Keeping Li-Ion battery on charger vs. plug the charger in and out regularly

Question

I have read on numerous occasions a set of recommendations to keep a Li-Ion battery happy: avoid discharging the device at all costs, don't charge it up completely, keep it as much as possible between 40% and 80% and don't charge it too quickly.

I'm wondering, however, which one of those two workflows are least harmful to a Li-Ion battery:

1. Keeping the device on (slow) charger most of the time the whole day while using the device, the battery level remaining at 100% most of the time, and dropping sometimes to 80%.

2. Unplug the (slow) charger regularly during the day, and then plug it again a hour or two later, the battery level constantly fluctuating between 50% and 100%.

I'm asking this question because while it is quite unpractical for me to keep the battery level of my phone between 40% and 80%, it is perfectly possible for me to either keep the phone on charger nearly all the time when I'm at home or at work, or to switch regularly between on battery and on charger modes.

Answer 1

One thing worse then overtemperature is overtemperature at charging.

One thing worse then overtemperature at charging is overtemperature at charging with charging >70-9x%.

That speeds up the side reactions of eventual metallic lithium deposits with dialkylcarbonate at high charge.

The reaction of either DMC or DEC with lithium was initiated at 180 K, a temperature much lower than their bulk melting temperatures, producing lithium methyl carbonate, methyllithium and lithium ethyl carbonate, and ethyllithium, respectively. At temperatures greater than 270−300 K, the lithium alkyl carbonates start to decompose with $\ce{Li2O}$, elemental carbon, and alkyllithium as products on the surface. 

Those reactions decrease the cell capacity and increase the cell internal resistance.

Things go worse in summer.

The rule number one is never heavily use the phone ( e.g. at gaming ), while charging it at high percentages.

If it must be charged, then when it is just topping, or better at low charge percentage or the best not being charged at all.

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Charging scheme is also a question of the cell static aging, that applies more to laptops than to short life cycle of mobile phones.

Another thing is that cells prefer rather more of shallow cycles than less of deep cycles. That may be unfortunately counteracted by durability of charging connectors.

Fully charged cell is aging several times faster than about half charged one. As at full charge, lithium ions may form a metallic lithium deposit instead of lithium graphite intercalate, which very slowly reacts with dialkylcarbonate solvent.

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I frequently charge my phone from my powerbank to avoid immobility of the phone during charging, what gives me the option to charge the phone whenever I want. I do not charge the phone overnight nor keeping it long time being fully charged. But the truth is, phone life is typically 2-3 years max, what does not have much chance to show consequences of not optimal cell maintenance.

My Lenovo T400 laptop from 2009 with 40-80% battery maintenance had its battery ( in late 2017) in better condition than T430 from 2013. (70-75% versus 60% capacity). The latter running RHEL Linux was fully charging for several years, until I found the way to achieve 40-80% mode as well. Both were running mostly with AC charger attached on the office table.