This one is about what it's like to run on actual batteries.
The very first time his battery died, it was on tv. In actuality he had no idea what happened at the time. He felt the mechanics of his robot form slow to a halt. He lost track of sight, sound and even time. How long was he in this darkened void? He vaguely remembered hearing a dull thinking knock, perhaps someone calling his name? He didn't know. It felt like an eternity tho till he felt the robot body power up and he could SEE again. When he saw the familiar face of Alphys, he felt safe... he gave her a nervous hug. And tried to never let his battery deplete. Especially before a performance.
What had happened, as others told him, was mid sentence, he just STOPPED. He fell limp. Luckily the camera crew were fast on their feet ND cut to commercial with the tagline "experiencing technical difficulties, even stand by" and ran MTT ads. Alphys, who had been WATCHING his show, saw what happened and rushed thru Hotland to where Mettaton was filming. She shooed the people who were poking him aside and wheeled Mettaton to the nearest outlet and plugged him in. She was scared she'd lost him... what would happen to his ghost? She gnawed at her claws with worry. He hadn't said anything at all while powered down... and she knew possesed items could speak... she found out the hard way when the angry training dummy outside Undyne's house yelled at her. But she breathed a sigh of relief when, once his battery was fully charged, Mettaton powered up and imediatly grabbed her in a hug. After that she installed a battery readout on his arm so it wouldn't happen again.
After Mettaton told her of what his experience was like when his battery died, she decided to make changes to how his systems work when his EX battery dies. Instead of being stuck in an endless void of nothingness, when his battery dies, he completely loses consciousness. That way it protects his mental state... he is grateful for this system... he still does not like it when his battery depletes as he never knew how long he was unconscious, and it happened rather a lot untill Alphys could build a battery strong enough for his new form, but it was a whole lot better this way. Plus atleast in this form he could be powered on while charging.