My son has muscular dystrophy and it is difficult to get a jacket on him. I'm constantly looking for ways to keep him warm.
You can get heated jackets for adults, but they're not easy to find for kids. So, I wanted to make a heated cloth liner that I could layer and put under any jacket to make it warmer for my son.
So, my concept was to make a rectangular piece of cloth with a hole in the middle so I could just easily slide it over his head and it would be the size of his upper body. At first, I was going to use 2 strips of 2 foot long 5/8" carbon fiber tape to heat it, but I changed that to 4 strips of 2 foot long 5/8" carbon fiber tape. I wanted to use the usb in his wheelchair to power it, and I wanted a rheostat to control the temperature and turn it on/off. Note that my concept changed during the making of this !
Step 1: Detailed Concept
The site https://www.carbonheater.us/ has all kinds of useful information for calculating the length of carbon fiber needed to reach a desired temperature based on the given power source.
Once I got the 5/8" width carbon fiber tape, I cut it into 4 strips of 2 foot length each. I used a voltmeter to read the resistance of a single strip and it was at 11 Ohms.
To get the total resistance using 4 strips in parallel, then is 1/(1/11+1/11+1/11+1/11) = 2.75 Ohms
Using I = V/R with voltage at 5V gives a current of 1.81 Amps which lines up with the graph for .6 meters.
I should expect to see a temperature of 33 degrees C or around 91 degrees Fahrenheit.
Step 2: Material List
Rheostat Temperature Controller
Silver Epoxy Adhesive
I didn't initially use these:
Step 3: Make a Pattern and Cut Out Liner
First I made a pattern for the liner out of paper and tried it on my son before using the pattern to cut the liner out of fabric. I cut two pieces out.
Step 4: Sew Carbon Fiber Tape
I wanted to try and distribute the heat from the carbon fiber tape through out the cloth, so I just sewed the strips in four parallel lines along the inside of the first piece of fabric.
I used an 18 gauge silicon flexible wire to connect to the carbon fiber. I think this is the largest gauge you'd want to use, or it will heat up too much. I laid the wire over the carbon fiber pieces and stripped the wire casing at each juncture between the wire and the carbon fiber and left plenty of extra so I could eventually connect it up to a power source. Next, I hand sewed the wire onto the liner to hold it in place. I used a conductive silver epoxy to glue the wire to the carbon fiber. It didn't ever really seem to dry or to hold firmly, so I ended up sewing forward and reverse over the wire with the sewing machine to better hold it in place. I read in other instructables that you really need the silver epoxy to make a good electrical connection between the wire and the carbon fiber tape.
You need to do this step along both ends of the carbon fiber tape. One wire will connect to the (+) and one to the (-) of a power source.
Before working on this project I knew nothing about testing for continuity, but I googled on youtube and learned how to use a multimeter to check for continuity with my wires. I was very happy that everything appeared to be making a connection.
Step 5: Wire Up Rheostat
I found this temperature controller on ebay.
Interestingly enough, the diagram that had on it what wire connects to what was mislabeled -- so pay attention if you get this off ebay! I have the correct diagram here.
The controller has 4 wires coming out of it. I used a crimping tool to put a 2-way connector on the wires that connect to the power source -- and a second 2-way connector on the wires that connect to the wires on the cloth.
I used a crimping tool to put a 2-way connector on the 2 wires coming out of the cloth. This connector connects to the controller wires. If you haven't crimped wires before, there are plenty of youtube videos for how to do it. This was the first time for me. It required patience and glasses :)
The last thing was to wire a usb connector to a 2-male connector so that I could plug this in to a usb port. I'm not going to explain this for reasons that will become apparent after the next step.
Step 6: Test #1
I plugged the 2-pin cloth into the 2-pin female of the controller, and the usb-to-2-pin into the other end of the controller. I plugged the usb into the wall and ... nothing... it didn't heat up one bit. The amperage for the USB said 2.5 amps -- which should have been enough for this project, but it didn't work at all.
At first I thought maybe I had a short, or maybe something wasn't connected. But I double-checked everything and it should have worked.
So, I cut the connector off the cloth to get to the bare wires. I connected these bare wires straight to a 12V wheelchair battery. It heated up right away. So the problem wasn't with the sewing job or the carbon fiber, but with the power source.
I needed to re-evaluate. I was very hesitant to connect directly to the wheelchair battery as a final solution. I didn't want to drain it or cause problems to the wheelchair.
Step 7: New Power Source
I realized that we have lithium batteries for our power drills. I also discovered that these same batteries are used to power Milwaukee heated jackets. Sounds pretty perfect -- it's already being used to power a heated jacket -- so should work on this, right?
I bought a battery holder to hold the battery and it uses a DC plug. So, I took an old transformer and cut off the DC connector. I crimped a 2-pin connector to the other end. Now, I can plug one end into the temperature controller and the other end into the battery holder.
Step 8: Test #2
I fully charged the battery and plugged everything together.
NOTE: I connected a multimeter to the system to check the current. I discovered that when you click the rheostat button to a lower heat setting, it pulses (turns on/off) in order to reduce the heat. I expected it to reduce the voltage (like a potentiometer) -- but it doesn't -- it just effectively turns it on/off at intervals.
Anyway, it heated up quickly. The temperature kept climbing to 115 degrees Fahrenheit. However, it drained the battery in about 20 minutes. Granted, it was an older battery that we had for awhile that is used for our drills.
Still thought it would last longer.
Since it seemed it was working, I sewed the second cut-out onto the first (inside-out leaving one side un-sewn) and turned it right-side out.
I bought a couple new batteries (12V 2500mAh) and charged one. I went for about a 20 minute walk on half setting. It was toasty warm and lasted for the walk. Although, when I was done, I was showing 2 out of 4 lights on the battery tester.
Step 9: Conclusion
My goal for being able to layer this under a jacket (like for instance this one: https://www.instructables.com/id/Adaptive-Jacket-M... ) is doable. I put the liner on, then a jacket. The battery can go in the pocket of the jacket. The temperature button rests on my son's lap.
Technically it works -- and when it does, it is nice and warm. It's like having a heated car-seat on your body.
But, there are two issues:
1. It sucks up the battery very quickly.
2. The crimped connection joints aren't the best. The battery is pretty heavy and when you are moving it jiggles the crimped connection and turns off the controller. This happens mostly for me when I'm testing it out and walking. For my son in the wheelchair, it doesn't jiggle as much and seems to stay on.