Lithium-ion suffers from stress when exposed to heat, so does keeping a cell at a high charge voltage. A battery dwelling above 30°C (86°F) is considered elevated temperature and for most Li-ion a voltage above /cell is deemed as high voltage . Exposing the battery to high temperature and dwelling in a full state-of-charge for an extended time can be more stressful than cycling. Table 3 demonstrates capacity loss as a function of temperature and SoC.
Spare or loose batteries are not allowed in checked-in baggage. If they are a size allowed as carry-on baggage (see below), they must be in the manufacturer's packaging or protected against a short circuit. You can do this by putting electrical tape across the terminals, or putting batteries in a strong plastic bag. Spare batteries must not come in contact with metal objects, such as coins, keys, or jewellery and steps taken to prevent crushing, puncturing, or pressure on the battery.