# generating heat from AA battery

Discussion in 'Power Electronics' started by rrocco, Jul 19, 2012.

1. ### rrocco

5
0
Jul 19, 2012
Hi,
What would be the simplest and safest way to generate heat from a AA battery, whether it is heating a resistor or the battery itself.
Looking to have the heat last at least a few hours before the battery is completely discharged.
Thanks.

2. ### Harald KappModeratorModerator

11,961
2,798
Nov 17, 2011
Connect a resistor between the two battery poles. Depending on the resistance the battery will generated more heat but drain fast or generate little heat but endure longer.
Details depend on the capacity of the battery, the temperature difference (aka heat) you want to generate and the time until the battery is drained.

Don't expect too much from a single AA cell.

Harald

1,114
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Aug 13, 2011
Last edited: Jul 19, 2012
4. ### rrocco

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Jul 19, 2012
Would the heat be generated by the battery getting hot, the resistor, or both?

Rob

5. ### CocaCola

3,635
5
Apr 7, 2012
Both by varying degrees depending on the resistor value...

1,114
159
Aug 13, 2011
Both. Batteries have internal resistance. When you connect a resistor to both terminals of a battery you create a series circuit composed of two resistances and a voltage source. Since a series circuit has the same current flowing through all components, the heating in components is proportional to the ratio of their resistances according to the power formula I^2R. If, for example, your battery had an internal resistance of 1Ω and you connected a 5Ω resistor, the heating in the resistor would be five times as much as the heating in the battery.

As you can see, in a normal consumer circuit using a battery, most of the resistance is in the load so only a small amount of energy is wasted in heating the battery. The opposite effect can be observed when a battery is shorted with a large conductor where most of the energy is dissipated in the battery. In some cases this can lead to an explosion of the battery.

Each battery chemistry has it's own typical internal resistance which you can find in the technical data from the manufacturers.

Last edited: Jul 19, 2012
7. ### rrocco

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Jul 19, 2012
I'll give it a try. Thanks for the link.

Rob

8. ### rrocco

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Jul 19, 2012
So, for safety one would want the heat to be coming from the resistor, not the battery. Is the heat the resistor generates affected by the current as well? For example, what would be the difference (if any) in heat production and duration of heat produced if one used a 1 watt, 5 ohm resistor versus a 1 watt, 3 ohm resistor? Thanks.

Rob

9. ### CocaCola

3,635
5
Apr 7, 2012
It will come from both, as has been explained you don't get to chose, you can only slightly adjust the spread...

Sure...

Amps = Volts ÷ Ohms
Watts = Amps x Volts

Plug in some numbers...

1,114
159
Aug 13, 2011
You're not going to explode a cell discharging a AA alkaline battery at a 500mA rate.

If we refer back to the example I gave earlier of a 5Ω resistor and a battery with a 1Ω internal resistance, the resistor produces 83% of the heat and the battery produces the remaining 17%. When you reduce the resistor value to 3Ω, the resistor produces 75% of the heat and the battery produces 25%. In addition to the ratio change, the lower resistance allows a larger current to flow so the total heating is greater than with a 5Ω resistor. To summarize, lower resistor values result in higher current, a higher percentage of the total heat produced occuring in the battery and reduced run time.

Why don't you get the discharge curve and internal resistance data for some batteries and do some calculations. If you're good with calculus, you can even factor in the diminishing voltage over time.

You haven't said anything about how this will apply to your project. Will such a small amount of heat work for it?

Last edited: Sep 16, 2012
11. ### rrocco

5
0
Jul 19, 2012
Thank you all for your replies. You made things clear to me now. I'll have to fool around with some batteries and resistors and see what happens. I believe only a small amount of heat will suffice my needs, but a single AA may not be enough. Thanks again.

Rob

12. ### donkey

1,300
56
Feb 26, 2011
if you can give us more detail as to what you're making we could brainstorm a little and maybe steer you in the right direction