[DonRotolo]

I was thinking the same, but the 12 volt variety of wires. Go to Home Depot or Lowe's and look at their wired lighting section: It's junk but you'll get the idea.

Run some #12 wire underground from a convenient window to the bench area. Put a waterproof electrical box (the kind for outdoor receptacles) on a pipe sunk a little into the ground, and a set of Powerpoles inside. Unplug to move the bench, plug in in the morning. Leave extra wire so you can replace the powerpoles once a year...

Batteries are a last resort, if there is AC power within 100 feet of the bench, run low voltage out to it. Very small likelihood someone's gonna mess with the wires.

Arm detection: Maybe some ultrasonics too, maybe for leg position?