The last time I charged my '15 SR was probably in November last year (so over a month ago). I always keep it unplugged when I'm not charging it for my next use. I had charged it to 100% back then, expecting to ride it, but that did not happen due to poor weather for about a month, other than for a couple miles running errands. The dash continued to show 100% until late in December, then we hit 65-70F unexpectedly, and I ran some more errands that brought the charge down to mid 80%. The bike has been parked in the garage since then and not ridden. It is indoors garage, which even in freezing outside temps stays in the 45F-55F inside.
Anyway, I just checked my bike with the app and it shows 3mv unbalance only, and 87% SoC (same as on the dash).
So, just sitting there, unplugged, my bike does not seem to loose charge or get cell imbalance. I will test the range in the Spring as I do not expect to ride it again until then. I won't be plugging it in though, unless I see the charge falling much below 40% SoC, which I suspect will not happen. At the rate it is loosing charge, I don't think it will loose more than a few % if that for the 3-4 winter months than I don't ride it.
For my Vectrix with Nissan Leaf cells conversion and no BMS, I too kept it off the grid for the winter. Similarly, the voltage on the battery stayed within just a couple of volts from when last charged. And the cells were always in balance (they are top-balanced, like on the Zero, but because there is no BMS to mess with their balance, these good quality cells tend to stay in balance and do not need rebalancing for many years).
Li batteries, unless there is a defective or poorly designed or implemented BMS, should be perfectly fine spending a few months off the grid. I see absolutely no reason to keep my SR plugged when not in use. Yup, everyone is free to do what they want, including follow the manual
, but in this case, for Winter storage, I think it makes a lot more sense to charge it to 100%, make sure equalization has finished (i.e., very small, under 3mv cell imbalance), then ride it not too aggressively until SoC drops to something like 60%, then park it for the cold season off the grid. Do check the SoC and cell balance by turning on the bike every now and then to make sure there is not surprise malfunction of some sort. Mine is charged a bit more at nearly 90%, but that's just because I did not get a chance to discharge it more. Will do next year