The tiny battery is an object under the Power tab of the Build Menu which stores a small amount of runoff energy from power generators, but can lose charge over extended periods of time. They do not require any research to construct. They are capable of holding a maximum of 10.0 kJ of energy, add 6.3 Watts of heat to their environment, and decrease the decor rating by 10 for a radius of two tiles.
"It isn't very efficient to let half your energy seep into the cosmos. Fight that entropy!"