Lithium Ion Battery Question

My bike generally sits most of winter without much maintenance or care, and a few years ago I bought a Li-Ion battery in the hope it lasts better over the winter than the normal batteries which don't survive a long discharge. This year however, for the first time, it was dead after winter. I jumped the bike and let it run for half an hour or so, about a month ago. Yesterday it struggled to turn the bike over, which I was surprised at as Li-Ion are normally strong and then simply stop when they're low on juice. Jumped the bike again, and took it for a ride. Stopped for an hour at the gym and the bike did start on the way home, but didn't feel like it really wanted to.

So, have I killed the Li-Ion battery over winter? If not, would putting a charger/optimiser on it help it recover full health again? And finally what do I need to do to stop this happening every year?!

Its an Exige (?) battery. Put the Optimate on it earlier and after a while charging is now telling me the battery is fine. Seems over the next few hours it will run several tests to ascertain the health, and so hopefully by tomorrow it will know if there is any issue beyond being drained over winter. Thanks for the help.

EDIT - Exide Battery - https://www.exidegroup.com/eu/en/battery/ELTZ10S

Not sure, I checked it about an hour and a half, maybe 2 hours after plugging it in and it was green then, but don't think it was minutes. Not 5 mins at least.