If you turn off the Wi-Fi and does not install any third party applications (potential under-covered background tasks), the i5 can operate for around 11 hours non-stop if you didn't use the screen very often. This is not a hype, I am sure we have i5 users sharing their experience on the battery duration of i5 to support my claim.
While Lithium battery installed in i5 does not have memory effect, it does has what we called inaccurate gauge issue, and this problem is quite common when a battery has been idled and/or drained for a long time. If you run into inaccurate battery reading or sudden power off when the duration period didn't add up, I suggest you do a gauge calibration to verify the problem. In fact this is something that we all should be doing occasionally. I normally do this after 15-20 charging cycle (2 times 50% charging is count as one cycle) An inaccurate gauge can lead to the fact that the the battery capacity values are are wrong. The battery may report that it still has reasonable capacity when in fact it has a much lower value, and this causes the computer to shutdown unexpectedly. You can google the terms "Calibrate Gauge" on Lithium Battery.
To calibrate the gauge of a lithium battery, you simply need to do a full discharge followed by a full charge, so this is what I advice:
(1) turn on the LED screen to maximum brightness and set the screen to "always on", keep it running until the player turn off itself.
(2) Charge the battery overnight, till you see 99% or 100% on the meter
(3) Set the screen to minimum brightness and 30sec duration only, set it to playback on any CD quality files (Wav or FLAC) continuously til the player power off again. Keep the screen off and check the battery reading every hour, and drop down the battery readings for record.
Hopefully the duration and accuracy of battery reading will go back to an acceptable level after the calibration. If not, we'll have some data to determine if this is a hardware problem or a software problem.