The temperature to which the phone is exposed also affects the integrated rechargeable batteries. In particular, it is difficult to tolerate high temperatures, which can damage not only the batteries but also the chips built into the device. The impairment that can occur mainly affects the maximum capacity of the rechargeable batteries: The hotter a lithium-ion battery is operated, the faster the maximum capacity decreases. This insight could be proven by various tests: For example, batteries with different charge levels – either 100% or 60% – and stored at different temperatures were investigated. At 0°C, no significant loss of battery capacity could be detected; however, if the cell phone was stored at normal summer temperatures of an average of 25°C, a loss of battery capacity of 4% at 40% charge level could be measured after one year and five times at a 100 % charge level. Losses at temperatures around 60°C were particularly high. Fully charged batteries already lost 40% of their original capacity within three months; a battery with a charge of 60% lost a quarter of its original capacity within a year. Therefore, the following applies to the use of mobile phones: The batteries should never be exposed to high temperatures. Especially during the warm season and on particularly hot days, overheating can quickly lead to long-term capacity damage. But even at moderate ambient temperatures, overheating of the batteries is easily possible, for example when the mobile phone is in the pocket. Apps that place a heavy load on the processor during operation should not be switched on. For example, if you leave your mobile phone while shopping in a car, you should choose a suitable cool place especially in summer, for example under the shelf. Smartphones that are lying on the dashboard or the rear shelf of the hat rack, on the other hand, overheat very quickly and that damages the batteries.
Otherwise, smartphone batteries should always be stored outside the mobile phone and kept in a cool place. If you put your batteries in the refrigerator, you can further improve the life time. But make sure to (slowly) bring it back to normal room temperature before using it.