Where have you heard this and why?
Many people said that it is not recommended to let the iphone run below 20% and that is not recommended to charge the iphone while it is on 80% or higher.
That the best charge for an iphone is between 21% and 79% ? And for tablets 11% and 79%?
For any phone in general charge as much as you want and don't worry about. The battery will outlast you. When I can I keep mine on the charger. Others that I know with a note 3 and s5 do as well.
Many people said that it is not recommended to let the iphone run below 20% and that is not recommended to charge the iphone while it is on 80% or higher.
I most definitely hope that my phone battery will not outlast me!
I would suggest you read some material on batteryuniversity.com. On their home page, their 'Most Popular Articles' section has a number of things specific to charging and prolonging the life of Li-ion batteries.
The principals of charging li-ion batteries are universal. Apple has no super special handle on the process. Although if they give guidelines for their products, it is probably worth heeding.
I try to let my iphones, ipads, and ipods drop to at least 20% before I charge. I may have topped off a device once or twice from an already charged state of 80% or higher. But I try to avoid that. Both of my iphone 5's, my ipad 3 and ipad air still go multiple days without having to recharge and charge back up pretty quickly. So I assume my batteries are in good shape still.
Many people said that it is not recommended to let the iphone run below 20% and that is not recommended to charge the iphone while it is on 80% or higher.
It is not worth any of my time to try and maximize battery longevity. Others may feel the same way: I don't know. My time is more valuable than the amount charged for a replacement.
Agreed. Not being able to use your $800+ device properly because you're fretting about the $20 battery is ridiculous. What's the WORST that can possibly happen? Aw no, a replacement in 18 months.
That the best charge for an iphone is between 21% and 79% ? And for tablets 11% and 79%?
OP just watch this: https://www.youtube.com/watch?v=LBKuOomv9Ko and then just use your device like normal.
The main point I will point out is that yes it is not good to let a battery go completely flat but modern devices have built in protection for that and that's why they turn off at low battery, when it says 0% the actual battery still has a little left in it but it turn off to protect itself.