I am wondering what the range for the charging input voltage. I've got a great idea for a battery powered charger that can attach to the wrist band, but I just need to know the input range, inorder to get the most out of the batteries.

It takes USB voltage in, 5V or range 4.75 V and 5.25

Eric,
Thanks for this info. I have a pretty good idea, just need to get the parts and test it out. Hoping for a couple days use instead of the 6 hours currently with android.

So i order so sample Step-up Convertors to build my test external battery. Still trying to decide if I want to use a lii-on or just some alkaline, but I will figure this out once I build the circuit and see how long it will last with different types of batteries.

Samples are in, they are the MAX631ACPA+ and MAX756CPA+. I got two of each so hopefully I can get something working right the first try. I knwo these IC's are not exactly small, but they will allow me to test the circuit design, and test battery life.
Right now I am going to be testing different alkaline batteries like the circle ones for watches up to 2 AAA's. I think the 2 AAA's will give a large battery life, as they are like 1500mAh each, but may be too big to mount to the watch band.

Also once I get the circuit working, I'm going to look into getting a li-on or li-py battery taht's like 500mAh and build a charging circuit on as well. I've found a couple battery charger kit's just need to make the actual board smaller.

If anybody has any suggestions on batteries, just let me know. I'm trying to make it last as long as possible, but also not run thru expensive batteries quick.

Ok so I built the board, and it puts out 5.1 VDC when I don't have a load on it. but when I connect my BB or my small 10 year old mp3 player it drops the voltage to 4.3. I know this is due to the size of my Inductor, which radio shack doesn't sell, so I am pulling all of them off my old circuit boards I have sitting around until i find the right one. This circuit is using the Maxim MAX631 Step-up

I do have a MAX756 as well, which may work better, just have to see if I have the right shotzky diode.

Also, does anyone know why a Li-ion that is rated for 3.7V only put ot 2.6 volts? I have some spare blackberry 1500Mah batteries that I my use, but they don't seem to want to play nice. They have 4 pins on them, with a label of + on pin 1 and - on Pin 4. When i measure on a voltmeter, I get 0 votls across pins 1 & 4, but get +2.6 volts across pins 1(-) & 2(+) and Pins 2(+) & 4(-).