I had an old nokia 3.7v lithium ion battery kickin around doin nothing so i decided to take it apart. inside is a small charging circuit with a cell. i found out the polarity of the terminals with a multimeter and found it at a voltage of 3.9v after not even being used for over a year! this is well within the limits of lithium ion batteries tolerance of 3v to 4.2v.
I also had this MP3 player kickin around that i had lost the external battery pack to, because this also doubles as a jump drive. i pluged this into a usb extension cable with 5v going to it and sure enough it powered right up! since the original battery was only a single "AAA" battery, this Must have some type of voltage regulator inside of it.
I took my usb extension cable and cut about 2" off the female end so i could solder this to the battery and i soldered the other end to a 5v power source. with the charging circuit on the battery already it keeps it within limits of voltage and heat, and runs the MP3 player WAAAAY longer then a single "AAA" battery. i insulated everything as best i could so i have a battery with a female USB plug on it and and a charger with a male USB end on it.
I made this because my job is very fast paced and my mini-disc player is a little too big and lunky to be running around with in a pocket.
i only thought of doing all of this an hour ago, so i fired of the soldering iron and let it rip. pretty cool eh?
to all would be experimental people, keep in mind, not all lithium batteries have a charging circuit on them and they can go up in flames in a heartbeat if mis-charged or discharged!!!! Point here is if your going to experiment, make sure you know how lithium batteries work and do all in a controlled environment to be safe in case of failure.