What is a failure from improper storage?
Improper/non maintenance is mainly concerning how you manage a battery when not in use. Lithium batteries are designed to be used. The longer they sit without use, the faster they are likely to fail. If a battery is not used for an extended period, it should be cycled monthly to avoid failure from prolonged storage without use. When not in use your battery should also be kept in a storage state which is 60-70% of its rated capacity/voltage or 3.75-3.95v per cell. Most of today’s smart chargers have a storage mode built in which will achieve this SOC(state of charge).
Storing at full charge for long durations will cause resistance to build in the battery and it will be unable to provide the expected runtime or power. Signs of this type of failure would be a high IR (Internal resistance) across all of the cells in a given battery and will eventually result in pre-mature failure.