ve materials, and covers up much of the active
material which remains, so that it is difficult to change the sulphate
back to active material. Moreover, the expansion of active material
which takes place as the sulphate forms is then so great that it
causes the active material to break off from the plate and drop to the
bottom of the jar.

2. Allowing a Battery to Stand Idle. When lead sulphate is first
formed, it is in a finely divided, porous condition, and the
electrolyte soaks through it readily. If a battery which has been
discharged is allowed to stand idle without being charged, the lead
sulphate crystals grow by the combination of the crystals to form
larger crystals. The sulphate, instead of having a very large surface
area, upon which the electrolyte may act in changing the sulphate to
active material, as it does when it is first formed, now presents only
a very small surface to the electrolyte, and it is therefore only with
great difficulty that the large crystals of sulphate are changed to
active material. The sulphate is a poor conductor, and furthermore, it
covers up much of the remaining active material so that the
electrolyte cannot reach it.

A charged battery will also become sulphated if allowed to stand idle,
because it gradually becomes discharged, even though no wires of any
kind are attached to the battery terminals. How this takes place is
explained later. The discharge and formation of sulphate continue
until the battery is completely discharged. The sulphate then
gradually forms larger crystals as explained in the preceding
paragraph, until all of the active material is either changed to
sulphate, or is covered over by the sulphate so that the electrolyte
cannot reach it. The sulphate thus forms a high resistance coating
which hinders the passage of charging current through the battery and
causes heating on charge. It is for this reason that sulphated plates
should be charged at a low rate. The chemical actions which are
necessary to change the sulphate to active material can take place but
very slowly, and thus only a small current can be absorbed. Forcing a
large current through a sulphated battery causes heating since the
sulphate does not form uniformly throughout the plate, and the parts
which are the least sulphated will carry the charging current, causing
them to become heated. The heating damages the plates and separators,
and causes buckling, as explained later.

If batteries which