I ran some tests of the standard NiMH rechargables a while back, in this thread:
https://www.mobileread.com/forums/showthread.php?t=74936
Not exactually "scientific" but they were specific to use in the JBL.
You should be able to expect between 25 and 30 hours of actual reading time per charge
of a set of the standard Made in Japan 2300mAh NiMH rechargables.
Luck;
Ken