TM 10-6140-200-14
Figure 2-2. Typical modified constant voltage charge
2.40 volts per cell gassing will be normal and at 2.50 it
inherently provides the correct starting rate and assures
will be rapid. The amperage at which gassing becomes
that this will automatically be reduced, as the charge
excessive depends primarily upon the state of charge and
progresses, to the recommended finish rate. The constant
electrolyte temperatures. As the battery approaches full
voltage source may be either a motor driven generator or
charge, it is necessary, therefore, to reduce the charging
rectifier. The characteristics of a typical modified con-
rate to a point at which excessive gassing is prevented.
stant voltage charge is illustrated by Figure 2-2.
This safe rate is the finishing rate. When proper charging
equipment is used the tapering of the charging current to
(2) Two-Rate Method. The two-rate method, in which
the finishing rate is achieved automatically. Manufacturers
the change from starting rate to finishing rate is accom-
instructions will normally prescribe the desired charging
plished by a sudden drop or step, requires a constant
rates.
current source, usually a rectifier. The starting rate is
allowed to continue until the battery reaches the gassing
2-9. Charging Methods and Equipment
point of approximately 2.37 volts per cell, corrected to
77 degrees F. When this voltage is reached, a voltage
a. The modern storage battery, used with a properly
sensitive relay operates to place additional resistance in
designed, modern, automatic charger, needs only normal
series with the battery to reduce the charge rate to the
water additions, occasional cleaning, and regular checking
recommended finish rate. A timer is then automatically
of both charger adjustment and battery condition to
set to control the interval of finishing charge. A typical
provide long, dependable service.
two-rate charge is shown in Figure 2-3.
b. There are three recommended methods for the normal
(3) Taper Method. The taper method can be used with
charging of motive power batteries:
is designed to provide the correct charge rate during a
(1) Modified Constant Voltage (potential) Method. The
constantly tapering charge. This is accomplished by
modified constant voltage method, in which the tapering
coordinating the rising battery voltage with the design
of the charging rate is accomplished gradually, requires a
characteristics of the charger. The shape of the current
source of direct current with a constant bus voltage of
taper curve will vary with different types of equipment
but the result is essentially the same. Solid state chargers
approximately 2.63 volts for each cell. This is for an 8
are available which deliver as much current as the battery
hour charge. Proper series resistance in the output circuit
2-4
