Has anyone tried rechargeable Lithiums? I did. Got less than 2 hours. Funny thing is the battery meter still showed 3 bars and the unit would turn back on but only for a few seconds. 
Those batteries are likely to have a voltage regulator inside them which seeks to maintain a constant voltage output during use, keeping the voltage-discharge curve relatively flat until the battery lacks sufficient current to keep the voltage regulator going just prior to full discharge, upon which voltage drops off rapidly. The recorder has settings for a few different battery types, which calibrates it's battery meter to the different discharge voltage curves of the supported battery types. Most of the supported battery types probably have voltage/discharge curves which is not nearly as flat but instead drop off of more slowly upon nearing full discharge, so the meter becomes inaccurate when using a type of battery that 's not specifically supported.
If you want to use a battery type not offered in the recorder's menu options, test a few times to determine comfortable run times and change out based on run-time.
If you can find the discharge curve for a battery type you'd like to use and compare that to the discharge curves of the battery types explicitly supported by the recorder, you may be able to choose one from the menu which has a similar-enough curve that the meter becomes somewhat more useful. But you'll want to test to determine how to interpret the meter so that you don't risk loosing part of a recording which is important to you.
