[Fig. 15b Old and new Exide covers]

From the illustrations of the vent and filling plug, it will be seen
that they provide both a vented stopper (vents F, G, H), and an
automatic device for the preventing of overfilling and flooding. The
amount of water that can be put into the cell is limited to the exact
amount needed to replace that lost by evaporation. This is
accomplished by means of the hard rubber valve (A) within the cell
cover and with which the top of the vent plug (E) engages, as shown in
the illustrations. The action of removing the plug (E) turns this
valve (A), closing the air passage (BB), and forming an air tight
chamber (C) in the top of the cell. When water is poured in, it cannot
rise in this air space (C) so as to completely fill the cell. As soon
as the proper level is reached, the water rises in the filling tube
(D) and gives a positive indication that sufficient water has been
added. Should, however, the filling be continued, the excess will be
pure water only, not acid. On replacing the plug (E), valve (A) is
automatically turned, opening the air passages (BB), leaving the air
chamber (C) available for the expansion of the solution, which occurs
when the battery is working.

Generally the filling or vent tube is so made that its lower end
indicates the correct level of electrolyte above the plates, In adding
water, the level of the electrolyte is brought up to the bottom of the
filling tube. By looking down into the tube, it can be seen when the
electrolyte reaches the bottom of the tube.

Vent Plugs, or Caps. Vent plugs, or caps, close up the filling or vent
tubes in the covers. They are made of hard rubber, and either screw
into or over the tubes, or are tightened by a full or partial turn, as
is done in Exide batteries. In the caps are small holes which are so
arranged that gases generated within the battery may escape, but acid
spray cannot pass through these holes. It is of the utmost importance
that the holes in the vent caps be kept open to allow the gases to
escape.

Case

The wooden case in which the cells are placed is usually made of kiln
dried white oak or hard maple. The wood is inspected carefully, and
all pieces are rejected that are weather-checked, or contain
worm-holes or knots. The wood is sawed into various thicknesses, and
then cut to the proper lengths and widths. The wood is passed through
other machines that cut in the dovetails, put the tongue on the bottom