Where can I increase my energy level?

Hint 1 of 4:

You'll need to find spare batteries.

Hint 2 of 4:

The first one you'll find is in the operating room on level 3.

Hint 3 of 4:

Just use the monitor at the operating table, and you'll take the battery out
of the patient who's on the table.

Hint 4 of 4:

There is another battery in the nose of the Icarus (underneath the control
room on level 3 -- use the wall of monitors in the depressed area of the
floor to access it).