I'm in the final design stage of an all-Victron LIFEPO4 upgrade for my sailboat. MultiPlus, 2 of the LFPSmart 12,8/200-a batteries, VE Bus BMS, BMV-712, battery protect, cyrix, Victron MPPT solar controller, new Balmar alternator regulator and Sterling APD...the works, very similar to the example system diagrams Victron provides. Given the amount this is going to cost, I'm keenly interested in designing the system for maximum utility AND maximum useful life. I'd like my children to inherit this LFP system :)
My typical usage pattern is to not really need the house bank most of the year, but for 4 - 6 weeks in the summer my family pretty much lives on the boat. So we see a period of intense use in the summer, but the rest of the year I would like to do what I can to keep the LIFEPO4 batteries in a "storage mode" if that will prolong their life.
My understanding is that when not in use for a while, they should be stored in a partially depleted state...60%, 70%, 80%, I've seen figures in those ranges, with the BMS and all other loads disconnected. I figured I could manage this, running my minor house loads at the dock (bilge pump, never really runs, lights and radio when I am at the boat in the winter working on projects) off the AGM starter battery, which has its own 10amp dedicated shore power charger (not the MultiPlus)
But I've also read in the Victron battery manual that they should be fully charged once per month to keep the cells balanced, which seems to defeat much of the point of having a system in place to store them in isolation at 75%-ish charge for extended periods of time.
What are your thoughts on habits and system design for maximizing LFP battery life?
Is it a false economy to try to maximize life by being fussy managing the storage charge level, disconnecting and reconnecting the BMS and then diligently charging to 100% every month? Is it better to just "set it and forget it" and keep the MultiPlus going all year long? I want to avoid trying to be too clever and cutting off my nose to spite my face, if it is not really worth it.
Part of my concern here is "what counts as a cycle" when we talk about things like 3000 or 5000 charge cycles as being the useful life of a LFP battery. I hate to think I am using up my precious finite lifetime charge cycles just bouncing up and down between 95% and 100% at the dock while getting zero utility from the system.