Why is my fitbit Alta randomly turning off like the battery just died?

Why is my fitbit Alta randomly turning off like the battery just died? I charge my fitbit Alta until fully charged but then after three or four hours of being used it shuts off like the battery just died. when I put it back on the charger it displays that the battery is only half drained. why is this happening and how can it be fixed. I have already tried the factory reset and the problem is still persisting.