A krellide power cell was a part of an old Vulcan transmitter.
The transmitter that Trip Tucker used to reach the Enterprise NX-01 when he was captured by the Andorians used a krellide power cell. (ENT: "The Andorian Incident")
Krellide is Rick Sternbach's homage to the aliens in the 1950s science fiction film classic Forbidden Planet.
(behind-the-scenes commentary, Enterprise Season 1
DVD, "The Andorian Incident")