9842Views20Replies

# Can a 9 volt battery generate enough current through Nichrome wire to heat it up?

My son is trying to create a battery powered electric blanket using one or two 9 volt batteries with some Nichrome wire. Will he be able to generate enough current to cause the Nichrome wire to heat up properly? He would appreciate any tips on what wire length he needs or what other options he has for a voltage source. He really wants to make it using a battery.

## Discussions

1 year ago

Tried with 4 1.5v AA batteries and 40 guage Nichrome wire. It was able to heat the filament. - https://youtu.be/NIxM9Aqq7Q8

2 years ago

a 9 watts nichrome powered coil won't be able to heat up a bigger surface area.These can be succesfully implemented as hot 9w nichrome soldering iron which heats up lesser surface area.Energy can't be tricked any way.

Let him use 12v 7ah battery sothat he can get 84W Power.Be careful while designing the coil and put maximum surface area.84W is pretty good power to burn human body that much

But,Apart from nichrome,there is induction heating which would not waste energy to atmosphere,but don't think it will be useful for a blanket haha:D.Do not try this without proper results to avaoid side effects to our body.

3 years ago

Assuming a 2 9V batteries in series, that would be about 18V, and assuming 500mA out of it (which is pretty much the maximum) I will guess that you can get about 9W of heat, if you size the nichrome wire correctly. Ohm's law tell you that you would need 36 ohms worth of resistance.

9W is not a lot really, a 10W resistor is about the size of a cigerete.

Answer 3 years ago

You're ignoring the output impedance of the battery. Its got an output impedance of around 75 Ohm.

Answer 3 years ago

Yeah, I know. That would even further reduce the voltage and thus the heat produced.

3 years ago

An average wattage for an electric blanket may be 200 watts watts is volts x current

The 9 volt battery will give you milli amps so No it doesn't have sufficient power available for even a modest heating element.

That isn't to say that a short bit of nichrome wire directly across the battery won't get very hot so take care.

Answer 3 years ago

To see if the 9v battery will actually heat up the Nichrome wire what length would you suggest? Or does the length matter?

Answer 3 years ago

If you REALLY want to try this just use a few inches. It WILL get red hot very quickly so DONT hold it in your fingers.

3 years ago

No, That won't work.

9V batteries can't supply enough current, And you'll set the mattress on fire if to much current goes through the wire (with a power supply)

3 years ago

A 9v battery can heat a thin nichrome wire hot enough to melt a mylar sheet to fire a 50 KV switch see picture.

You however do not have enough energy in a 9V battery to heat a square foot of fabric using Nichrome wire.

Nichrome wire is available as 18 gauge to a tiny 36 gauge so thin that it tears with little force. Also available as flat toaster ribbon wire.

You need to search for a nichrome chart that can draw 50 ma for the length of wire you plan to use. Remember that the 9v battery can deliver 50ma for about ten hours.

And you want to heat a blanket on 9 volts x .05 ampers => o.45 watts of power.

Go read the wattage of a regular blanket, the best I was able to discover was a low 50 watts, thats about 100 times more then your battery can do.

Now you can decide if this will work...

Answer 3 years ago

I think the kids are starting to realize that the 9v batteries in series may not work now. We have 26AWG Nichrome wire to work with. As an experiment, do you think a few of the batteries would be able to heat up some length of wire?

Answer 3 years ago

Nichrome-A 26 gauge has 2.57 ohms per foot I= V / R = 9 / 2.57 = 3.57a Unfortunately a 9v battery can only deliver o.500a for less then an hour at about 0.65 watts...

A 20 foot segment of NiCrA wire has 51.4 ohms & I = V / R = 9 / 51.4 = 167ma with a run time of 2.25hrs at 1.5 watts.

That is still 33 times less then the 50watt heat blanket.

................................................

A 100 foot segment of NiCrA wire has 257 ohms & I = V / R = 9 / 257 = 35ma with a run time of 17hrs at o.154watt heat blanket .

That is 324 less then the 50watt heat blanket.

..............................

Your son might consider LiOn 9v batteries that cost twelve times as much, weigh half of the Alkaline and deliver ten times the power.

.

3 years ago

This is probably going to sound like an echo of what everyone else said.

Trying to heat an electric blanket with one, or two, 9-volt batteries is kind of like trying to water the garden with a single cup (250 mL) of water, per day.

It becomes reasonable if you shrink the size of your garden to a single flower pot.

Similarly, you could shrink the size of the blanket, although I think it is going to need to be reduced, in area, by a factor of about 100, so the resulting "blanket" will be about the same size as the batteries themselves.

Actually such things already exist, in the form of battery-powered mittens, or socks. If memory serves, a typical product is a pair of socks, with each sock powered by a single D-cell battery. I'm guessing the run-time for such a D-cell powered sock is probably an hour or two.

A similar way to kill D-cell batteries, is just to wrap the cell in single layer of aluminum foil, and put it in the pocket of a jacket, or hoodie. Preferably, this pocket is big enough to fit one, or both of your hands if it's the front pocket on a hoodie, because the goal of this setup is to make a

hand-warmer. So basically, your hand is wrapped around this shorted D-cell, receiving warmth from it. The hand might also provide some feedback, unwrapping (disconnecting) the cell in case it gets uncomfortably hot.The shorted-battery as hand-wamer is a well-known recipe, so you can probably find existing tutorials, Instructables, or even Youtube videos on that topic.

Alternately, you could try to find a bigger battery.

By a bigger battery, I mean one which stores more

energy, measured in units of watt*hours, which is roughly equivalent to battery voltage (in volts), multiplied by current capacity (in ampere*hours).The Wikipedia article on "List of battery sizes" can give you these numbers for a wide variety of battery sizes. For example, the watt*hour capacity of a single 9-volt, alkaline battery, is:

(battery voltage)*(current capacity) = (9.0 V)*(0.565 A*h) = 5.085 W*h

about 5 watt*hours, or 10 watt*hours for two of them.

The other part of the problem is the size of the load you're trying to power, in watts. A typical electric blanket uses something on the order of 100 watts. So just a naive calculation of how much run time you could get from a 10 W*h battery, is:

(10 W*h)/(100 W) = 0.1 hour = 6 minutes

And that's just the energy=power*time accounting. A more practical concern is the 9-volt battery is not capable of delivering its energy that quickly, and most of energy is going to go into heating the battery itself. So you're basically back to the idea of shorting the battery and using it as a hand-warmer. Moreover it will be a short-lived hand warmer.

Regarding questions about how to build a heating element out of nichrome wire, usually the way that works is you start with a desired amount of power dissipation in mind, e.g. 100 watts, and also a battery voltage (or average battery voltage considering you expect this to decrease with time). Then assume the heating element is a resistor R, so the power it dissipates is P = V^2/R. Thus R = (V^2)/P.

A long wire has resistance proportional to its length, and there are tables listing resistance per unit length (e.g. ohms/meter) for different thicknesses (gauge, AWG) of nichrome wire. There is a slight temperature dependence, but I think it's only about like 10 percent greater, for wire that is red hot, compared to room temperature. So that's how you build your resistor, with R to match the P = V^2/R amount of heating you want.

Answer 3 years ago

Thanks for all the suggestions. Will a few 9v batteries in series be able to heat up some of the Nichrome wire enough so that they can take some temperature measurements?

I am going to suggest that they switch from 9v batteries to a power brick. Maybe from a spare laptop charger? Would that be a better voltage source for this? The blanket size can be adjusted if necessary.

Answer 3 years ago

If you want to use batteries, a series stack of D cells, or even AA cells, will give you more energy per unit money (e.g. per dollar), than will the boxy 9-volt. I mentioned the Wiki page on battery sizes, but I neglected to link to it, here:

https://en.wikipedia.org/wiki/List_of_battery_size...

If you want to use a power brick, also called AC adapter,

https://en.wikipedia.org/wiki/AC_adapter

that is an even better idea, in terms of repeatability, and low cost. The only problem is it's not portable. You can't strap it to your leg, and then go for a walk, the way you can with some D-cells.

http://www.webbikeworld.com/heated-motorcycle-clot...

But, you know, if you just want to witness the miracle of electricity turned into heat, then, a AC adapter would work well for this. Also low voltage DC (i.e less than 20 volts or so) is much safer to play with than the 120 VAC (or 240 VAC depending on what part of the world you live in) mains power.

If you have a multimeter, that will come in handy for measuring the cold-resistance of your length of nichrome wire. Also if you have the ability to measure resistance, this is handy for discovering the resistance per unit length, of found, salvaged, nichrome wire.

Like for example if you have some nichrome wire you pulled out of a broken toaster, or hair-dryer, and you do not know for sure the gauge of this wire, then the published tables for resistance/length for different AWG gauge, those tables won't be that helpful.

However, using the lowest resistance setting on your multimeter, you can just directly measure the resistance between two points on a length of nichrome wire.

I think the actual amount of resistance you want is going to be on the order of 10 to 100 ohms. Note that the resistance in the leads of the multimeter itself may be as high as 1 or 2 ohms, so measure that first, just the resistance in the leads, then subtract that number from subsequent measurements.

Regarding the power brick, if you have a laptop charger that says it can supply 19 volts, at 2 amperes max... well, 19 volts divided by 10 ohms is 1.9 amperes. Maybe that's getting kind of close to the limit, since 1.9 is just barely less than 2. So maybe use 20 ohms instead. That would give you a current near 1 A, and the total power across the whole length of wire is (19V)*(19V/20 ohm) = 18.05 W, roughly 20 watts.

Also note, the amount of temperature (or rather temperature difference between the element and its surroundings) you get from an electric heating element is roughly proportional to power divided by surface area. For example a wire that is a meter long, dissipating 20 W over its full lenght, might just get warm. In contrast, 20 W, dissipated over a wire 10 cm long will probably be burning hot.

The usual trick for dispersing the heat from a hot wire, to get the same heat but at lower temperature, is simply to immerse it in something that conducts heat well, but electricity not so well. For example surrounding the wire with dry sand.

I think professionally made heating elements, like those in an electric stove, are nichrome wire embedded in magnesium oxide,

https://en.wikipedia.org/wiki/Magnesium_oxide#Othe...

but just for experimental purposes, I think sand will work.

3 years ago

Its not going to work.

Answer 3 years ago

This is a good summary. Concise. To the point. ;-)

Answer 3 years ago

Why beat about the bush ?

3 years ago

Electric blanket?

What size? Like a cigarette box?

You have 9V and a few milliamps to work with, unless your wire is much finer than a human hair it will just short out the battery.

To create heat you need energy and there is not enough energy inside a 9V battery to do anything like that.

3 years ago

It may be able to start heating the wire but it won't last long. 9V batteries don't have a lot of capacity. You'll want to better power source.