The restriction on batteries under 100 Wh for airplane travel is primarily due to safety concerns. Here's why this limit is important:

Risk of Fire: Batteries, especially lithiumion batteries, can pose a fire risk. These batteries store a significant amount of energy, and if they shortcircuit, overheat, are damaged, or improperly handled, they can ignite. In flight, a fire could be catastrophic due to limited firefighting resources and the difficulty in managing an onboard emergency.

Controlled Environment: In an aircraft, it's crucial to maintain a controlled environment. Batteries above 100 Wh are more powerful and, if they fail, can produce a more intense fire that is harder to control.

History of Incidents: There have been several incidents of battery fires on airplanes, prompting regulatory bodies like the International Air Transport Association (IATA) and the Federal Aviation Administration (FAA) to impose stricter regulations on battery transport.

Standardization of Safety Measures: The 100 Wh limit is a standardized measure that balances the need for passengers to carry batteries for personal devices and the overall safety of the aircraft.

Mitigating the Effects of Pressure and Temperature Changes: Air travel involves significant changes in pressure and temperature, which can affect battery performance and safety. Limiting the battery size helps mitigate these risks.

For batteries that are between 100 Wh and 160 Wh, special regulations and airline approval are typically required, and there are usually restrictions on the number of such batteries a passenger can carry. Batteries over 160 Wh are generally not allowed on passenger aircraft. This regulation helps ensure that the risks associated with transporting batteries by air are minimized.