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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
You'd be lucky to get 0.5W for 3 hrs. You might try a 4.7Ω or 3.3Ω, 1W resistor.
http://www.powerstream.com/AA-tests.htm
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.
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?
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?