I have a project (Wii Wireless sensor bar) that i am working on for a friend. I plan to power 4 infrared LEDs with a 3.7v 810mAh lithium Ion cell phone battery charged by a dismantled charger made for that phone (you can charge the phone or a spare battery). What i want to do is use the device (4 powered LEDs) whether it is plugged in or not. I have a feeling that there will be two different voltages depending on if it's plugged in or not. What is the simplest way to get a constant voltage to the LEDs?