And with modern Li-ion cells, battery life is remarkable. Knowing Li-ion cells are better off in the 40-80% range, I store the device with the battery at that charge level. Each year for the past 14 years, I buy sets of Panasonic eneloop batteries to replace disposable AA batteries as they die. Energy Density and WeightThere are cases where the energy density, portability, and weight tradeoffs of traditional battery cells don't work out: laptops, tablets, smartphones, watches, extremely lightweight computer mice... I can understand when you need a more exotic or thin layout that doesn't lend itself well to cylindrical battery cells.