Low voltage, low temp DC heating element

[dk101]

Hi all,
I put together a few basic circuits back in college, but don't really remember much. I'm trying to figure out how to make a heating element that can be powered by a battery and used to wrap around a container and keep it just a few degrees above freezing. I'll probably add in something to cut power on and off at specified temperatures as well. I don't have any real hard requirements on what battery or electronics I use to do it, although smaller/cheaper is always better, and I do want it to be reasonably portable. Any suggestions on how to get started? Also, I have tried some google searches and searching on this site, but I'm having trouble finding much info about low temp applications like this one. Thanks.

 

[chemelec]

The Heater Depends on Temperature difference, Size of container and Possible Insulation around it.
Insulation is IMPORTANT, If you want LOWER POWER DRAW.

And If Outside Temperature is just a Degree or Two lower, Not much of a Problem.
But if it is 10 or 20 degrees below freezing, Much More of a Problem.

The Heating Element could be Nichrome Wire, or even a suitable length of smaller gauge, High Temperature Magnet Wire.
Since you don't need the wire RED HOT, It is all about Resistance, Current and the resulting Temperature.
I have created Many heating Elements with this Magnet Wire and its Insulation stands up Very good, even at temperatures that burn your fingers.

You also need a Circuit to Control this current flow, based on Ambient Temperature or the Containers Temperature.

I have a fairly simple circuit that may work.

 

[Reloadron]

I would start with a Google of "silicon heating mat" which should get you to manufacturers like Watlow. Then start looking for lower voltage units and assorted control used with these mats. Next you need to take into consideration everything chemelec covered. The problem with most heater designs and batteries is making heat using electricity takes power and depending can take a lot of power. This makes a need for larger batteries and you can see where this is going. You can find heaters used in automotive for things like heating baby bottles that work on 12 volts. Anyway, start with thinking about everything chemelec mentions.

Ron

 

[chemelec]

dk101, You can probably Make a good heating coil, Much Cheaper than Buying one.
Where do you Live, Just your Country?

 

[Externet]

'Low' temperature means nothing. Give degrees. 'Small' battery means nothing. Tell what you have or plan.

Try a plain incandescent light bulb, of the wattage that does the desired effect, at the voltage that does it, with a battery that does the desired effect.
No need to play with nichrome windings nor complex devices.
If the battery is an automotive/motorcycle one, use automotive bulbs or cigarette lighter.

A silicone coated waterproofed incandescent light bulb base and soldered connections allows to submerge the bulb for intimate contact to liquid. Insulate the container and all heat losses paths.

 

[chemelec]

With the High Temp, Insulated Enameled Wire, It can Wrap around the Container, Giving a Nice Even Heat.
Than Covered with some Insulation, makes it Even Better to Retain this heat.
And it is Cheap.

 

[dk101]

Thanks for all the input! I'll try to answer the questions that were posed. I live in the US. I think I answered the question about degrees in my original post, but to reiterate, I would like to keep it just a few degrees above freezing. It can be hotter, but does not need to be. I plan to monitor the temp with some type of sensor and automatically switch the unit on and off to keep it in an acceptable range while conserving power. As for battery sizes and insulation, I definitely plan to insulate the unit well, and I would like to keep the size of the battery to something smaller than a car battery. I don't really have a specific size in mind, just something that the average person can carry pretty easily. Maybe keep all battery dimensions to 6 inches or less? Just kind of making most of this up as I go, so it's all pretty flexible.

 

[chemelec]

dk101, If you have more Specifics on How COLD the Air Might be and What Size and type of a Container it is?
Than I could give you Much better Detailed info.

Or if you want to Discuss this by Phone, I have FREE Long Distance calling to the USA.
Just email me with your number.

 

[gary350]

How many Watts do you need?

Will a children's riding toy yard car rechargeable battery work for you, dimensions are about 3"x 6"x 6" ?

I have done a lot of work with electric heaters over the past 45 years. You can buy heating elements of all sizes, shapes, lengths, watts, voltage, and special order heaters too. You can buy heaters with a built in thermocouple for temperature control. You can connect a 120v or 240v heater to a 12v battery and it will get warm. What physical heater size will work for your project?

You can connect a heater, on/off switch, fixed temperature thermostat and battery in series for a very simple circuit. A cheap low cost $3 thermostat will hold the temperature plus and minus 5 degrees F each side of the target temperature for a small light weight portable circuit.

This is a photo of a cartridge heater with threads, you can buy then with no threads, 3/16" diameter minimum diameter, length 4" to 20 feet long, any voltage any watts you like. You can by these on Ebay very reasonable prices.

Your not going to find may adjustable thermostat for heat that goes below 50 degrees. Look for refrigeration thermostats for lower temperatures.

**broken link removed**
Hot water heating elements come is all sizes and watts.

Check out this like for heaters.

https://www.google.com/search?q=hea...X&ved=0ahUKEwj-rfuH3u_JAhUCKWMKHUPsB3kQsAQIJw


This is a fixed temperature thermostat.

 

[dk101]

The battery you suggested should be fine as far as size/weight. The container I am planning on heating is along the lines of a metal can of shaving cream or a can of spray paint. It will likely be outside so I'm not sure about what temps will be. I would like to wrap something like the insulated enameled wire that was suggested around, or at least in the area around, the container to achieve fairly even heating (so I'm not sure about using something like the rigid elements that you suggested, gary350). I just need to keep it from freezing. If we need to assume some coldest possible air temp, lets go with zero degrees F. That should be a good starting point.

 

[alec_t]

What is inside the container? Don't forget you also have to maintain the heat of the contents, so you need to consider the total thermal mass of insulation + container + contents, as well as the U-values of the container walls and insulation. A 35F temperature differential may need more heat than you'd think.

 

[dk101]

Contents thermal properties similar to water. I'm trying to do some basic calculations with it. Guessing between 0.2-0.25 kg of liquid.

 

[chemelec]

I Would really Recommend a Wire Wrap, Like I suggested, With Insulation Over it.
It will give a NICE EVEN HEATING with little Loss of wasted heat.

 

[Reloadron]

The battery you suggested should be fine as far as size/weight. The container I am planning on heating is along the lines of a metal can of shaving cream or a can of spray paint. It will likely be outside so I'm not sure about what temps will be. I would like to wrap something like the insulated enameled wire that was suggested around, or at least in the area around, the container to achieve fairly even heating (so I'm not sure about using something like the rigid elements that you suggested, gary350). I just need to keep it from freezing. If we need to assume some coldest possible air temp, lets go with zero degrees F. That should be a good starting point.

If you just want to keep something from freezing just for the heck of it try a google of "automotive baby bottle warmer". Think a 12 volt DC tiny electric blanket which can be had relatively inexpensive, maybe less than rolling your own. Considering my kids are in their mid 30s it has been awhile since I used one but they worked well for warming baby bottles and could likely easily be tailored for what you are looking to do.

As to calculating heaters and such you are looking for watt density:
You can start here. That link has an overview but I doubt you need to get that fancy.

Ron

 

[chemelec]

I find that the "You can Start Here"gets Quite Confusing for his application.

Just a Simple Wire Coil around the bottle, With a suitable Resistance Value of a PTC Thermistor in Series with it would work Very Good.
As the Temperature Rises, So does the Resistance of the thermistor, Till it Reaches a Stable Medium at a suitable temperature.

Both the Coil and the Thermistor Add Heat to the bottle.

Really CHEAP and SIMPLE to construct.

 

[Externet]

Use one or more fish tank heater. If not available on 12V; choose a high power 120VAC one fed by 12V.

----> http://img1.banggood.com/thumb/large/upload/2012/lixia/SKU095412_2.JPG

Or, a tea warmer

----> http://g03.a.alicdn.com/kf/HTB1z7FM...sion-font-b-Heater-b-font-Tea-Coffee-font.jpg

Or. a film heater

----> http://i01.i.aliimg.com/wsphoto/v0/...ument-auxiliary-font-b-heater-b-font-font.jpg

 

[Reloadron]

I find that the "You can Start Here"gets Quite Confusing for his application.

Just a Simple Wire Coil around the bottle, With a suitable Resistance Value of a PTC Thermistor in Series with it would work Very Good.
As the Temperature Rises, So does the Resistance of the thermistor, Till it Reaches a Stable Medium at a suitable temperature.

Both the Coil and the Thermistor Add Heat to the bottle.

Really CHEAP and SIMPLE to construct.

I started with simple. I only added watt density if the original poster was curious. I made that pretty clear I thought. There are likely a dozen ways for the original poster to acheive their end means. They certainly have enough suggestions to help with making a decision.

Ron