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I usually charge my phone all day and night long. All day long, because at work I connect my phone with my laptop, and all night long because when I go sleep, the phone will be charged overnight too.

But I got worried this may reduce battery life. So I went searching online, and the best article I can come up for best practice for charging Apple device is this. In the article, it just says that the device is smart, and can automatically stop the charge once it's almost full, to prevent overcharging. Does that mean it's actually completely okay for me to charge the phone all day long? I still can't find any article that can give me the straight answer to this question.

This questions risks inviting a lot of opinion, which also explains why it seems you can't find an article that can gives you a straight answer.

However, to try and keep this to the facts, I refer you firstly to what Apple has to say about their batteries. You should also read this.

In a nutshell, the key indisputable points are:

• 
  Charge your iPhone whenever you want. Many people believe you need to let them discharge 100% before recharging. This is not true.



• Apple lithium-ion batteries use fast charging to reach 80% of their capacity and then switch to slower trickle charging to complete the charge.




• Ambient temperature is one of the biggest factors in battery health/life. iPhones are designed to work at their best when ambient temperatures are between 0° to 35° C (32° to 95° F).



• Store your iPhone where the temperature is between -20º and 45º C (-4º to 113º F). Don’t leave it in your car, because temperatures in parked cars can exceed 45º C (113º F).

Also, the latest research confirms one of the most important determinants of what ruins a battery is time. (You could purchase a brand new iPhone battery and leave it unused in its original packaging for five years and then find its capacity is nowhere near what it should be when you start using it)

Basically, you can continue to do as you have been without fear of causing damage to your iPhone battery.

The battery life is mostly impacted by deep (Complete) discharging.

That is why the Cycle count is monitored as indicator of battery life.

You probably noticed that battery gets hot when been completely discharged. That is because the internal current is increasing to maintain the Voltage. That leads to edit currents and growth of Whisker. Whisker are build up between the positive and negative poles. Those whisker eventually lead to end of life of the battery.

The second factor limiting life is the ambient temperature. High temperature is not good.

If you do not let the battery discharge all the way is the best practice.

It is a "Smart" battery (it has a circuit build in) and it will newer overcharge.

If you like to learn more visit Battery University.

You better not charge it all day. Keeping it at high voltage increases stress for the battery.

Lithium-ion suffers from stress when exposed to heat, so does keeping a cell at a high charge voltage. A battery dwelling above 30°C (86°F) is considered elevated temperature and for most Li-ion a voltage above 4.10V/cell is deemed as high voltage. Exposing the battery to high temperature and dwelling in a full state-of-charge for an extended time can be more stressful than cycling. Table 3 demonstrates capacity loss as a function of temperature and SoC.
https://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries

The mentioned table notes that at room temperature a battery at 100% charge loses 20% capacity after a year, while a battery kept at 40% loses only 4%. This is why new batteries will always have a 40-50% charge.

I know that this 40% rule is only mentioned in instructions when storing a battery long term, but it is also valid for batteries in use. This is why Dell has something like Desktop Mode (When desktop mode is enabled, the battery charges between 50 percent and 100 percent while the power adapter is connected.) and Asus has Maximum Lifespan Mode (If your ZenBook operates almost exclusively on your desktop and is nearly always plugged in, the most conservative power profile limits the maximum charge to 60% and starts charging at 58%. This will ensure your battery has the longest possible lifecycle while still keeping a sufficient charge for light usage while unplugged.)

It could be that Apple caps the charge at 80% capacity, but I can't find any reference to that. On the Apple battery page they also recommend storing at 40%.