I like to go camping in all seasons. For winter camping, a thick sleeping bag and ground pad are important to staying warm. But that wasn't enough on my last trip, and I still got cold at night. While tossing and turning in my sleeping bag trying to get warm, I thought about how convenient it would be to have a rechargeable battery operated heater that is also portable. However, the applications extend well beyond just camping. Sometimes I get cold while working in the garage or basement. Because this is portable, it can be used wherever I am cold. While researching for heating elements, I came across Kipkay's video where he used carbon fiber from http://www.carbonheater.us/ for his heater.
Materials and Tools
- Heater Filament
For this project, I will be using carbon fiber from http://www.carbonheater.us/. It is very soft and feels like normal fabric which means it is also very flexible. I'll be using the 15mm filament which has a resistance of approximately 17Ω per meter.
- USB Battery Bank
I have a YD-T016 solar rechargeable power bank. It has 2 USB ports and an advertised capacity of 12000mAhr.
- Wire Cutters
- Old USB cable
- Soldering iron
- electrical tape
- conductive glue/epoxy
- 0.75A fuse for safety
- Sharpie permanent marker
- Sewing machine -or- needle and thread
- Exercise armband
(optonal) mating connectors for quick disconnect in case of fault
If you don't have a soldering iron, the assembly is still possible. Soldering is preferred because it also adds mechanical strength to the connections.
Step 1: Design the Heating Pad
The location of the heater is important for optimal performance. For this project, I will make a band that wraps around the bicep near the armpit. There are big veins/arteries in your arm that will help distribute the externally warmed blood throughout your body. While sleeping in a mummy bag on my camping trips, my elbows tend to be at my sides. This also positions the heater physically closer to my core. There will be wires involved, but they won't have to run all the way up my legs or arms like they would for heated socks and gloves. Shorter wires will also be less likely to tangle and be uncomfortable.
Let's build the arm band heater first. Since it will be wrapping around the arm, a good starting point would be to use an exercise/sweat band. These are meant to fit around your wrists, so they might be a really tight fit around your bicep. It should fit a little tight so it won't slip, but not too tight that it reduces circulation. A better approach might be to use an adjustable version. After a quick search on eBay, I found a couple options:
- P hot Fashion Sports Wrist Sweatband Tennis Squash Badminton GYM Wristband Gifts
- 5pcs movement Terry Cloth Cotton Wrist Sweat Band Sports/Workout/Running yellow
Battery bank has to be stored somewhere else, but that reduces bulk and weight on your arm
Personally, I don't like the idea of having a bulky battery pack attached to my arm while I try to sleep. So I'll deal with the longer wires connecting to the battery pack.
A comfortable length around my bicep is 14" (35.5cm). If that's the length of the filament, it will have a resistance of approximately 17Ω/m * 0.355m = 6Ω. When powered from 5V from my USB battery bank, it will draw a current of 5V / 6Ω = 833mA. My battery bank can deliver a maximum of 1A, so that still leaves some headroom as a safety margin. But how warm will this get?
Estimating Filament Temperature
Looking at the Temperature vs Length curves on http://www.carbonheater.us/, it is hard to tell what the temperature will be. I entered the same data into Excel and made my own graphs shown above. As you can see in the Temperature vs Length chart and the Temperature vs Power charts, the lines only match up with the corresponding voltages. But how hot will it get if I use a 5V supply instead? After all, it is very common nowadays with USB, phone/tablet chargers, and those rechargeable battery banks. In order to figure the temperature with a different voltage, I played with extra calculations in Excel. I eventually learned that I got a positive correlation between all supply voltages if I plotted Temperature vs Volts/cm. This technique allows one general equation to approximate the temperature for a given voltage and length. To figure out what that equation is, I again used Excel to plot the same chart with the best-fit-line. Keep in mind, however, that this equation is a rough approximation and is accurate only to about ±15%.
So, let's move on to estimate the filament temperature.
With a Volts/cm = 5V/35.5cm = 0.141, I calculated an approximate temperature of 57.5°C (135.6°F). That is a little too warm for me. So I'll increase the length to 61cm which drops the temperature to 37.9°C (100.2°F). Much better!
A filament that is 61cm long has a resistance of about 10.4Ω and will draw about 0.482A from a 5V supply. My battery bank claims to have a capacity of 12Ahr. At this load, my battery bank could theoretically power the heater for about 24.9 hours! In reality, though, I expect the battery life to be much less than this. The capacity of batteries is hard to define because it depends on the amount of current draw and the temperature and it also decreases with age. Performing your own battery life test will be a much better way check the capacity of your specific battery bank.
Step 2: Assembly
Cut the heating filament to length (24").
Mark the filament placement on the armband
I know I want the filament to be 24" (61cm). But the arm band is about 3" wide and 6.5" in circumference (un-stretched). So what will be the best way to attach it? I will be using a zig-zag pattern. This will allow the filament to stretch along with the armband when I pull it onto my arm. Imagine cutting the armband so it can lay flat like a rectangle. The 6.5" circumference is now the length of the rectangle. Look at this picture to see what I mean and how I arranged the filament. With a filament length of 24", I can have 9 equally spaced zig-zags that are 2.667" long. The black dotted line in the middle will not be covered by the filament. This gap allows for clearance to reduce the chances of the filament from shorting.
I drew the zig-zags on a piece of paper and taped it around the armband. I flipped it inside-out and marked the end points using the sharpie. Then I connected them with lines except for one. These will be the two ends of the filament that the wires will connect to. Here's the picture of the marked armband.
Place the heater filament and pin it down
I don't have the filament yet, so I can't show a detailed picture.
Sew the filament onto the sock
Lay the filament down and use pins to secure it in place. At the points, fold the filament over itself. Now that the heater is in place, you can sew the ends down but leave middle parts detached. This helps it to stretch out a little bit better when wearing it.
Connect wires onto the filament and secure in place using conductive glue
For an example of the best way to attach wires to the filament, check this link out for some pictures. I tied the heater wires in a knot that should be sewn into place. This serves as a strain relief to reduce the chances of the wires breaking off from the filament if they're pulled too hard.
Cut the micro end off the USB cable and connect the fuse in line.
As seen in this picture, the heater extension wires are tied together in the middle. Also, you can see the fuse connected in series with the red wires.
Wrap the solder connections with electrical tape.
Step 3: Operation
Once I get some filament, I would like to do the following tests:
- Wrap the heated armband around some rolled up socks to simulate the size of my arm.
- Secure an thermocouple (or MCP9808) temperature sensor directly on the filament.
- Measure and log the heater voltage, heater current, and temperature.
- Determine how it takes to heat up and what the max temp is.
- Determine how long my fully charged 12AHr battery bank can last.
Step 4: Closing Notes
- Over current is handled by fuse. It's there to protect the battery bank and your arm from being damaged.
- Be sure to put the armband on before plugging into power. Otherwise, the filament could short and blow the fuse
- Use higher capacity battery pack so it can last longer. Recharge via solar during the day time.
- Since this is meant for cold weather, it is important to also keep your battery warm. Cold batteries have a harder time delivering the power, so keep the battery pack in your pocket or in your sleeping bag with you.
- The filament and wrap are washable, but only by hand due to the wires and connector
- make custom rechargeable pack with adjustable output voltage to compromise between temperature and battery life
- add micro for automatic timeouts
- add thermistor for temperature control
- Use a USB wall-wart for heat without a battery pack
- In case it still gets too hot, add a quick disconnect makes it easy to kill the power to the heater to avoid possible burns.
Alternatively, an exercise arm band would also work since it is adjustable and also provides a holder for the battery bank.
- Battery bank can be stored on your arm and eliminate long wires, increases bulk and weight on your arm
If I toss and turn, I might get the wires tangled up or possibly disconnect the heater. Therefore, this arm band would be better suited to wear when I'm up and still want extra warmth.
Other carbon fiber filament projects
Step 5: Update - Received My Filament!
Update: 18 Nov 2016
I received my carbon fiber filament, so I decided to update this Instructable. When I was pinning the heater onto the armband, I realized that this arrangement would not really be stretchable at all. When the armband did stretch, the filament did not. This caused the height to shrink and become narrow. This ended up being too tight and uncomfortable, so I redesigned it altogether.
I decided to cut up an old hand towel and use that instead of the armband. Since this isn't a stretchy material, I added some velcro. This provides some tension to help keep this thing in place as well as adjust it for maximum comfort.
I laid out the 24" of filament in a square wave pattern and pinned it down. Click here for a picture. I cut about two inches of insulation off a wire and bent it into an S shape. Then I put it on top of the filament and used a thin piece of wire to sew it down. It's kind of messy, but you can see a close up picture here. All that was left was to use regular thread to sew it down along all the outside edges.
So now that I had it all put together, I hooked up my USB battery bank. Unfortunately, it didn't work at all. It consumed about 300mA of current and didn't get close to being warm enough.
The End...for now
Clearly, something went wrong. My results weren't what I was expecting. I will continue to work on this until I can get a functional armband heater. Once I do, I'll create a new Instructable and link it here.
Update: Rev 2 Instructable
I re-worked the design and got it working. I created a new Instructable for it here: https://www.instructables.com/id/Portable-Personal-Heater-Rev-2/