Portal 2: The Cake Is Still A Lie

[The Linker]

There were dozens of potatoes lying around. It's certainly possible he could, now being massive and in control of everything, retrieve one of the potato batteries from down near the neurotoxin chamber.

Aye, it was foreshadowed by a room just before you get to GLaD0S: all the science projects set up. Basically just a dozen potato batteries and one baking soda volcano.