I"ll just add: a real simple way to make a heater is to use a power resistor. For example, if you have a 5-watt, 10 ohm resistor, and you connect it to a voltage source, you can use some simple euqations:
P=V^2/R
If you use ~7V for V, then P=49/10 ~= 5W. The current is given by:
I = E/R = 7/10 = .7A. So your battery must be capable of providing .7A or 700mA. That's actually alot of current for most batteries, so that's why batteries are't used often for heating applications.