Energizer.com FAQ
What is the difference between alkaline and rechargeable batteries?
Although there are many different types, batteries are generally divided into two types?primary and secondary. Primary batteries are used once and thrown away. Secondary batteries can be recharged and used again. Generally speaking, alkaline batteries are primary batteries. Rechargeable batteries contain reversible chemical materials, allowing them to be recharged again and again.
IE, in the process of making electricity, in normal batteries the electrodes and electrolyte (zinc & alkaline, respectively) are distorted and largely destroyed. In rechargeable batteries:
Nickel-cadmium rechargeable cells are constructed of alternating layers of porous, negative cadmium anode and porous, positive nickel oxide cathode separated by absorbent layers, all permeated with electrolyte and all inside a nickel-plated steel case.
Both the cadmium anode and the nickel oxide cathode material are contained in plates. The plates are conductive, increasing the efficiency of the system. Because of this design, the anode structure is not seriously distorted as the cadmium is oxidised, nor is the cathode structure as the nickel is reduced. This gets over the biggest problem of recharging batteries - the distortion.
Recharging pushes a current through the battery therefore making it "useful" again.