Well my player is the one in my picture, it supports Nokia batteries.. but it came with a fake BL-4C (without the MAH specification :roll: ). I managed to get a BL-5C real Nokia battery with 850mah
(nice)
I had to mod the player case to make the new battery fit, as it is a bit wider [don't know if this is the correct word].
This are my results (tested with music and backlight off at 6sec):
Fake nokia battery:
8hs of continuous playing (not bad at all!)
But this is the best:
Real nokia battery:
23hs of continuous playing
Now I know that the fake nokia battery is about 300mah as I have the 850mah battery as a reference..
Also with the same player a 700mah battery did about 18hs of continuous playing..
So if you have one of these players and you can get a real nokia battery just do it! it's a great upgrade for your player!!!
Sorry for my english! I know it's not perfect...
EDIT: Spelling mistake 
ops: