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I've read that lithium-ion cells are best preserved by charging them to be less than 80% charge and discharging them no lower than 20% charge.

I'm wondering which has the most impact on cell life (performance degradation vs cycle count) / which is most sensitive between the two?

e.g. does a lithium-ion cell degrade slower by being charged to 95% and discharged to 20% or by being charged to 80% and discharged to 5%?

More specifically, I am wondering:

  1. When should I ideally daily start and stop charging my phone and laptops using Li-Ion batteries, when using 60%, 80% and 90% of the capacity within the day.
  2. How should I set up my PV system to stop charging / stop using power from my LFP battery bank, for the same respective scenarios (planning to use 60%, 80% or 90% of the battery capacity each day before charging).

Recharging would occur daily over the course of maybe 2 hours for the phone/laptop and 6 hours for the LFP battery, discharge would depend on usage but the planned discharge capacity would occur within the 24h cycle for both (minus respective charging durations).

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    \$\begingroup\$ What research via google have you done on this so far? \$\endgroup\$
    – Andy aka
    Feb 8 at 10:25
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    \$\begingroup\$ Not a full answer but my findings from reclycling batteries is that deep discharge decreases capacitance but to a high degree increases self-discharge. Staying at 100 % SoC seems to only hurt capacitance by a little. So I would go for a 35-95 % range over a 20-80 % SoC one. \$\endgroup\$
    – winny
    Feb 8 at 15:33
  • \$\begingroup\$ Thank you for your answer winny! \$\endgroup\$
    – Hans
    Feb 8 at 16:16

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The answer depends on the Li-ion cell chemistry, and, more importantly, on the State of Charge (SoC) when the cell is not in use.

Specifically, a cell that is kept a long time charged at 95 % SoC degrades faster than one that is kept a long time charged at 80 % SoC. Therefore, if the cell is kept unused for a long time, 80 % is better than 95 %.

On the other hand, a cell that is cycled constantly is better charged at 95 % than 80 % because the cell degrades faster when discharging at 5 % that id does at 20 %. Also, a cell that is charged to 95 % holds more energy than one charged at 80 %. Therefore, you only need to discharge it to, say 25 % (not 20 %) to get the same energy out of it compared to charging it at 80 % and discharging it to 5 %.

At 95 % SoC, Li-ion cell with LCO (cobalt) chemistry will degrade faster than a LFP (LiFePO4) cell, because the voltage is higher and the electrolyte will dissasociate faster. Therefore, there are fewer concerns at high SoC levels with LFP cells than with LCO cells. On the other hand, an LFP cell that is held at 100 % SoC without turning off the charger will experience an ever-increasing series resistance, which is bad. Finally, LTO (Titanate) cells are pretty much impervious to many of these issues.

So, it's complicated. The designers or traction batteries spend years of research to answer this question.

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  • \$\begingroup\$ Thank you for your answer Davide! Sorry for not detailing enough: my scenario is one wherein the lithium battery is cycled daily, such as a smartphone Li-Ion or photovoltaic LiFePO4 battery. My thinking is how come so many websites advuse to cycle between 20% and 80% charge, since it seems to me unlikely that on cycling the battery "wears out" equally whether overcharged or overdischarged (i.e. why exactly 20% at both ends)? ... \$\endgroup\$
    – Hans
    Feb 8 at 15:11
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    \$\begingroup\$ lithium battery It's a "LI-ion" battery, not a "lithium battery. Lithium batteries are primary and cannot be recharged. \$\endgroup\$ Feb 8 at 15:12
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    \$\begingroup\$ so many websites advise to cycle between 20% and 80% charge, Well, without knowing your specific situation, a website can only give a general suggestion. it seems to me unlikely that on cycling the battery "wears out" equally You are correct. It matters where the 60 % range is centered. That's why, as I said, "The designers or traction batteries spend years of research to answer this question." \$\endgroup\$ Feb 8 at 15:14
  • \$\begingroup\$ And if it isn't the case then what are better numbers. As if I want to cycle while using 60% battery capacity (as in those recommendations: what are most likely optimal levels to stop the discharge / stop the charge at)? And what if I want to be using 80% battery capacity, is cycling between 15% and 95% a charge a better way to do it vs 10% and 90%? And what if I want to be using 90% of the capacity each day, what are then most likely optimal values on each end? From your answer, it seems going deeper on the charge side may be the way to go. All I need is a rough estimate. \$\endgroup\$
    – Hans
    Feb 8 at 15:16
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    \$\begingroup\$ From your answer, it seems going deeper on the charge side may be the way to go I don't believe I said that. I said that it depends. If you edit your question and tell us 1) Cell chemistry (e.g.: LFP) and 2) the profile of a cycle (SoC vs time), then one of us may be able to give a specific recommendation. \$\endgroup\$ Feb 8 at 15:18

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