I mentioned this to a colleague lately who looked at me like a strange animal for having:"worry about cell degradation" even on my list.
No, it's not. NiMH have practically no degradation from cycling, which is why Toyota still uses them in the Prius. The tradeoff is in energy density.
Li-Ion batteries are not meant to be held at 100% State of Charge (SoC). There is a non-linear relationship between average SoC over the life of the cell and the rate of degradation. Optimal charge for extended life in a typical 4.2v cell is around 3.9v, or 65% of available stored energy. Keeping an average SoC near 100% can cause up to a 5x loss in battery lifetime. See the link below [0]. Specifically "Table 4: Discharge cycles and capacity as a function of charge voltage limit."
http://batteryuniversity.com/learn/article/how_to_prolong_li... [0]