TARDISes contained power cells. After the Doctor's TARDIS seemingly perished when it fell into Pete's World, the Tenth Doctor found a "insignificant little" power cell clinging onto life. By providing ten years' worth of his own life energy, enabling it to power the TARDIS back up within 24 hours. (TV: Rise of the Cybermen)
In the meantime, the Doctor used the power cell's energy to disintergrate a group of Cybermen when they refused his surrender. However, he could not do so repeatedly as the cell took four hours to recharge. (TV: The Age of Steel)
