have been discharged to the voltage limits are
allowed to stand idle without being charged, they will, of course,
continue to discharge themselves just as fully charged batteries do
when allowed to stand idle.

3. Starvation. If a battery is charged and discharged intermittently,
and the discharge is greater than the charge, the battery will never
be fully charged, and lead sulphate will always be present. Gradually
this sulphate forms the large tough crystals that cover the active
material and remove it from action. This action continues until all
parts of the plate are covered with the crystalline sulphate and we
have the same condition that results when a battery is allowed to
stand idle without any charge.

4. Allowing Electrolyte to Fall Below Tops of Plates. If the
electrolyte is allowed to fall below the tops of the plates, so that
the active materials are exposed to the air, the parts thus exposed
will gradually become sulphated. The spongy lead of the negative
plate, being in a very finely divided state, offers a very large
surface to the oxygen of the air, and is rapidly oxidized, the
chemical action causing the active material to become hot. The
charging current, in passing through the parts of the plates not
covered by the electrolyte also heats the active materials. The
electrolyte which occasionally splashes over the exposed parts of the
plates and which rises in the pores of the separators, is heated also,
and since hot acid attacks the active materials readily, sulphation
takes place quickly. The parts above the electrolyte, of course,
cannot be charged and sulphate continues to form. Soon the whole
exposed parts are sulphated as shown in Fig. 209.

As the level of the electrolyte drops, the electrolyte becomes
stronger, because it is only the water which evaporates, the acid
remaining and becoming more and more concentrated. The remaining
electrolyte and the parts of the plates covered by it become heated by
the current, because there is a smaller plate area to carry the
current, and because the resistance of the electrolyte increases as it
grows more concentrated. Since hot acid attacks the active materials,
sulphation also takes place in the parts of the plates still covered
by the electrolyte.

The separators in a battery having the electrolyte below the tops of
the plates suffer also, as will be explained later. See page 346.

5. Impurities. These are explained later. See page 76.

6. Addin