Here is my take on lithium storage.
1) don't float the cells.
2) for long-term storage, don't worry about charging up to 100% before layup
3) but also don't maniacally pursue some ideal state of charge for storage.
In my case, there was some draw-down of the pack after my last charge session.
- The boat was charged to about 100% (Elcon charger finished its cycle without the BMS interrupting).
- I scheduled my haul-out with the marina
- The boat sat for about a week, with the charger off, and some house systems running, such as the DCDC converter, bilge pump, etc.
- for the trip to the service dock, we did a little tour around the marina and out (to the first marker and back)
- once hauled out, the boat sat for a couple of weeks before being covered. I left the cells in at this time to run house lights and other things while working on the boat.
- finally removed the batteries before closing up the boat.
All told, I probably ran down to 90% SoC.
While not 40% SoC, I doubt that it will impact the overall life or capacity of my pack since my yearly cycles will be in the neighbourhood of 20-30 cycles per season. So if my battery abuse reduces my pack life from 2000 cycles down to 1000, I still have 30 seasons of use ahead of me. If I get only 300 cycles, I still have 10 seasons, which I would judge to be more than acceptable. Other factors that age the pack into oblivion will likely have more of an affect.
I have done the tour of the googlewebs on the topic and found much "collective wisdom" and testimony but little in the way of reported data. Mind you, how does one test the impact of 10 6-months storage cycles at 100%SoC vs 80%, 50%, 40%, 20%, while at the same time, not taking at least 5 years to collect that data?
Perhaps by seeing the effect of 1-month, 2-month and 3-month storage intervals and projecting to 6-month storage cycles. I would be very interested in reading such a report. Here's one that looks interesting, but behind a paywall...
http://jes.ecsdl.org/content/159/8/A1283.abstract
Mind you, I also really like my lithium bank and would like it to last as long as practical (as opposed to possible). I probably will store it at a lower SoC next season. And since I have plans to repackage the cells into a monolithic battery box, removing them from the boat may become impractical. The spec sheet for my cells indicates that charging while the cells are below freezing (<0C) is a big no-no. However, discharging is fine down to -20C. This past winter has been rather cold -- consistently below -20C, which is the low end of the storage temperature range on the battery spec sheet. So I would like to see data from battery suppliers about what happens to cells that are stored outside of the "approved" ranges as well instead of just listing the upper/lower bounds of what they deem acceptable. Do the cold temperatures cause the electrolyte to crystallize and stratify, thereby reducing the chemical reaction area? Is there a knee in the curve that happens around -20C? Can it be pushed to -25C without too much ill effect? What about storing cells at a punishing -40C for 6 months. What will that do to cycle life if done for one season, two seasons, etc?
/Jason
