Power packs or power books were a common form of generating power. They were similar to the Earth batteries.
The Movellans relied on external power packs to survive. If these packs were removed the Movellans simply shut down. (TV: Destiny of the Daleks) According to an unsubstantiated report from the Space Security Service archive on Micawber's World, the Daleks used their duplication program to infiltrate the Movellan forces and sabotaged the power pack production facilities; infecting all new power packs with a virus that connected them to the Dalek pathweb. After a suitable time had passed the Daleks sent a signal through the pathweb which caused the Movellans to turn on each other. The Movellans never recovered from their civil war and were later wiped out by Davros' Imperial Daleks in a conflict which Daleks called the Pa Jass-Gutrik, the War of Vengeance. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe)
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)
Petronella Osgood built a pacifier which required a power pack. (AUDIO: Vanguard)