An update? What?!
There’s plenty of hack-worthy update material lurking around my (new!) workbench, (not to mention several skateboard upgrades), and it’s been over a year since I’ve had time to post on my own site—it’s hard to keep this place interesting while I’m busy writing for Hackaday.
But you’re not reading this to hear excuses.
I’ve had this Gerbing heated vest for a few years now. I got it on sale a few years ago and quickly discovered that despite buying the smallest size, I was swimming in the thing. Using it as a base layer was out of the question. To be fair, the vest did warm me up, and I could hide it under a larger jacket, but the heat output felt lackluster at best, and I had to hug my sides to determine whether it was still working from time to time.
If I could trim the bulk and stick the heating elements inside a closer-fitting layer, this would be a daily gadget for the cold season. Time to rip a seam and dig out the coils.
Inside were four heating pads: two in the front and two in the back. I stared at them for about a week, and then put them in a box. I thought about what garment(s) would be best for a base layer. Which would insulate well and allow me to easily attach these pads? How do I make the pads removable to wash the base layer? I couldn’t made up my mind. The box sat for ~2 years.
With another winter looming, I wanted to give it another shot. No overthinking the design this time. Just make it work. I found a snug-fitting undershirt and had Jenny sew some snaps onto both it and the pads.
I wasn’t sure whether the snaps would work for positioning the pads, or if they would be strong enough to hang on. No catastrophes so far.
Putting the shirt on is easy. Taking it off is a smidge perilous. I added some tape to tame the wires for now. I don’t want to sew them in—that would defeat the “removable electronics” function. I’ll look into better solutions later, though suggestions are welcome.
So, that’s it? You just stuck some wires into a shirt and called it a hack…and you didn’t even sew the snaps on yourself?!
Fear not! My typical fury at a minor annoyance led to the complete overhaul of the battery pack, and hopefully justifies this post. Nothing earth-shattering follows, though, other than my workflow for complex 3D printed parts. This post may ramble.
The Gerbing battery is a palm-sized pack with two 18650’s in series and a PCB with a few key functions, including an on/off button, adjustable power settings (via PWM@ 25, 50, 75, & 100% duty cycles), and it has the necessary charging circuitry for the Li-ions. There’s nothing wrong with the pack. Not really. In fact, it’s pretty neat.
My gripe is how long the battery lasts (under 2 hours when it’s on 100%) and how long it takes to recharge (3-4 hours from dead). Again, those numbers are tolerable. Decent, even. But you know what would be better?
Say you had a couple of extra 18650’s lying around…
Maybe you have more than a couple. Sure, some of them can’t hold much of a charge and a few of the Ultrafires are probably imitation 18650’s with junk innards, but many..many of them are in full working order, and I have more lithium chargers than I can shake a soldering iron at.
The Gerbing pack is sealed up and the batteries are soldered in. Additional packs are $25-50. No thanks. I would rather charge up a handful of 18650’s, throw them in my bag, and replace as necessary. The walk to my office at the university takes 30 minutes each way, and over the course of a day I’m sure to encounter at least one poorly heated classroom (good luck controlling thermostats on campus). As an added bonus, I can keep the thermostat down at home and stay toasty for significantly less kWh usage. I want the ability to pop 18650’s like an addict. This battery pack needed an overhaul.
I started simple, modeling a rough base shape with recesses for the 18650’s, then adjusting for shrink and adding some wiggle room. Print a test piece. Test it. Hey, it works. Now to build out the rest.
A 3D printing workflow can be finicky. After you measure the components and CAD up a piece, I’ve found that it’s a good habit to print early and often, refining your design along the way as necessary. My battery pack would need close-fitting sliding parts and some wiring kung-fu, and it’s difficult to predict what minor differences a printer might spit out. Even a well-calibrated printer has to deal with ABS shrinkage. Sometimes you forget to include a feature. Sometimes your perimeters come out wrong and you have to tweak the widths. Iterate, I say!
My rough pack design has two halves: a “column” and a “sheath.” The finished piece is fairly complex (for an FDM print) because of what I need to accomplish. The new pack must:
- House the original PCB and retain button / LED / power jack functionality.
- Open to remove / replace 18650’s as necessary.
- Have the option to clip onto a belt, etc.
- Securely enclose all connections out of sight. It doesn’t have to be waterproof, but there should be zero gaps and the pieces should fit snugly.
I worked out the design on paper and started by printing the rough shapes for various pieces. First, I got the endcaps sorted and tested how they fit with the PCB housing. I’m designing with zero expectation of printing the entire battery pack in two easy chunks. Instead, I want to get everything out in pieces to play with shape and fit. Afterwards, I’ll refine the design and then use a (hopefully) familiar cheat to assemble the pieces.
You can see how the bits came together below. The column piece holds the PCB vertically and the sheath slides over the column.
Rather than print the entire sheath out and hope it will fit, I printed a couple of cross-section pieces and tested the slideability. Once I got the rough sheath shape worked out, I finished up the rest of the column.
I didn’t remember to take progress pictures of how I built out the battery springs, but it’s pretty straightforward. I soldered new wires from the PCB to two springs and enclosed the wiring beneath an insert, which holds everything in place.
Now for the cheating. The more complex your design, the less likely you will be able to print it in one (or two, or three…) pieces. I don’t mind toiling away in CAD to overcome the printer’s limitations, but some builds aren’t worth the trouble.
There’s an easier way. Grab your bucket of acetone (if you print with ABS you should have some ABS juice handy. That’ll be smashing). With the individual column parts complete, I fused them by painting each with acetone and mashing them together. Be forewarned, the finished product won’t look flawless unless you take your time and avoid painting exterior sides. I don’t care how the pack looks, though, I just want it to work.
Fusing pieces is simple. Grab two parts that fit together and paint any point of contact between them with acetone, then clamp them together. Let it sit for about thirty seconds and you should have a sturdy new part to work with. The assembled column below is comprised of 4-5 pieces: the rectangular PCB enclosure box, the vertical “rail,” the endcap with recessed circles, and an insert (exsert? technically it’s beneath the endcap).
For some added strength, you’re better off doing some friction welding with a rotary tool. You’ll end up with some bumpy bits here and there, but they can be sanded down. If the gaps between parts are too large for acetone to bridge them, friction welding is probably the better option.
Now to finish up the sheath. The cross-section received one final revision to include a slot for the clip, then I printed the whole sheath except for its endcap, which needs the same battery spring assembly as the column (Again, no pictures. Sorry!) I realized at this point that I had no easy way to remember the orientation when replacing the batteries, so I printed a red circle to indicate polarity and then assembled the sheath the same as before: acetone fusion, baby.
The end of the sheath is where the batteries connect in series. Normally that’s not a big deal, but if I wanted to keep the same wiring as the original pack I needed to find a way to tap in and connect back to the PCB. The solution was to place two exposed tabs that fit together when the sheath covers the column. You can see one of the exposed tabs in the last picture above, (between the red and black circles), and in the second image below. The column’s tab is in an earlier picture a few paragraphs back.
I designed the column to gradually increase in thickness toward the end, which securely wedges it into the sheath and keeps the pack closed. That’s….about it. Here are some final pictures.