Ideal charging / discharging percentage for maximum battery life?

One of my friends has a relatively new Samsung smartphone. He has this app that notifies him whenever the charging is up to 80%, and tells him to unplug the charger.

He tells me that it will increase the number of charging cycles for his battery. He says that if he lets it charge all the way up to 100%, it will mean he has to replace the battery of the phone more often.

Is this true?

Somehow, I feel this is a thing of the past. I thought the internal architecture of batteries these days was made so that it wouldn't drop too low or too high. (In other words, when the phone is at 100%, it isn't really 100%. It's what the battery can safely be charged to in order not to degenerate too much.)

Here's a related answer which taught me something about this.

Answer

In the comments to linked answer, it was brought out that batteries are not allowed to stay at 100% charge since it is harmful to the battery and the battery % rapidly drops to around 90% as explained in this XDA post. This is a precaution implemented by OEMs ( though not sure if all OEM's do that)

Related to this is battery longevity on terms of Voltage per cell

Source: How to Prolong Lithium-based Batteries

Most Li-ions charge to 4.20V/cell, and every reduction in peak charge voltage of 0.10V/cell is said to double the cycle life.... In terms of longevity, the optimal charge voltage is 3.92V/cell. Battery experts believe that this threshold eliminates all voltage-related stresses;

This voltage as per Table 4 ( above ) corresponds to battery charge level at 58 ℅ . This is clearly impractical and the recommendation of your friend to charge to 80%, more than doubles the discharge cycles .

This was on the charging part. Table 2 shows another interesting aspect discharging and it's impact on battery life

IMO , if one badly wanted to maximise battery life, ideally, keeping practical usage in mind - charge when the battery drops to 50% and stop charging at 80 or 90%

Thanks to Dmitry in comments who says and subsequently made an interesting point in his answer for the end user is number of charge cycles*capacity at full charge: that's how long a battery will last for a given usage pattern

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Edit: For rooted devices, there is an app on XDA which disconnects charging at a given percentage. This is now open source. Approach followed appears to be similar to my solution here Automatically limit (stop) battery charging at a given percentage

Edit 2: Discovered this magisk module which does the same bit going by the description it is better than module listed above. Not tested either of them as I am using own solution

Edit 3 This theory is disputed by some and not decisively concluded one way or other. I believe it to be valid. Adding this to be objectively clear