The energy cell was a piece of technology providing energy for various devices.
The Na'kuhl Vosk claimed that in the 29th century energy cells the size of a coin existed. (ENT: "Storm Front")
In 2268, Pavel Chekov speculated that whenever Apollo rested, he was in effect recharging his energy cells. (TOS: "Who Mourns for Adonais?")
The energy cell usage of a Federation phaser rifle, manufactured by the Romulans in 2367, remained constant at 1.05 megajoules per second. (TNG: "The Mind's Eye")
In 2369, Worf connected his communicator's energy cell to a holographic telegraph machine to create a short-lived force field. (TNG: "A Fistful of Datas")