sh acid flows, or
diffuses into the plates. Furthermore, the sulphate, which is more
bulky than the lead or lead peroxide fills the pores in the plate,
making it more and more difficult for acid to reach the interior of
the plate. This increases the rate at which the voltage drops.

The sulphate has another effect. It forms a cover over the active
material which has not been acted upon, and makes it practically
useless, since the acid is almost unable to penetrate the coating of
sulphate. We thus have quantities of active material which are
entirely enclosed in sulphate, thereby cutting down the amount of
energy which can be taken from the battery. Thus the formation of
sulphate throughout each plate and the abstraction of acid from the
electrolyte cause the voltage to drop at a constantly increasing rate.

Theoretically, the discharge may be continued until the voltage drops
to zero, but practically, the discharge should be stopped when the
voltage of each cell has dropped to 1.7 (on low discharge rates). If
the discharge is carried on beyond this point much of the spongy lead
and lead peroxide have either been changed into lead sulphate, or have
been covered up by the sulphate so effectively that they are almost
useless. Plates in this condition require a very long charge in order
to remove all the sulphate.

The limiting value of 1.7 volts per cell applies to a continuous
discharge at a moderate rate. At a very high current flowing for only
a very short time, it is not only safe, but advisable to allow a
battery to discharge to a lower voltage, the increased drop being due
to the rapid dilution of the acid in the plates.

The cell voltage will rise somewhat every time the discharge is
stopped. This is due to the diffusion of the acid from the main body
of electrolyte into the plates, resulting in an increased
concentration in the plates. If the discharge has been continuous,
especially if at a high rate, this rise in voltage will bring the cell
up to its normal voltage very quickly on account of the more rapid
diffusion of acid which will then take place.

The voltage does not depend upon the area of the plate surface but
upon the nature of the active materials and the electrolyte. Hence,
although the plates of a cell are gradually being covered with
sulphate, the voltage, measured when no current is flowing, will fall
slowly and not in proportion to the amount of energy taken out of the
cell. It is not until t