second hand running too fast?

My son made a wooden clock face in his shop class.So the timing mechanism is simple and runs by small battery. Worked for many years but with 2 moves the second hand maoves about 10 seconds at a time. The hour and minute hands are okay. The second hand was just dropping from 1200 to 6:00 but after putsing with the hands the second hand now runs, as I said 15 seconds in one movement. Can you help?