How does a pendulum powered by a battery in a clock work?

The time it takes a pendulum to make one full cycle - go from the left to the right and back - is proportional to the sqrt of its length divided by the acceleration due to gravity. So the pendulum is the time keeping part of the clock - the rest of the clock is used to dsiplay the time. In an idealized model, once you put the pendulum in motion it keeps going. But in reality, it encounters friction, air resistance and some of its kinetic energy is used to move the clock mechanism. So you need to give teh pendulum a little "kick" after each cycle. The battery in the clock drives a motor that pushes the pendulum just a little bit at the start of each cycle to replace the energy lost during the previous cycle. Grandfather clocks do this with a system of weights on a chain driving the motor, or with a spring driven motor.