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
218
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
630
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
630
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
630
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
630
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
3,466
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.
 
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