Discussion in 'General Electronics Discussion' started by Xephera Geeb, Nov 18, 2014.

1. ### Xephera Geeb

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Sep 6, 2014
So I wanted to see if I could make a really quick and cheap heater for my pockets by getting a 9V battery to discharge over a small resistor, producing heat. After I did the calculations I realized the best value would probably be around 10Ω, as it allows the battery to last a couple hours, but also produces enough heat to keep a small pocket of air warm.

My issue is that I don't have any 10Ω resistors. The smallest ones I have are 100Ω, so I was wondering if there's any way to make a 10Ω resistor out of household materials, that can both survive being used at 1 watt, and will convert most of the electrical energy to heat energy.

If there's anything that would come to mind that would help!

2. ### chopnhack

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Apr 28, 2014
Well, you could put ten 100Ω resistors in parallel - what watt ratings are the resistors you have?

3. ### BobK

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Jan 5, 2010
Your 9V is not going to last more than about 15 minutes discharged into a 10Ω resistor.

E = I R
9 = I * 10
0.9 = I

That is too much current for a 9v. At 0.5A they have 300mAh, and it gets worse at higher currents, so say 200mAh at 0.9A, or 0.22 hours or 13 minutes.

If you use the very expensive lithium one, you might get 3X that.

People keep thinking little 9V batteries can power anything they want. The reality is that they have less energy than a single AA cell and less than 1/4 of the energy in a single D cell.

Bob

Last edited: Nov 19, 2014
4. ### davennModerator

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Sep 5, 2009
nicely put, Bob

5. ### Colin Mitchell

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Aug 31, 2014
"People keep thinking little 9V batteries can power anything they want. "
A 9v battery has enough energy to start a car.

6. ### Harald KappModeratorModerator

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Nov 17, 2011
Not the ubiquitous 9V block battery (PP3). Unless you're talking about model cars

7. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
According to someone who has done some back of the envelope calculations it might take between 300j and 18000j to start a car (depending on engine capacity, compression, and other factors.

At the very low end this is similar to the energy storage of a 9v battery.

The only issue is that it is not possible to discharge such a battery in a reasonable time without losses due to it's internal resistance.

Then again, if we could convert the entire mass of the battery into energy we could probably send the car to the moon

8. ### Colin Mitchell

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Aug 31, 2014
"People keep thinking little 9V batteries can power anything they want. "
"A 9v battery has enough energy to start a car."

This highlights the difference between ENERGY and POWER.

9. ### BobK

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Jan 5, 2010
Yes, and I did not say it did not have enough power to run his heater, in fact I calculated that it could run it for 13 minutes. It is more energy that is needed to run it longer since energy = power * time.

If he had asked if it had enough energy to start a car, I would have recommended (theoretically) that he charge a very large capacitor with the battery over several hours, then try to start the car from the capacitor.

Bob