Diggsut... Yeah you can loose up to couple of % per year doing it BUT.... Most bigger solar installs you will also have the powerwall fully charged for up to 8 hours a day.
So if we look at the numbers. Lets say you have used cells and you let them sit at 15c that I do... Lets say I leave them for 3 months. If you look at that white paper you will see that I loose a whopping 1% or so.
So if we take this further and if you read that white paper you send you will see that sitting at lower SoC does not save you that much in terms of degradation during those 3 months. If you add how much you actually degrade using the cells later on especially for those pushing cells harder you degrade ALOT faster doing that
If you pull all those numbers on the table this is the result:
* Sitting at full charge some month does not kill the cells
* Spending that extra time discharging and wasting that capacity in the cells cost MORE in time and energy wasted than actually just buying some extra cells to compensate the minimal loss
* Sitting att full charge during that time you can even better weed out self dischargers. That self discharging cells actually saves you more energy than what you have lost letting them sit there comparing using the batteries over some years.
* If a cell actually wont be able to sit at full charge for lets say 3 months... Its not even worth having in a powerwall. Because 3 months full there equals 9 summer months sitting full in my powerwall... And this I cannot do anything about unless i want the battery bank to sit half charged and loose valuable DoD capacity.
Do I have numbers of this? Yes i calculated and tested this. Im not going to write all numbers here because thats a white paper it self
Yes - Leaving them fully charge degrades the batteries slightly faster than middle/close to empty.
BUT just lowering the temperature where they sits do ALOT and really... If thats your biggest concern loosing energy you should consider buiing better wiring or inverter/charger. Thats where you save alot more energy.
Would letting them sitting fully charge cause a "fire" -> NO.
We have a couple of side-tracks here becaue LiPo on other hand are more sensitive and thats because of the case. If LiPo would sit in hard-case its not a problem there either. I have done my fair share of RC last 10+ years and I have many cells sitting for all winter fully charged for testing. Some brands doesnt like it as much when using soft cases since they do wanna puff a bit but so far not problem.
I have killed more cells due to abusing them when in use than leaving them as is
Lastly: If you dont know when you will use them or its a very long time like half a year or years then of course discharge them down to 40-50%. its just better. But for a month or 3 during the build its ok