A krellide power cell was a type of power cell.
It was found as part of an old Vulcan transmitter.
The transmitter that Trip Tucker used to reach 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")
In 2367, the USS Enterprise-D had to escape a dark matter nebula. Jean-Luc Picard decided to pilot a shuttlepod to help escort it, but lost control. Its krellide power cells were damaged, making manual control difficult to maintain. (TNG: "In Theory")