylinders and rectangles to get to the model shown above and below. This model required a bit of fine-tuning, both to perfect the model and to get it to 3D print properly. Both geekboy1011 and I have XYZ Da Vinci 1.0 printers, so we each attempted to print the model. The first and second iterations (the first two images) below were printed horizontally on my printer, and both are slightly lopsided as a result of insufficient support material added by the slicer. The washer pocket on the first version was too small to fit our washers, and wasn't thick enough to stay together during printing. The washer pocket was deepened and thickened on the second version, but the print quality was still subpar. Geekboy1011 tried printing the batteries vertically, with the negative terminal against the printer bed and the positive terminal at the top. Since the quality was much better this way, the third and fourth versions were sliced and printed vertically, using Cura rather than Slic3r for slicing. The only difference between versions 3 and 4 is a deeper cylinder of plastic behind the positive terminal to give the 0-80 machine screw more support plastic. The fourth version below worked well once the screws and washers were inserted, so wires were soldered to the positive and negative terminals of two batteries, and successfully powered the blue TI-83 Plus in the photograph below.

If you'd like to 3D print your own AAA battery inserts to power your own devices, the STL and a readme file with further instructions are available at the link below. We'd love to hear your feedback if you try printing these, and come see us at World Maker Faire in New York City if you'd like to see these in action.

3D-Printable AAA Battery Forms

From left: first, second, third, and fourth iterations of the batteries, along with the final product in a blue TI-83 Plus. Click to enlarge.

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