User Manual

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Float Service Life
NP batteries are designed to operate in float/standby
service for approximately 5 yrs (NP+NPH) 7-10 yrs (NPL)
based upon a normal service condition in which float
charge voltage is maintained between 2.275vpc ± 0.005
volts per cell in an ambient temperature of approximately
20°C. Figure 33 shows the float service life characteristics of
NP batteries when discharged once every three months to
100% depth of discharge.
In a normal float service, where the charging voltage is
maintained at 2.275vpc ± 0.005 volts per cell (see Fig. 34),
the gases generated inside an NP battery are continually
recombined into the negative plates and return to the water
content of the electrolyte. Therefore, electrical capacity is
effectively not lost due to the “drying up” of the electrolyte;
the loss of capacity and eventual end of service life is
brought about by the gradual corrosion of the
electrodes. It should be noted that this corrosive process
will be accelerated by high ambient operating
temperatures and/or high charging voltage. When
designing a float service system, always consider the
following: LENGTH OF SERVICE LIFE WILL BE DIRECTLY
AFFECTED BY THE NUMBER OF DISCHARGE CYCLES,
DEPTH OF DISCHARGE, AMBIENT TEMPERATURE AND
CHARGING VOLTAGE.
The relationship between the number of cycles which
can be expected and the depth of discharge is readily
apparent. If an extended cycle life is required then it is
common practice to select a battery with a larger capacity
than the one that is required to carry the load. Thus, at
the specified discharge rate over the specified time, the
depth of discharge will be shallower and cyclic service
life will be longer.