ttery voltage, which opposes,
or "bucks" the charging voltage, and reduces the voltage which is
effective in sending current through the batteries. Such a charge is
called "tapering" charge because the charging current gradually
decreases, or "tapers" off.

The principle of a "tapering" charge is, of course, that a discharged
battery may safely be charged at a higher rate than one which is only
partly discharged, because there is more lead sulphate in the
discharged battery which the action of the current changes back to
active material. As the battery charges, the amount of lead sulphate
decreases and since there is less sulphate for the current to act
upon, the charging rate should be reduced gradually. If this is not
done, excessive gassing will occur, resulting in active material being
blown from the grids.

A battery which has been badly sulphated, is of course, in a
discharged condition, but is not, of course, able to absorb a heavy
charging rate, and in handling such batteries on a constant potential
system, care must be taken that the charging rate is low. Another
precaution to be observed in all constant potential charging is to
watch the temperature of batteries while they are drawing a heavy
charging current. A battery which gasses soon after it is put oil
charge, and while still in a discharged condition, should be taken off
the line, or the charging line voltage reduced. With constant
potential charging, as with constant current charging, the two things
to watch are temperature and gassing. Any charging rate which does not
cause an excessive temperature or early gassing is safe, and
conversely any charging rate which causes an excessive battery
temperature, or causes gassing while the battery is still less than
three-fourths charged, is too high.

[Fig. 48]

Fig. 48. Hobart Bros. Co. 3 K. W. Constant Potential Motor-Generator
Charging Set

The Constant-Potential Charging Set manufactured by the Hobart Bros.
Co., consists of a 3 K.W. generator rated at 7.5 volts, and 400
amperes. This generator is direct connected to a 5 H.P. motor, both
machines being mounted oil the same base plate. Figure 48 shows this
outfit. Note that for the charging line there are three bus-bars to
which the batteries are connected. Twelve volt batteries are connected
across the two outside bus-bars, while six volt batteries are
connected between the center bus-bar and one of the outer ones.

The Tungar Rectifier