My lawn mower's LI battery is connected 24/7 and still works after 10 years, using it every summer. My phone battery recently died after almost 3 years but my previous phone battery still works 5 years later, both 24/7 on charge unless in mobile use. I think/thought the prevailing advice was that car batteries like lead/acid should be cycled (actually I know this) and the same for NICAD and for LI it has always been best to charge 24/7. Sometimes batteries including LI go bad, it just happens.