Plenty of people would commit a mistake of forgetting to take out batteries when they are full and leave them in the charger overnight. This could lead to the overcharge of the battery if the charger has no overcharge protection.
Although lithium-ion batteries heat themselves while they're being charged, constantly charging a full battery could result in overheat. 3.6/3.7VLi-ion batteries typically charge to 4.20V (+/–0.05V of tolerance is allowed). Keep charging batteries when the voltage reaches 4.2V could add more stress on batteries and even cause battery explosion. Therefore, batteries should be kept at the peak cut-off voltage as short as possible.
Normally lithium-based batteries are safer at a lower charge. This is part of reason why authorities will demand air shipment of Li-ion at 30 percent state-of-charge rather than at full charge. Furthermore, it would reduce the life span of battery to always charge them over cut-off voltage.
Not only can high temperature cause overheat of battery, but also cold weather can also have a negative impact on battery life as well, so we always recommend you to keep your batteries charged within safe temperature in case of any accidents.