Read this thread.
https://www.mobileread.com/forums/showthread.php?t=9937 Apparently some people get the full 7500 page turns while others don't. Did you charge the reader when you first got it, and for how long? I charged mine till the red light turned off. I read, I think, 3 rtf books (around 300 pages each) before I lost a bar (in the middle of the third book). I don't know, I suppose that about average.
Also, did you read pdf? rtf? lrf? I think pdf takes more battery power than other formats.