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:
- 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.
- 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).
Best Answer
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.