One important classification for batteries is by their life cycle. "Primary" batteries can produce current as soon as assembled, but once the active elements are consumed, they cannot be electrically recharged. The development of the lead-acid battery and subsequent "secondary" or "chargeable" types allowed energy to be restored to the cell, extending the life of permanently assembled cells. The introduction of nickel and lithium based batteries in the latter half of the 20th century made the development of innumerable portable electronic devices feasible, from powerful flashlights to mobile phones. Very large stationary batteries find some applications in grid energy storage, helping to stabilize electric power distribution networks.