Heat is the main enemy of the lithium-ion batteries inside power banks, earbuds, smartwatches and phones. It shortens their life and, when a battery is already damaged, raises the risk of overheating or fire. Most of the protection is simple: keep batteries out of parked cars and direct sun, charge them somewhere cool on a hard surface, store them about half charged, and stop using any battery that swells, smells, leaks or gets unusually hot. This guide goes deeper than the short safety notes in our power bank guide.
What heat does to a battery
Apple's battery guidance puts the comfort zone for its devices at 16 to 22 °C and warns that ambient temperatures above 35 °C can permanently reduce battery capacity, which means less use from every charge. Charging in high heat does more damage, and even storing a battery somewhere hot can harm it for good. Cold is different: battery life drops in very cold conditions, but Apple says that effect is temporary.
Devices also protect themselves. When an iPhone gets too hot, Apple says charging can slow or stop, the screen can dim and some features pause until it cools down. Treat that as a signal to move the device somewhere cooler, out of the sun, not as a fault.
| Source | Use and charging | Storage | Charge level for long storage |
|---|---|---|---|
| Apple (iPhone and iPad) | 0 to 35 °C ambient, ideally 16 to 22 °C | -20 to 45 °C; below 32 °C for long-term storage | About 50 percent, topped up every six months |
| Anker power banks | 0 to 35 °C | 0 to 35 °C | About 60 percent (three indicator lights), recharged every three months |