Power packs were a common form of generating power. They were similar to the Earth batteries.

K9 ran on power packs. They depleted quickly but automatically renewed while not in use. (TV: The Creature from the Pit)

When Nyssa found a number of power packs in London, it proved the crash must have been light and passengers may have survived as the power packs were fragile. The Terileptils used them to power their mind controlling bracelets. They were easily removed however, quickly disabling the mind control capabilities. (TV: The Visitation)

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