time limit

Accelerator’s brain had been injured on August 31.
He required that calculation assistance device to live an ordinary life as an esper. The
normal mode that allowed him to walk, talk, count, etc. could last for 48 hours. However,
the esper mode that allowed him full use of his vector control power would drain the
battery in only about fifteen minutes due to the massive amount of calculations that
needed to be carried out every instant. It was quite a restrictive item.
That meant he effectively had only fifteen minutes of safe time.
Other than during those fifteen minutes, he was a weakling who could not even walk
without recharging that battery once every 48 hours.

- Toaru Majutsu no Index Volume 12, Chapter 2 Part 5