I wanted to know what what was going on so I did some research. Batteries can be charged more than 4.2 volts but for safety reasons it is limited to 4.2V, but that doesn't mean it's healthy for the battery. As a matter of fact, when only charging by just 0.10 volt less you increase the usable life of the battery by around 2x. Charging to 4.2 volts may get, for example, 300-500 cycles. Charging to 4.1 volts, in the same battery, will get 600-1000 cycles. Less is more in this case. Being fully charged is stressful on the battery. Charging to 80-90% is better than charging to 100%. Simple fact.
Read about it here...... Fourth chart down the page:
There is also a chemical instability that occurs in the battery at the anode or cathode end when left at full charge or almost empty for extended periods. When you've seen a lithium battery puff up from being stored at full charge or discharged too much or too long it is the chemical instability taking over causing the physical swelling. I can't recal exactly what it is, but my take away (and experience) says don't keep the battery charged at full for extended periods of time.
A simpler read on the subject from 'Wired'.
At the end of the day you will more than likely have a new device before the battery dies when using the device regularly, but I caution against leaving the thing plugged in ALL THE TIME if you care about having any sort of charge time when you need it unplugged.
Edited by x RELIC x - 3/23/15 at 2:49pm