There are two distinct types of storage battery. One is called the
"lead" or "acid" storage battery, and the other the "alkaline" or
"nickel-iron" storage battery. Each of them simply acts as a reservoir
to hold the energy of the electric current which is led into it, and
each of them, under proper conditions, will give back that energy.
As the lead storage battery is the oldest in point of discovery and
invention, we will describe it first.