Re: To charge only when it is less than 20%?
Lithium-based batteries (including Li-Po) indeed do not have battery memory effects, but they still suffer from battery wear. With this, one must try not to let the battery undergo a charge-discharge cycle that is greater than 50%. That means that you should start considering charging your phone once it reaches 50% if you intend to charge it all the way back up to 100%, or from 20% to 70%.
Originally Posted by lmws214
Even further, Lithium-based batteries last longer (not in terms of power delivery, but in terms of service life) when the charge is maintained at around 70-80%. But this would be extreme already, and would be obviously inconvenient for the user... yet it must be note mentioning that this is the reason some devices have the option to "extend battery service life by not charging the battery fully to 100% every time".
Personally, I keep a powerbank with me and charge my phone once it starts reaching 50-60%.
Memory effects in battery technology may be long gone, but right now battery wear is real. Unless we find better and more economical ways of containing ion-migration for portable energy storage, this would be our best practice: avoid a charge-discharge cycle greater than 50%.
New technology is coming, though. Expect revolutionary changes in battery technology in 3-5 years (unless some marketing giant hinders it). Businesses (of course, including Apple) would release ambiguous statements as to how to exactly maximize the battery life of their devices because doing so would decrease after-sales revenues from replacement batteries or servicing units. But that is my advice - coming from someone whose research is partly in the field of energy storage.