On Sunday, January 29, 2017 at 2:05:25 PM UTC-8, Steve Koerner wrote:
SF: Last spring I charged up my 2 year old K2 batteries after about 3 months off the charger and then tested them. I was quite surprised that my capacities had dropped to a range of 2 to 3 AH over the winter period. I too had stored my batteries indoors and fully charged before I'd unplugged them over the winter period.

I got in communication with a factory engineer. The engineer had me do a couple cycles of discharge and long time charging -- on float for 10 days. His explanation was something about rebalancing the cells. I don't entirely understand the issue. Yet doing as he directed did bring my capacity back up to 8.5 AH. They worked fine all season.

Your situation sounds so similar that I'd suggest trying the same. Let us know if it fixes your K2s.

The types of cell balancing used in these cheaper batteries requires getting to a fairly high charge voltage for awhile to work. If you just plug them in (or into a lower voltage charge) until the green light comes on and then unplug, the cells may never get balanced. More expensive batteries than we typically use have active cell balancing and don't require this. Li batteries are much more susceptible to cell balance issues than LA batteries, because you can just overcharge LA batteries to balance (an "equalizing" charge). Li cells have to be protected from both over charge and over discharge, so you can only charge to 100% of the best cell, and only discharge to 0 or maybe 10% of the weakest cell. As they get further out of balance with time and cycles, the apparent capacity goes down far more than the real capacity.

The 3000 cycles noted by many manufacturers refers to the cells under ideal circumstances. In a cheap battery with a cheap BMS and a cheap charger, you might not achieve the ideal.