Automatically adjusting PWM to maintain steady power as battery discharges?

Thread Starter

seanspotatobusiness

Joined Sep 17, 2016
210
I would like to make my own heated clothing that could completely eliminate the need for heating my home (aside from making sure water pipes don't burst - they could potentially get their own heaters for that purpose). I would want it to be easy to power with a battery whilst moving around but also be able to plug it into a PC ATX power supply that already powers a few things in my bedroom.

I don't know how much power would be needed to stay comfortable in a cold UK winter with zero heating of the house but I'm willing to go through some iterations to get it right. I may start with a maximum power of something like 60 watts and of course I will wear plenty of insulation over the top. I am planning to use a PWM controller to drop the power to comfortable levels when 60 watts is too much.

The problem I'm hoping to address in this thread is that when I switch over to the battery, the power will vary a lot (up to 42% or 72% (depending on your perspective) for li-ion batteries) depending on the state of charge of the battery. Of course I could address this by using a buck or boost circuit as appropriate to keep a steady 12 V but that would mean wasting precious power in the conversion. I was wondering whether there was some simple solution (not as simple as just manually adjusting the PWM controller as the battery discharges; I'm not a savage!) that would automatically adjust the PWM controller according to the voltage of the battery?

Thanks for reading.
 

Juhahoo

Joined Jun 3, 2019
256
I just wonder what happens to the house when you cut out the heating in the wet winter of UK... very moist and humid air, will eventually ruin your interior, and eventually your health when all nasty stuff starts growing.

If we forget the house, you need a temperature sensor and control loop that adjusts the power to keep a steady temp, a thermostat. It doesn't care what battery voltage is, it will take power as much is needed to keep a steady temp.
 

MrSalts

Joined Apr 2, 2020
2,377
Many people imagine projects like this for various goals. Most never start. Those who do start soon realize that safety/burns becomes an issue. Then they start putting more temp sensors into the plan, more insulation between skin and heater and finally realize how sweaty they are wearing this stupid thing and give up. Some also realize it takes a bit more research and confidence in their electronics skills before they decide to strap a lithium battery to themselves while the sleep or, more importantly, strap a homemade device that contains a lithium battery to their spouse or child.

Be sure to read the datasheet of your lithium battery before you start. What? No datasheet is available on the cheapest battery on Aliexpress.com?
 

Thread Starter

seanspotatobusiness

Joined Sep 17, 2016
210
Thanks very much. I will be mindfull of the potential for damp and monitor and maybe control humidity.

Using a thermostat is surely the way to go. I discounted it at first, thinking it would be too hard to know what temperature to set it at but now I realise it's arbitrary and I just need to keep going up or down until comfort is acheived. I will use bimetal temperature switches to cut power at say, 60 °C for safety.

At first I thought of a cheap W1209 thermostat module but I think a difficulty I will have would require multiple modules because it's going to be hard at first to know just how much heating I will want in different areas of my clothing and scaling all regions up and down together would mean somewhere hot or somewhere cold if I guess wrong. For this reason, I think I'll make my own thermostat using an ATmega328P with four or five zones that I can control separately and together. I suppose I will just use MOSFETs instead of relays.

I don't have a spouse or child and I don't intend to use this while sleeping; a conventional electric blanket would work fine for that. I don't really want to derail the thread discussing the battery but I will use a quality BMS with balancing and appropriate techniques and quality cells when assembling the battery. Obviously I will never use so much power as to induce sweating; the run-time of the battery is too important to permit power to be wasted on sweat.
 

MrSalts

Joined Apr 2, 2020
2,377
Obviously I will never use so much power as to induce sweating
You are not the first to say that but, once you put heater wires in your clothing, you'll realize you get hit spots so you'll start to put some insulating layers between the heater ribbons and your skin. Then what? Sweat. Inefficiency (because less heat transfers to your body with the insulation between body and heater ribbons.
 

MrSalts

Joined Apr 2, 2020
2,377
I won't be using heating wires, I'll be using wide carbon fibre strips so the heat will be well spread out. Any more baseless assumptions?
I'll assume that:
- your batteries won't last nearly as long as you think they'll last.
- recharching is a bigger PITA than you think it will be.
- durability of your heater strips will be more of an issue than you plan.
- you don't like people pointing out gaps in your plans
- you'll lash-out in your next post.
 

Thread Starter

seanspotatobusiness

Joined Sep 17, 2016
210
You made it a point to be an asshole in your very first post and even went back and edited it to be even more of an asshole (I'll be using Samsung cells bought in the UK) so don't talk to me about lashing out; you're a troll. Pointing out potential problems is exactly what I wanted because it could save me a lot of wasted effort but the things you're pointing out are absolutely baseless and aren't intended to help but derail and insult.

I already calculated the amount of energy in my batteries (150 Wh) so know exactly how long they can last at different power outputs. I don't need or expect it to last all day. Like I said, I will connect to a mains-based power supply when I don't need it to be portable. It won't be in the least annoying to charge. The BMS will take care of the sets of cells when I plug it in. I won't be charging the cells individually.

Carbon fiber is good enough for the Milwaukee heated jacket and they charge hundreds of dollars for that so I doubt it will fail in an unreasonable time. Any breaks in particular fibres will just get conducted around by neighbouring fibres like stranded copper.
 

MrSalts

Joined Apr 2, 2020
2,377
You made it a point to be an asshole in your very first post and even went back and edited it to be even more of an asshole (I'll be using Samsung cells bought in the UK) so don't talk to me about lashing out; you're a troll.

Pointing out potential problems is exactly what I wanted because it could save me a lot of wasted effort but the things you're pointing out are absolutely baseless and aren't intended to help but derail and insult.
if you'd put the details in the first post, people wouldn't have to make assumptions. If you want people to point out potential problems, ya gotta give the details. When you don't, well, people's minds tend to wonder and fill in the blanks in their own - and assume. When we assume, you lash out and call me a turd slicer (see, your version of the body part will soon be deleted and mine will likely stand).

Cheers.
- Watching with interest
 

Ian0

Joined Aug 7, 2020
6,316
Just a thought - does the resistance of carbon fibre change with temperature? If so, you could perhaps measure its resistance to determine its temperature, and you would then get an average temperature rather than the temperature at a specific place where you put a thermostat.
 

Parkera

Joined May 3, 2016
96
I am glad to see that you are willing to go for quality components rather than the cheapest possible. You are after all fundamentally dealing with personal safety when you try to design and make a heated body suit. But - ignoring the two snarky points, what MrSalts says is 100% correct. There is a host of problems with your project, which can be solved, but at a cost. That is why quality heated body suits for hunters sell for $500. I’m not sure what the power requirements would be for a heated suit, but battery power evidently is practical for at least a few hours. Using inverters or converters to change voltage levels would just lower the overall efficiency. The heaters themselves are 100% efficient in that they convert 100% of the power into heat. Just size the resistance of your heaters to work with the voltage available. To keep power demands at their lowest, use the best insulation available on the outside of the heaters and use the least insulation possible on the inside of the heaters. At no point allow a heater to directly contact the skin. Perhaps start with high quality winter clothing as your base and install the heaters between the inner lining and the insulation.

It does not make any difference whether you use nichrome wire, iron wire, Kanthal wire or carbon fibers – it still has to get just as hot. If you want fast heating response (a few minutes at most) to a specific temperature, then you have to dump (relatively) large amounts of power into those heaters. How much power depends on the temperature difference and the time constant which is largely determined by the insulation used in the clothing. To control the temperature you have to use a thermostat. While an on-off thermostat is simple and works, there may be too much hysteresis for precise control of the temperature in this application. That will be dependent on the physical arrangement of the heaters to your body, the quality of insulation, your metabolism at the time, your level of physical activity and ambient temperature (I will assume no wind factors since you are inside your house). You may have to use a PID controller.

To prevent hot spots and burns to the skin and a general overheating of your body, you have to use multiple thermostats located at each pressure point as well as area thermostats. Think hard and long about how many potential pressure points there are. (A pressure point is formed where ever a slight force is pressing against your skin.) This includes your butt, elbows, back, shoulders, arms, etc. Where ever you sit, stand, lean or carry is a pressure point. No need to dwell further, you get the idea. Unless you are the military, the only practical solution is to trade heating response time for safety.

You mentioned using 60°C bimetal switches for control. First, bimetal switches are not particularly precise devices; most have a temperature tolerance of 5° to 7° for the better ones. Repeatability is typically 1 or 2 degrees and degrades over time. Second, a comfortable skin temperature is about ½ of that. 60°C perhaps will not melt your skin off, but it WILL BURN YOU! Use an IR thermometer to measure your skin temperature when you are in a comfortable environment. That is the temperature you want to have under all conditions at that point or area of your body. You will probably notice that every point is not at the exact same temperature. A change in temperature of 0.5°C, up or down, will begin to feel uncomfortable at that location. If the temperature is above the comfort temperature for any reason, you need to find a solution to remove heat from that area. Your comfort tolerance will be a factor of how far you are from that temperature and for how long you are not at that comfort temperature. A comfort map of your body could be coded into a microprocessor and each point appropriately controlled.

The comments made about the care and preservation of your house are very true. Unless a building is heated, it will deteriorate VERY quickly. While the problems caused are associated with humidity (the RH needs to be between 40% and 60%) you can’t control the humidity level without heat. That is why the lowest setting on thermostats is typically about 50°F or 10°C. That setting also prevents freezing of pipes if you can’t drain the water from the plumbing system. If you maintain the heat in the house to 15°C you will save on heating costs and the energy consumption of your heated suit will also be much less.

If you do the research and get real numbers for each of the points I and others have made, you will learn a tremendous amount of knowledge about several different disciplines. Once you work out the tradeoffs and find solutions to each of the problems for your project, you may very well decide to simply purchase a heated hunting suit, but you will know much more than you do now.

Good luck with your project.
 
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