Sounds similar to an issue I had with my 5 amp from 2015. About 6 months past warranty battery would not charge . Charger ran for a few seconds then showed full charge. Mower showed red. I tried all the trouble shooting I could find (including the standard charger that came with chain saw). Still no use. Bought another 2 amp battery. Can mow my yard with the two smaller batteries. At some point I ran the big battery till it quit, and set it next to the other two. I am a packrat, with intentions to take things apart (sometimes repair) before I discard. Hey there might have been some good cells left in the battery. Anyway, this spring (after January, I'm in the south) I got ready to mow and throw a battery on the charger. I put the big 5 amp on without looking close. It charged. I now use it for mowing.
Bottom line. Something in the course of running down and sitting for a while made a change in the battery's brain.
Unfortunately, this (June) is not a good time for idle batteries.
I did try to contact EGO when battery went. I did not win the callback lottery.