1. All batteries have an aging characteristic whether they are charged or not (around 6% loss per year). For example, a home solar array or EV should prioritize

1. All batteries have an aging characteristic whether they are charged or not (around 6% loss per year). For example, a home solar array or EV should prioritize the largest practical capacity budgets allow. Thus, the performance remains more consistent throughout the entire service life. This also reduces the discharge load stress on the cells reducing wear, and exposing greater energy capacity through efficiency gains. 2. There are several types of Lithium batteries with unique charge cycle limits. A common 4.2v cell is ideally rated for 8000 cycles if shallow cycled, and slow charged. Accordingly, deep cycling a LiPol or LiIon below 60% into the dead pre-conditioning zone or in the Constant-Current mode will reduce the life to well below 4000 cycles. 3. 80% or 100% full is meaningless because accessible energy depends on temperature (outside 4'C to 50'C is bad), chemistry, and loading. Typically, even the cheapest Lithium charge management ICs have a hysteresis built into the cycle, and will let the cells stand-by after charging until the cell internal resistance self-drains a bit of power in time before charging again. 4. Li batteries do not like trickle-charging or rapid-changing, and in a way the "100%" indication icon often means just that. Over-boosted the cell into the 112% capacity range to provide a short-term capacity boost. This wears out the cell 15% faster, but many tends to do this... so who would know the difference. I'd wager the Apple team added the 80% mode to turn off this boost trick, at the cost of "boosted" capacity. 5. There are consumer chargers for Lithium cells with temperature and impedance monitoring while charging. These are highly recommended if you have 21700 or 18650 cell based equipment. Unlike other options these chargers will show you a rapid impedance change when the cell has an internal problem (dendrite fusing in polymers etc.) Read the manufacturers app notes for your cells. They will often disclose the conditions their chemistry is stable (Samsung and Sony are usually the best). There are so many various types of cells out there... most of them are crap. This text is just a starting point, and should be assumed as another ill-informed internet opinion until validated under your own efforts. YMMV Best regards =)