Nicad Nickel-Cadmium rechargeable battery.
Ni-cads can be recharged hundreds of times and are therefore the most cost effective power supply. They are ideal for the travelling photographer as you only need to take two sets with you (one for the flash and one to keep on charge). Ni-Cads offer faster recycling times, but less firings, and fail more abruptly (usually without warning and usually in mid roll!). They normally require 14 hour charging times and can develop a ‘memory'. You must discharge them fully before recharging otherwise they ‘remember’ and will only take a part charge next time.