In a battery backup application, the battery will normally remain in a fully
charged, standby condition most of the time. It will discharge to the load only
when the power supply fails. Under this standby application, the battery should
be charged at the recommended float voltage.
Assuming the standby float
charging voltages
of 12 V / 24 V sealed lead acid (SLA) batteries to be 13.5 V to
13.8 V and 27 V to 27.6 V ( at 60ºF to 77ºF) respectively,
the output voltage of the
power supply should be adjusted as follows:
Power supply voltage for 12V SLA battery backup: 14 V
(The output voltage of this unit for the load and for float charging of the battery
will be 14 V minus drop of 0.2 V to 0.4 V across diode D1 i.e. 13.8 V to 13.6 V)
Power supply voltage for 24V SLA battery backup: 27.6 V
(The output voltage for the load and for float charging of the battery will be 27.6
V minus drop of 0.2 V to 0.4 V across diode D1 i.e. 27.4 V to 27.2 V)
As long as the power supply is operating normally, it will supply the load as well
as charge the battery through resistors R1 and R2, each 1Ohm, 35W ( R1 and R2
are connected in parallel and will provide a net series resistance of 0.5 Ohm).
NOTE: Please note that as the power supply is required to feed the load as
well as charge the battery, the current rating of the power supply should be
at least 20% more than the sum of the maximum load current and the
maximum charging current.
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