My understanding of the ideal SOC for storage is based on two things:
(1) I read the objective, peer reviewed research literature which all seem to point towards longer life at around 40-60% SOC for long term storage (hey, passages can be long and boring!)
(2) My conversation with a senior Victron tech to clarify whether they suggest the same.
What was explained to me "off the record" is that the research literature is correct that 40-60% maximizes lifepo4 life. But a manufacturer's primary goal is to minimize customer complaints and concerns and a secondary goal is to maximize longevity. And he jokingly said "and if batteries last forever, we'd go out of business".
Charging to 80% is the foolproof way to go, for sure as it reduces the likelihood of low voltage battery death in the off chance that the charger/power source is cut off. The cost of this is reduced longevity.
I opted to follow the objective research. Time will tell whether that research is correct.
2007 A54 #69