I don't know is this board is active right now, but I'll post my battery hack. Now it's in experimental stage.
I have iPod nano clone s1mp3, and when i resembled it i saw that the battery is standard Li-Ion 3.6V, and capacity is 270mAh.
S1mp3 is strange device, so you can't be sure what is the color of + wire:
I guessed that red wire is +
, and i was right.
Now i just have soldered old nokia battery on the wires, and taped it around s1mp3.
Size of old battery and nokia batery compared (voltage is 3.6V, classical Li-Ion battery, all the diferencies are about the capacity