s a true indication of the condition of a
battery. Where a battery does not give satisfactory service even
though the specific gravity readings are satisfactory, the latter are
not reliable as indicating the amount of charge in the battery.

As long as a discharge current is flowing from the battery, the acid
within the plates is used up and becomes very much diluted. Diffusion
between the surrounding electrolyte and the acid in the plates keeps
up the supply needed in the plates in order to, carry on the chemical
changes. When the discharge is first begun, the diffusion of acid into
the plates takes place rapidly because there is little sulphate
clogging the pores in the active material, and because there is a
greater difference between the concentration of acid in the
electrolyte and in the plates than will exist as the discharge
progresses. As the sulphate begins to form and fill up the pores of
the plates, and as more and more acid is abstracted from the
electrolyte, diffusion takes place more slowly.

If a battery is allowed to stand idle for a short time after a partial
discharge, the specific gravity of the electrolyte will decrease
because some, of the acid in the electrolyte will gradually flow into
the pores of the plates to replace the acid used up while the battery
was discharging. Theoretically the discharge can be continued until
all the acid has been used up, and the electrolyte is composed of pure
water. Experience has shown, however, that the discharge of the
battery should not be continued after the specific gravity of the
electrolyte has fallen to 1.150. As far as the electrolyte is
concerned, the discharge may be carried farther with safety. The
plates determine the point at which the discharge should be stopped.
When the specific gravity has dropped from 1.300 to 1.150, so much
sulphate has been formed that it fills the pores in the active
material on the plates. Fig. 23 shows the change in the density of the
acid during discharge.

[Fig. 23: Variation of Capacity with Specific Gravity]

Changes at the Negative Plate. Chemically, the action at the negative
plate consists only of the formation of lead sulphate from the spongy
lead. The lead sulphate is only slightly soluble in the electrolyte
and is precipitated as soon as it is formed, leaving hydrogen ions,
which then go to the lead peroxide plate to form water with oxygen
ions released at the peroxide plate. The sulphate forms more quickly