Actually li-on battery's should avoid full charge.... Check out my signature .. In the thread I have multy links regarding Battery's by MS.
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To clarify this missing information statement. Leaving it plugged in will not matter. How the battery is charged is set by the manufacturer which is why your phones turn on when you charge them. Also, some li-on batteries do need to be discharged fully periodically to calibrate them.
I plug mine in so I can do wifi stuff with Zune (and podcasts). Technically the best thing for Li-Ion is to avoid deep discharges and full charges. If you could run it somewhere between 30% and 80% that would last the longest. But there's no way to do that conveniently so I just plug 'er in every night without thinking twice.
We just got lithium powered drills at work. The real odd thing I noticed about them is they stay at 100% power until they die. They're not like the old nickel cadmium batteries that slow down as the charge gets lower.
It doesn't matter if you charge it 6 times a day as long as it isn't at 100% and still charging. I usually stop it around the high 90s.
im pretty sure there is a discrepancy between what the % says and actual charge. By that I mean there should be measures to keep the battery from charging to much. Its also probably why the battery drains quick through the first 10%. Unless you have a crappy 3rd party charger than all bets might be off. I'm not sure if it is controlled from the phone side as well