i have a mp4 player with firmaware version 9.0.50.000 2006/10/30 and had a lot of problems with the charge duration ,at the begining of the problem the battery was only 1 1/2 hour but i was start to worry about it whe the battery performance was only 30 minutes was very frustrating,so i put my creatitvity to work, i open my mp4 very carefully and i start to analize all the components (in fact only the battery ) and i note that the battery have two terminals connected to a small cisrcuit and another pair of cables to the main circuit of the player ,so i start to search in my cellphone cementery and i found a battery from a siemens cellphone(lithium ion 3.7 v and 700mha) and i start to solding cables in the positive and negative terminals in the battery and thoes cables goes solding tho the terminals of the old battery i (you can try to use a long cable to put the battery in the back of your player) and try the polarity of the cables to match, connect it to the charger and if the charging indicator is on ,your mp4 player is working,and the best is the battery leght in my case the battery last more tha 3 days playing all day long with dj hedphones great new remember try this at your own risk ,
if you need a diagrams of all the procedure my e mail is
wipeout25@hotmail.com
if you have success with this please let me know.