Hi! My brother has a small micro-heater that runs on a single AA battery. The battery life tends to be rather short so we started using rechargeable batteries. The rechargeable batteries are marked as 2000mAh and since they're AA and Nickel-metal Hydride I'm guessing they operate at 1.25v. I want to mod it for use with a rechargeable cell phone battery. But I have a few questions about that. 1. The cell phone battery is marked as 3.7v, 930mAh, 3.4Wh and is a Li-ion battery. Is it safe to use a Li-ion battery? The NiMH batteries tend to heat up, will this damage the cell phone battery? 2. Assuming P=IV and V=IR hold true here, would I just have to add a resistor to decrease the current and send the same power to the heater? Is it really that easy? 3. Would there be any difference in sending a higher voltage and lower current to the heater if it receives the same power? 4. Given the cell phone battery is running with a higher resistance, would it last longer than two AA batteries? If its not, then this project isn't worth it.