Temporal Research Centre
Battery bulb
Grows on the Ultimate plant in pot C and appears only once the plant is fully grown: one hangs on the plant and six fall around it. It goes in the battery slot of the time machine in the main hall, in the Present.
What it is for
Puzzle page: Greenhouse
Where the walkthrough uses it
5-5 Back to the Future· step 1Hook Gun player
Take the battery bulb (the Hook Gun can pull it to you).
5-5 Back to the Future· step 2Hook Gun player
Leave by the greenhouse exit: it opens with the plants done and the bulb in hand. The decontamination takes you back to the main hall.
5-5 Back to the Future· step 3Hook Gun player
Put the bulb in the slot by the time machine by hand (the Hook Gun can't place it).
Where to find it
No item spot is random: the game files place every item in the same spot in every game. Some items exist several times at once, such as the fuses in the Time Maze. From the game files: the area is loaded at these checkpoints. A door on the way can still be locked at some of them.
How to find it
In the round room with the tall arched windows, around the big round grated pot in the middle (pot C). They only appear once the Ultimate plant in it is fully grown: one hangs on the plant, six fall onto the floor around it.
5 mSeen from above with the ceiling removed. Yellow ring: this spot. Small labels: other spots of the same item close by (select one to go to it). 
The spot seen from nearby (rendered from the game's own level geometry).
Achievements

All Grown Up
Cultivated a Battery Bulb
How it unlocks (spoiler)
Finish the greenhouse plant puzzle (5-4 Overgrown) and grow the Battery Bulb.
Items:
Greenhouse seeds
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