I've had the player since 2006 or 2007 and I've never changed the battery. I wasn't sure if 13 hours was abnormally low for a battery this old, but if that's about what you're getting, then I guess I'll just live with it. Regarding turning on the screen while testing the runtime, the Rockbox wiki (http://www.rockbox.org/wiki/BatteryRuntime
) has this to say:
The best method for determining if the device is still on (if this cannot be determined by looking at it) is to listen briefly at the headphones. Pressing a button will light up the backlight and this is a considerable power use.
I'm not sure how much of an effect me turning on the screen had, but I might try benching the battery a few more times without turning on the screen to see what happens. The battery is fully charged now and Rockbox is reporting a 100% charge.