science project help: batteries and heating element

No, the 1500mAH figure really means the capacity of the battery, which has little relation to the maximum current that can be supplied.

A C battery probably can provide 10A maximum (maybe more, maybe less, but lets assume 10A). Then the power output is 10A * the voltage, which is 1.5V. Thus, the battery can provide 15W by itself, flat out.

However, assuming a perfect usage of the capacity, that means the time it can do so is

1.5Ah / 10A = 0.15h or about 9 minutes.

In reality, using a battery in this way will drain it even faster than that.

If you put 10 of these is a series circuit, you can get 150W for 9 minutes.

If you need 30W, then you can do it for 45min before all the batteries are used up.

Unfortunately, your soldering tool is 30W at 120V. That means that the current through it when it's operating is 0.25A, so it's resistance is

120*4=480 ohms. If you put 10 C cells together, it'll be 15V. Thus, the power dissipated by the iron will be

15^2 / 480 = 0.468W. It probably won't even get hot.

Thus, your idea isn't going to work without 120V, which means you need

80 batteries. However, they will last alot longer than 9 minutes at a 250mA drain.

A cigarette gets to between 400 and 700 centigrade. That's pretty hot. However, you might be able to use some kind of resistance wire to get that temperature. There exist electric cigarette lighters, which clearly get this hot, on a couple of AA cells.

--
Regards,
   Robert Monsen

"Your Highness, I have no need of this hypothesis."
     - Pierre Laplace (1749-1827), to Napoleon,
        on why his works on celestial mechanics make no mention of God.

10 x 1.2 is not 120 ....

When batteries are in series, the voltage adds, not the current capacity.

No. It's still only 1500 mAh. It doesn't matter what's in series with them: the cells are flat in 1 hour if 1500 mA flows.

Correct.

10 * 1.2 = 12 volts 100 * 1.2 = 120 volts

In addition to other comments:

What is this "heat" to be used for? Are you trying to achieve the same temperature as the tip of a cigarette, but can accept a smaller area? Or are you trying to get the same net energy output per unit time as is released by a cigarette tip, but don't really care if the temperature is exactly the same in doing that? Or ... ? And how long does it need to operate?

Jon

i want to heat a small stream of air, about 2 leters. - roughly as hot as the smoke of a cigarette. it only has to be on intermitantly for periods of 5 seconds.

thanks for your help guys. max

Two _liters_ of air? It passes by as a stream? What is the volume per second, here? What is the cross-sectional area where the heater is to be?

Is this to actually replace a cigarette in someone's mouth? Is the rate of flow to be determined by someone "pulling" on a tube?

Jon

Hi, Max. Apparently you've already run into one of the unofficial rules of school science projects -- "You can't plug anything in".

There's another rule, which is unofficial but is enforced almost as rigorously -- you can't do anything which has the capability to hurt somebody.

I'd really suggest getting some guidance form the teacher or sponsor, and get some assistance if necessary in changing your project to something else. If you mention that an integral part of the project is generating an airlow at close to 800 degrees F, I'm sure they'll encourage you to try something else.

Good luck, and feel free to come back and ask for help once you've got the next project idea.

Chris

thanks chris but i'm not in high school.

jon, yes, the rate of flow is to be determined by a suction on a tube and i'm not sure what the cross sectional area around the heating element will be.

i was imagining a ceramic tube of maybe a 1/4 inch and a heating coil of about 1/8 inches in diameter.

what are you thinking about?

Sorry, Max. Didn't mean to offend, but nearly all the "science project" posts here have to do with science fair stuff for grade school or high school.

If you could back away from the trees for a minute, try to look at the forest to get an idea of the power you'll need. If you didn't have the line voltage restriction, I would recommend something like the small Ungar electric heat gun I use for shrink tubing. That's 250 watts, which will give you an idea of the current required at 12VDC (20 amps plus). This doesn't look like a job for "C" batteries.

You could probably "home brew" this with a small 12VDC marine battery, a bunch of heavier gauge nichrome resistance wire, ceramic or mica forms to keep the wire in place, and a small 12VDC fan. I would think that something like the nozzle of a heat gun or small hair dryer would be what you want to make. If you have access to a trickle charger, you could use an automotive battery.

If you want to go this route, you'll have to preheat the nichrome wire before you turn the fan on to let it get up to temp. I'm also not sure how much control you can have over the temperature at startup, but with a little experimentation, you can get close to a desired temp for the whole 2 liters of air. Be sure to weld hi-temp electrical connectors to the nichrome wire if you want good non-resistive electrical connections.

Good luck Chris

Is the suction rate fluctuating, as if a person were drawing on it? Or is it mechanically determined so that the flow rate can be predicted with fair accuracy? (The two liters you mention is suggestive to me of our lungs, which if you blow out fully and then draw in to refill might be just about 2 liters.) Does the temperature of the air need to be controlled? Or can it be allowed to vary widely, depending on the rate of air flow across it? How long is the tube and at what point does the temperature need to be established? What range of ambient air temperature at the intake needs to be tolerated?

Well, I'm still just trying to get a picture of the intentions, here. It helps to get an accurate answer, if there is an accurate question in mind. I'm still not even sure if what you are trying to control is the temperature of the air or something else. For example, you may just want the average temperature of the total 2 liters to be some value, but since it is all going to be mixed into some container (lungs, for example), it isn't important to you to maintain this for each tiny bit of flowing air and you can accept a wide variation, so long as the mixed temperature is about where you set it. I've no idea if you can use the mixing chamber, if it should exist, for your temperature measuring point. Etc.

It is simply hard to have any precise thoughts, without all the details in hand.

Jon

thanks chris for your advice. you too jon.

a marine batery would be too big for the device i would like to build. i want to build a portable device, no bigger than a nerf football. i thought making a series of 4 C batteries would allow for the ideal size, but i got tripped up with my equations.

so there is a line voltage restriction. and that restriction is what ever we can fit into a small set of rechargeable batteries. 4D ? 6D? a special order battery? (is there any place to do this?)

no fan is needed. a person will blow into one end of the football, through a heated chamber at the end and want to be able to brown a piece of paper on the other side, but not ignite it. the paper would be held very close to the end of the heat chamber. room temperature air. could be fluctuating, as long as its hot enough to dry out paper. does that satisfy your questions jon about the air temperature?

so chris, is a nichrome wire a heating element? any idea where i can get a wire that draws low current but puts out the most heat? would soldering the wire's together work? i've never welded something so small.

thanks again for the dialogue, i hope i answered your questions well. max

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Hi, Max. Nichrome wire (also called resistance wire) is the stuff used in electric heating elements for space heaters, toasters, hair dryers, etc. The nickel-chromium alloy has a reasonable bulk resistivity, as well as being resistant to oxidation at high temperatures. Also, the bulk resistivity is somewhat constant over temperature range (usually

+/-10 to 20% from cool to red hot). These qualities make it suitable for heating applications. The least expensive source for nichrome wire is discarded toasters, space heaters, hair dryers, and power resistors. Nichrome wire can also be purchased from a number of sources.

Since nichrome wire glows red hot when heated, solder isn't a good idea. It will melt. Also, nichrome wire doesn't "wet". Your solder will just sit on top of the wire like a blob of glue, and will open up the first time its heated. Resistance wire is typically welded to a hi-temp electrical connector in order to get a good connection (even "gas-tight" electrical connections suffer when subjected to extreme thermal expansion/contraction cycles).

Before you get too far here, I think you should do some science. Look at the thermal mass of air, determine what kind of power source you require to heat two liters of air to exactly what temperature (paper eventually burns at 451 degrees, but you'll need air much hotter than that to cause instantaneous browning of a sheet of paper). Also remember that exhaled breath can be assumed to have 100% humidity, which will drastically increase the heating requirement. Experiment by following Mr. Kirwan's advice, and set up a heating element with a bench power supply and try to get what you want. You'll probably go through several iterations on your heater setup. This will give you a handle on your power requirements. You'll have to look at finding a method to ensure the safety of the person who's blowing through the football (i.e. not burning their lips off).

Once you're there (these are not insurmountable problems), you can then take a look at your power source. Just as a WAG, you're going to need at least several hundred watts to your heaters for those five seconds or so. There is no way 4 or 6 "C" or "D" batteries can provide that, much less rechargeables. If I absolutely had to take a stab at doing this job in this way, and money was no object, I might consider a bank of Polapulse batteries (Polaroid P100). Each one can reliably provide

5 amps at 6V for at least 12 seconds. If you need only 300 watts (almost certainly you'll require a lot more than that), you should be able to fit 10 of these in easily. They're small and can provide high currents for limited times, but they're fairly expensive (and you would have to replace the bank of batteries after every other use). But it's theoretically possible you might be able to fit the whole thing into a spheroid the size of a nerf football. If you even consider this route, be ABSOLUTELY sure you thermally insulate the batteries as well as your volunteer's lips from heat (the MSDS shows possibility of hazardous gases if they burn, and you're looking at a high probability of inhaling here), or have your volunteer quarterback stop the clock by hurling the football out of bounds immediately after use.

Polaroid data sheet on P100 batteries:

?PRODUCT%3C%3Eprd_id=845524441761387&FOLDER%3C%3Efolder_id=282574488338439&bmUID=1107857297463&bmLocale=en_US

MSDS on Polapulse batteries:

Summary: Hot idea, but severe practical problems with power requirements. Safety issues that cannot be ignored. Cost a major constraint. Seriously consider either abandoning project or drastically revising specifications (Mr. Kirwan's butane idea above looks to me to be a far more practical way to accomplish this).

Good luck, and be careful Chris

Oh, my. Sounds like a gag or a magic trick. Blowing into the end of an innocuous device that makes it look as though you are just blowing air and instead you are toasting some paper.

By the way, paper is fairly dry and your breath will probably be a lot moister. What I'd now like to know is what does it take to get paper to brown and what does it take to hit the "flash point" before, during and after browning. My hunch is that there isn't a lot of margin to work with, safely. One advantage that comes to mind here is that breathing out means that the air won't have as much oxygen in it for the side of the paper on the receiving end of the flow. That may help the flash-point margin some. Then there is the fair moisture content and possibly mucous particles and bacteria, etc. You plan to keep this clean?

A calculating approach to take is to think this way:

• Less than 2 liters of heated air blowing across a specified area of paper, must be sufficient to brown it, as desired.

• Time for those 2 liters (or less) to be blown will be about ... ? ... 6 or 7 seconds. Volumetric air flow is thus (assuming a typical volume of about 0.8 liters) something like 150 cc/s.

• How large of an area of paper must be browned?

• How far away will the paper be held? (The further the air has to travel after being heated, the more you'll lose in turbulence and mixing before it reaches the paper.)

• Can the air flow be focused through the use of some kind of mechanical "lensing" to a known distance ahead? (I don't know.)

• What is the rate at which heat will diffuse away through convection outward from the heated spot through the paper and also to the air behind the paper?

• What temperature is required in the air used in order to compensate for the losses and still heat the paper at the required rate?

But I don't think all that is needed. Forget theory and just go empirical.

Probably the easiest way for you to find out what works will be to arrange some nichrome wire to cover the near-exit aperture of your tubing, set up an AC desktop-type powered DC supply that will allow you to set the current used to heat the nichrome, and then start running some tests with different current settings and trying out your abilities to heat the paper adequately. One you figure out the necessary heating, you can write down the current setting and the voltage used across the nichrome and the product of these will tell you the wattage. That will then tell you what kind of battery arrangement you'll need and how long it will last.

Another possibility might be to consider using a halogen flash lamp with a parabolic or (more likely) a spherical reflector. You might be able to brown paper in a single flash pulse from one. Forget the air flow, entirely.

Another possibility might be to use myriad tiny ceramic capillaries (perhaps only 1 cm long, each) stacked into a honeycombed grid. A simple chemical flame (butane, for example) would jet from a tiny tube behind the grid, ignited by a piezo-igniter, and would be immediately diffused into an even heat by the capillaries, so that when the flow exited it would be very uniform without hot spots in it. Chemical energy in hydrocarbons is very dense and probably a lot better than some battery would be. However, I'm not entirely sure how to arrange this with using a breath blown through to automatically adjust the tiny flame to correspond with the flow rate -- some kind of venturi?

Jon

interesting ideas guys.

what about my 30w soldering tool though? i've done some tests with this and it provides more than enough heat. this identifies how much electricity this would take. is there no way to provide this amount of current with rechargeable batteries? what about batteries for an electric car or drill? it seems to me these devices could provide a few short bursts of current to heat a coil.

i'm going to run some tests on various wire and the soldering tool. how can i adjust the voltage coming out of a computer power supply to do that jon?

and the butane/ ceramic setup sounds interesting. would heating the metal element with butane happen faster? what about heating the metal element with fire, letting it "charge" for a few seconds, till its red hot - then blowing once the fire is out? that sounds like it would be a lighter device, with maybe even grater danger problems if it holds a tank of gas and ignites in a small hand held shape. also i'm concerned about the user breathing in butane fumes - all though as i recall this gas burns pretty clean.

but for the electrical tests - can i get some kind of electricity valve to connect to the power supply ?

thanks chris and jon for your participation - you've really helped me flesh out this idea. max

Power = Voltage^2/Resistance

The resistance is fixed. Thus, since heat = power, you have to have the same voltage, ie, 120V to get the same heat. This has nothing to do with the maximum current a battery can supply; without the same voltage, the current will be limited by that resistance.

--
Regards,
   Robert Monsen

"Your Highness, I have no need of this hypothesis."
     - Pierre Laplace (1749-1827), to Napoleon,
        on why his works on celestial mechanics make no mention of God.