My personal answer: charge when you can!
Lithium ion batteries age according to the rate at which you use them. Whenever the battery goes down, then goes back down, whether it's 10% or 90%, it gets old the way it was used. It is therefore not important for your battery to run out 10% or 50% before charging. Every time you use your phone, the battery will discharge and age.
So: Basically, load it whenever you have the opportunity to keep it as complete as possible. This will be convenient for you and should have no effect on the battery compared to letting it become really weak before filling it again.
What about letting him plugged in?
Honestly, I do not know anymore. Logically, the facts about lithium ion batteries imply that keeping it plugged is the best solution because it does not go up or down, but stays at 100%, but I'm not sure whether it's practically true or not. Apple makes no mention of keeping it plugged or not into the linked document below, which is annoying because IMHO is the biggest question there is.
Personally, I try to plug it in whenever I can, but for practical reasons, I rarely keep it plugged in while I'm using it, which is the main period when it gets empty of any way.
The answers to this older question about AskDifferent about "overload" imply that there is no reason to think that leaving it plugged is bad for the phone, but there is not a lot of hard evidence.
I think the most important thing to keep in mind is: Do not leave it plugged in if what you're doing makes it hot. Since heat is the main enemy of short- and long-term battery performance (as described below), mixed charging that warms the phone with intense activity (for example, games) is probably a bad solution.
Leave the screen off unless you need it.
Remember that the screen is the biggest drain of the battery. It can do non-screen things all day, but every time the OLED is turned on, the battery is exhausted, so if you end up watching the podcast application, open the phone just for admire its beauty, then stop doing it and your battery will last longer each day (and will be fuller over the years, because anything that prevents the battery from breaking down in the short term makes it inherently stronger in the long run) .
Apple's answer: Wave!
Apple has a detailed article that covers this in detail, here are some quotes:
Maximize battery life and service life
"Battery life" is the amount of time your device will run before needing to be recharged. The "battery life" is the life of your battery until it is replaced. Maximize both and you'll get the most out of your Apple devices, whatever they are.
Your device is designed to operate in a wide range of ambient temperatures, with an ideal temperature of 16 ° C to 22 ° C (62 ° F to 72 ° F). It is especially important to avoid exposing your device to ambient temperatures above 35 ° C (95 ° F), which could permanently damage the battery capacity. In other words, your battery will not power your device as long with a given charge. Charging the device at high ambient temperatures can further damage it.
The iPhone, the iPad, the iPod and the Apple Watch work best at ambient temperatures of 32 ° to 95 ° F (0 ° to 35 ° C). Storage temperature: -20 ° to 45 ° C (-4 ° to 113 ° F).
The brightness settings must be taken into account:
No matter how you use your device, there are two ways to conserve battery life: adjust the brightness of your screen and use Wi-Fi.
Dim the screen or turn on the automatic brightness to prolong the battery life.
- To darken, open the Control Center and drag the Brightness slider down.
- Auto-Brightness automatically adjusts your screen to the lighting conditions. To enable it, select Settings> General> Accessibility> View accommodations and set Automatic brightness to On.
When you use your device to access data, a Wi-Fi connection consumes less power than a cellular network. Therefore, keep Wi-Fi enabled at all times. To turn on Wi-Fi, go to Settings> Wi-Fi to access a Wi-Fi network.
Do not forget to activate the low power mode when you forget and the level becomes very low!
Activate the low power mode.
Introduced with iOS 9, Low Power mode is a simple way to extend the battery life of your iPhone as soon as its charge level drops. Your iPhone lets you know when your battery level drops to 20%, then to 10%, and lets you turn on the low power mode just by touching it. Or you can activate it by going to Settings> Battery. The low power mode reduces screen brightness, optimizes device performance, and minimizes system animations.