Questions about DC heating circuit

[jpanhalt]

Using a battery in this design can be viewed as converting electrochemical energy into electricity into heat. Why not cut out the middle man, electricity?

For many year, heaters based on the catalytic conversion of a fuel (sometimes ethanol or methanol) to heat. A quick search led to this flameless, propane fueled heater: **broken link removed**

One advantage to using a fuel is that the "recharge" time is quick and the heat produced per pound is probably going to exceed what you can get with a battery. A little flask of ethanol fuel is easy to carry around, inexpensive, and relatively non-toxic. It doesn't explode like lithium batteries, and it doesn't leak acid or lead salts like lead-acid batteries can do.

You could also skip the whole heater concept and add the ethanol directly to the coffee for a little warm up.

BTW, that Coleman heater is not an example of something you can use directly for coffee. I provide it only as an example of the technology.

John

 

[SaraBMB]

Did you check its physical size? I used a 12v 4.5Ah battery in a motorcycle, it was about 6" x 3" x 4" high and weighed about 3 pounds. It's hardly goign to "fit into the base" of your portable coffee heater.

Also I think you might be still underestimating the amount of energy needed to heat the coffee, and underestimating the heat losses.

30W will keep the coffee hot ok, if applied well to the cup and the cup is in some insulating (styrofoam?) holder that surrounds at least the bottom half of the cup.

But if you still want to heat the coffee up from cold to drinking temperature, 30W is still too little power especially if your styrofoam insulating system etc is less than perfect. We worked out earlier in the thread that would take hundreds of watts, depending how quickly you want to heat the coffee and how it the final temp is.

Hello Mr RB,
Don't worry the battery size I found is acceptable, it's 4"*3"*3", it can fit underneath my mug. The amount of energy might not be exactly what's needed, I agree with you. However, it can still work for me. Maybe it's not perfect, but it's still good. Thank you.

 

[SaraBMB]

Hi Sara,


Well when you connect resistors in parallel the total resistance decreases. So if you need 5 ohms and you are going to use three resistors, then 3 times 5 is 15 so you need 15 ohms for each of the three resistors.

The type of wire wound resistor only needs to be able to handle the power, and because of the way we are doing it each resistor needs to handle 1/N times the total power multipled by 2, so for three resistors that would mean 30*2/3=20 watts each resistor. The bolt down resistors are easy to use when you can bolt them down to some metal like aluminum. But in any case you may need to insulate the bottom so that most of the heat goes to the top surface to heat the liquid.

Hello again MrAl,
Thanks for the clarification! I think I am ready to start the project now.

Here is everything I am going to use and do, so you can assure me if I am going on the right way.
Okay so the circuit now will consist of:
1- Sealed Lead Acid battery that has 12V and 4.5 Ah. Like this one in Amazon:
https://www.amazon.com/UltraTech-UT...sr=1-18&keywords=Rechargable+Gel+Cell+Battery
2- Three resistances in parallel, each one of them is 15 ohms and able to handle 20 watts.
I will insulate the bottom of the three resistances and attach the top into the metal surface. Is that correct?

If it's correct, then there is another thing I would like to ask you about if you may. I was thinking of recharging my battery by solar energy! So whenever I walk around with my mug in a sunny day, it can charge the battery a bit. To charge it fully I can leave it outside for a long time till it charges for example. I made a diagram combining the circuit I consulted you about (the heating circuit) with another circuit that charges battery by solar energy. My idea is to cover my mug with a flexible solar panel, like these: https://www.siliconsolar.com/product/flexible-solar-panels.html and attach it to the battery I am using for the heating circuit.
The diagram of the whole thing is attached to my post, please tell me what you think.

The components of the charging circuit are:
First, a high voltage solar panel, higher that the battery I am using but I don't know how much exactly yet, maybe about 20V. Second, a resistance. Third, a voltage regulator chip LM317T, so it regulate the voltage produced by the solar panel and doesn't allow the battery to heat up.

Waiting for your opinion

 

[audioguru]

I bought a solar panel on sale for $15.00 (half price). It is designed to trickle charge a car battery.
Its current is only 0.2A at noon on a sunny day when pointed directly at the sun so it will not need an LM317 current regulator to slowly charge your battery.
It will charge your battery in about 4 or 5 days.
Its size is 18" long by 6" wide so you will not want to walk around with it on your head.

The 15V/50mA flexible solar panel you found is also huge and will have such a low current that it will be useless.
A small solar panel to cover your mug will also be useless.

 

[Mr RB]

Hello Mr RB,
Don't worry the battery size I found is acceptable, it's 4"*3"*3", it can fit underneath my mug. The amount of energy might not be exactly what's needed, I agree with you. However, it can still work for me. Maybe it's not perfect, but it's still good. Thank you.

Ok, cool, you understand it's a big heavy battery.

Before committing to a final design and build have you considered doing some testing?

I've built a lot of hotplate/heatbox type devices over the years and it's a really good idea at the start to get thermal data on the actual physical device.

What I would do is get a few cheap resistors, glue them to the bottom of a former that will hold your coffee cup, then connect it to a typical variable voltage DC supply and do some testing with a thermometer. That will give you vital information as to the amount of voltage and current you need, and amount of insulation you might need to add. Once you have that thermal data you can easily choose the right resistors to suit the battery etc.

 

[MrAl]

Hello again MrAl,
Thanks for the clarification! I think I am ready to start the project now.

Here is everything I am going to use and do, so you can assure me if I am going on the right way.
Okay so the circuit now will consist of:
1- Sealed Lead Acid battery that has 12V and 4.5 Ah. Like this one in Amazon:
https://www.amazon.com/UltraTech-UT...sr=1-18&keywords=Rechargable+Gel+Cell+Battery
2- Three resistances in parallel, each one of them is 15 ohms and able to handle 20 watts.
I will insulate the bottom of the three resistances and attach the top into the metal surface. Is that correct?

If it's correct, then there is another thing I would like to ask you about if you may. I was thinking of recharging my battery by solar energy! So whenever I walk around with my mug in a sunny day, it can charge the battery a bit. To charge it fully I can leave it outside for a long time till it charges for example. I made a diagram combining the circuit I consulted you about (the heating circuit) with another circuit that charges battery by solar energy. My idea is to cover my mug with a flexible solar panel, like these: https://www.siliconsolar.com/product/flexible-solar-panels.html and attach it to the battery I am using for the heating circuit.
The diagram of the whole thing is attached to my post, please tell me what you think.

The components of the charging circuit are:
First, a high voltage solar panel, higher that the battery I am using but I don't know how much exactly yet, maybe about 20V. Second, a resistance. Third, a voltage regulator chip LM317T, so it regulate the voltage produced by the solar panel and doesn't allow the battery to heat up.

Waiting for your opinion

Hello again Sara,


Everything sounds good so far but im afraid the solar recharge part is going to cost you a small fortune. That's because you'll need a decent solar panel that can put out at least 400ma at the required voltage of around 14.2 volts or higher. Since the max power point of a solar array is not at it's open circuit voltage, you probably need a 20v panel or better (as you noted) that can put out 600ma short circuit current. That puts the wattage of the panel at 12 watts.
If you try to use less than 400ma it may charge a little, or it may not charge much at all. Also working against us here is the angle of the sun relative to your panel front surface. The power that gets to the panel varies as the cosine of the angle, so at 12 noonish you get full power and any other time you get less unless you have the panel track the movement of the sun.
Anyway, taking that into consideration and assuming we get 8 hours of decent sunlight, we'll probably see an average of 300ma over the 8 hour period if we are lucky, and also that the battery charges at least some with lower current than 400ma, and also the charge acceptance of lead acid, it will take 3 or 4 days of bright sunlight to charge the battery even with a panel that good (20v, 600ma assuming the max power point is close to 14v at 400ma).
So this starts to sound a little expensive unless you can get cheaper panels on sale or something. And it may end up being a little bigger than you want to have to carry around with you

Are you into hand crank generators ?

 

[jpanhalt]

Hello again Sara,
Are you into hand crank generators ?

That would kill two birds with one stone. Exercise equivalent to a nice brisk walk while keeping your coffee warm.

Source = Wikipedia
On firm, flat ground, a 70 kg (150 lb) person requires about 30 watts to walk at 5 km/h (3.1 mph)

John

 

[Mr RB]

Or a "tealight candle", cheap, small, portable, instantly "rechargeable" and produces about 40W according to Wiki;

**broken link removed**

 

[audioguru]

Try walking around outside on a windy day carrying a candle.

 

[SaraBMB]

I bought a solar panel on sale for $15.00 (half price). It is designed to trickle charge a car battery.
Its current is only 0.2A at noon on a sunny day when pointed directly at the sun so it will not need an LM317 current regulator to slowly charge your battery.
It will charge your battery in about 4 or 5 days.
Its size is 18" long by 6" wide so you will not want to walk around with it on your head.

The 15V/50mA flexible solar panel you found is also huge and will have such a low current that it will be useless.
A small solar panel to cover your mug will also be useless.

audioguru,
I didn't consider the sizes of the solar panel before now, I didn't think that they are this big :\
I tried searching them after your post but the best one I found with a reasonable size was 6V and 100mA, which is useless as you mentioned..
Thanks for notifying me about that.

 

[SaraBMB]

Hello again Sara,


Everything sounds good so far but im afraid the solar recharge part is going to cost you a small fortune. That's because you'll need a decent solar panel that can put out at least 400ma at the required voltage of around 14.2 volts or higher. Since the max power point of a solar array is not at it's open circuit voltage, you probably need a 20v panel or better (as you noted) that can put out 600ma short circuit current. That puts the wattage of the panel at 12 watts.
If you try to use less than 400ma it may charge a little, or it may not charge much at all. Also working against us here is the angle of the sun relative to your panel front surface. The power that gets to the panel varies as the cosine of the angle, so at 12 noonish you get full power and any other time you get less unless you have the panel track the movement of the sun.
Anyway, taking that into consideration and assuming we get 8 hours of decent sunlight, we'll probably see an average of 300ma over the 8 hour period if we are lucky, and also that the battery charges at least some with lower current than 400ma, and also the charge acceptance of lead acid, it will take 3 or 4 days of bright sunlight to charge the battery even with a panel that good (20v, 600ma assuming the max power point is close to 14v at 400ma).
So this starts to sound a little expensive unless you can get cheaper panels on sale or something. And it may end up being a little bigger than you want to have to carry around with you

Are you into hand crank generators ?

Hello again MrAl,
um yah I see now :/
Maybe I can afford the cost, but the size is a huge problem for me. So yah I think the idea is not worth it. Thanks for all the info..

I like the the hand crank generator idea very much! It it is an environmental friendly way of charging too and it is easier to afford.
May you please tell me more about it and how it can fit in my circuit?

 

[SaraBMB]

Mr RB, I like your idea of testing, but problem is that I don't have any materials at the current time. I want to buy whats needed for my circuit only, thats why it is important for me to know the values from now. Thanks anyways. and LOL about the candle idea! haha imagine that! specially at a windy day like audioguru mentioned

jpanhalt,
yes exactly! that's why I like the idea very much. Thanks to MrAl!

 

[MrAl]

Hi again Sara,


I do like the candle idea thanks to Roman, and we'd just have to figure out how to prevent it from being bothered during windy days.

As jpanhalt mentioned, the hand crank would give a little exercise too yes, so somewhere in the world we'll one day see this nice girl walking around with these huge Popeye forearms

Seriously though, they used to use hand crank radios quite a bit so that you didnt need power like when a storm hit and you needed emergency radio. You'd have to find one that can put out around 400ma, and that might be a little bigger than you want to have to carry too, but that's life i guess. Another idea would be to ride a bike and have a generator on the bike that charges the battery, if that's practical. Normally you would just charge the battery when you got home that evening with a wall wart.

Just to note, i keep a hand crank flashlight around because we get bad storms around here sometimes that knocks out the power and i dont want to ever be without a light. I have many lights ready for use, but the hand crank light doesnt need batteries. It has an internal Li-ion battery (very small battery though) that charges with the crank. The crank maybe puts out 100ma that's about it.

 

[SaraBMB]

As jpanhalt mentioned, the hand crank would give a little exercise too yes, so somewhere in the world we'll one day see this nice girl walking around with these huge Popeye forearms

Seriously though, they used to use hand crank radios quite a bit so that you didnt need power like when a storm hit and you needed emergency radio. You'd have to find one that can put out around 400ma, and that might be a little bigger than you want to have to carry too, but that's life i guess. Another idea would be to ride a bike and have a generator on the bike that charges the battery, if that's practical. Normally you would just charge the battery when you got home that evening with a wall wart.

How big could they be?

 

[MrAl]

Hi,


What the crank or the forearms?

The crank is whatever you can find that puts out 400ma min.

 

[SaraBMB]

Hi,


What the crank or the forearms?

The crank is whatever you can find that puts out 400ma min.

LOOL! Well it would be good to know how big my forarms will be as well

Umm I see, I'll try searching for a good crank then. I hope I find a reasonable size.
Thanks a lot MrAl :3

 

[ClydeCrashKop]

You made me think of this Zippo hand warmer.
**broken link removed**
The right size and wind resistant. They don't say how hot it gets.
Provides warmth via a flameless catalytic burner; simply fill the warmer with Zippo lighter fluid (sold separately) and hold a lighter to the burner for a few seconds to light.
Once the hand warmer is lit, stash it in the included fleece bag and then hold it between your hands or keep it in a jacket pocket for warmth throughout the day.
3.9 x 2.75 x 0.5 inches 2.8 ounces
https://www.rei.com/product/838510/zippo-hand-warmer
They may be extinct or hard to find now because of the chemical type hand warmers.

 

[SaraBMB]

Hello again all,
I was searching for the materials in some electronics stores. I found a reasonable size battery that is 5 Ah instead of 4.5. If I used it, would I need to change my calculations and buy different resistances values? or is it okay keeping them the same, total of 5 ohms, cuz 0.5 Ah doesn't make much difference?

 

[audioguru]

Hello again all,
I was searching for the materials in some electronics stores. I found a reasonable size battery that is 5 Ah instead of 4.5. If I used it, would I need to change my calculations and buy different resistances values? or is it okay keeping them the same, total of 5 ohms, cuz 0.5 Ah doesn't make much difference?

Are you still looking at an old fashioned very large and heavy Lead-Acid battery?
Does your cell phone use one? NO!

Modern batteries are LITHIUM!! They are small and light weight.

 

[SaraBMB]

Are you still looking at an old fashioned very large and heavy Lead-Acid battery?
Does your cell phone use one? NO!

Modern batteries are LITHIUM!! They are small and light weight.

Waht do you mean by that, what are you suggesting? As far as I know, those lithuim batteries are not able to provide me with the ammount of power i need.