Leave phone connected to charger when full or disconnect, drain, reconnect, repeat?

Question

I’ve heard that lithium batteries are most stable at 50% charge and anything above or below that causes their capacity to degrade.

If I know I’m going to be on my phone for a few hours and a couple of full charge cycles, is it better to disconnect my charger once the battery reaches 80-100% and reconnect it at 20%, repeating this several times, or is it better to leave it sitting at 100% for several hours?

Which causes the least degradation to the battery, if any?

I always use a 2.4A battery pack or a 2.1A wall charger to charge it, if that makes any difference.

I have my iPhone set to vibrate and show a notification when the battery is full to remind me to disconnect the charger (within Shortcuts > Automation.)

Answer 1

Li-Ion (and their subset Li-Poymer) batteries are much more complex than "above and below 50%".

What is pretty much known to degrade them:

1. Keeping them charged near 100%. But this is relative, because these 100% are left at the vendor discretion - some vendors just derate the battery and say it is 100% charged at 4.10V when others would consider the same battery at the same voltate as being like 85% charged and pump it all the way up to 4.35V. And these numbers are relative either because different chemistries have them somewhat off.
   
   There are settings/applications that can limit the charging to less than 100% (of whatever the phone vendor thinks is 100%).

2. Keeping the battery at high temperature. Especially bad when combined with p.1

3. Discharging them with high current when nearly empty. Again relative, because one can trade off capacity vs longevity and discharge a battery down to 3.1V or 2.8V. Worse when done to a cold battery, here the higher temperature somewhat helps.

4. Charging the battery at high rate (worse when the temperature is also high).

What to do when the device is under a constant load and external power is available:

1. Get a powerful charger. Important because a lot of chargers cannot meet the peak demand and in some cases their power is supplemented by the battery. The battery is recharged back when the power demand is less, burning up the possible charge/discharge cycles.
   
   If a powerful enough charger is connected, the battery is asked for power just rarely (or never). Look for chargers supporting high-power charging protocols like USB-PD or QuickCharge (the phone has to support them too).

2. Limit the battery charging to 60-90% by whatever means available (and accept that you will have 60-90% battery when unplugged).

Note: a lot of modern phones (incliding recent iPhone models, as well as other recent high-end phones) have this charging limit enabled by default and top up the battery all the way to 100% only off-hours. This gives a very good compromise between having all the battery capacity vs preventing accelerated battery aging under heavy phone use.

The same approach is used in battery-powered vehicles.

3. If available, limit the charging current to ~50% of its factory value (this may or may not help, depending on how this limit is implemented in the device).

p.s. whatever strategy is used, topping the battery to 100% and discharging it near empty once in a while is important for maintaining proper calibration of the battery state-of-charge indicator.

The algorithm that estimates the remaining energy self-calibrates at 0% and 100% points. It gradually loses accuracy if 0% and 100% points are not reached for long time.