Lithium-ion batteries, like those of the iPhone, have a longer life if you do not charge more than 80%. But, to last the day, you probably want a full charge. With iOS 13, Apple can offer you the best of both worlds.
iOS 13 will charge 80% and will wait
Apple has announced iOS 13 at WWDC 2019. Buried in the list of additional features Apple's goal was to "reduce the full load time of your iPhone". Apple will prevent your iPhone from recharging more than 80% until you need it.
You may be wondering why Apple wants to keep your iPhone at 80% charge. Everything is based on the operation of lithium-ion battery technology.
Lithium-ion batteries are complicated
Batteries, in general, are a complicated technology. The basic goal is to store as much energy as possible in as little space as possible, then release that energy safely without causing fire or explosion. It is an act of priorities that juggles.
Lithium-ion batteries complicate things by being rechargeable. The old rechargeable technology was suffering from the effect of memory: the batteries lost track of their maximum capacities if you reload them constantly after only having them partially discharged. Lithium-ion batteries do not have this problem. If you continue emptying your battery before recharging, you should stop. You damage the health of your battery.
You should not keep your battery at 100%
One cycle is to exhaust an amount that goes to 100%. Apple
Lithium-ion batteries charge up to 80% faster than previous battery technologies. For most people, 80% is enough to spend the rest of the day. So you have what you need earlier. It also does not have the dreaded memory effect that causes the battery to lose all its capacity.
However, instead of having a memory problem, the Li-ion has a problem of maximum charge cycle. You can only charge the battery very often, and then the capacity starts to decrease. It's not just about charging from scratch to 100%, it counts as a full charge. If you charge 80 to 100% five days in a row, this 20% charge adds to a "full charge cycle".
Not only does it drain the battery to zero, then charging it at 100% will hurt your battery life, but charging the battery is not good either. Staying close to 100% can overheat the battery (which can cause damage). In addition, to prevent your battery from "overcharging", the charging is stopped for a moment, then it starts again.
This means that if you charge your device all night after reaching 100%, it drops to 98 or 95%, then recharges to 100% and repeats the cycle. You use your maximum charge cycles without even actively using the phone.
The solution: the rule of 40-80
For all these reasons and more, most battery manufacturers recommend the "40-80 rule" for lithium-ion. The rule is simple: try not to let your phone run out (less than 40%), which could damage the battery, and try not to leave your phone fully charged (over 80%) all time.
The weather conditions aggravate one or the other of the scenarios. Therefore, if you want your battery to retain its maximum capacity longer, keep it at about 80%.
iOS 13 is sitting 80% during the night
The latest iOS updates include a health function of the battery This allows you to check the capacity of your battery and have a history of your battery usage. This feature is a useful way to find out if you stick to the 40-80 rule.
But Apple knows you do not want to start the day at about 80%. If you travel a lot or are often out of reach, this extra 20% can easily make all the difference for your iPhone to be at the end of the day. Staying at 80% risks losing a valuable asset, your phone. That's why society wants to meet you in the middle.
In iOS 13, a new charging algorithm will keep your iPhone at 80% during a nighttime charge. This algorithm will determine what time you wake up and start the day, and then restart the charge sequence to give you a fully charged battery when you wake up.
This means that your iPhone will not spend the whole night charging, it does not need (and risk of overheating), but when you start the day, your battery needs to be 100% charged. It's the best of both worlds to give you the longest possible battery life, both to keep the battery full and to get through the day.