If the power supply fails, the load will be supplied by the battery through the
Schottky diode D2. As in the case of diode D1, there will be a forward voltage
drop of 0.4 to 0.5 V across D2. Thus, when the battery is supplying the load, the
voltage seen by the load will be 0.4 to 0.5 V less than the battery voltage. Note
that diode D2 is connected in parallel with the resistors R1 an R2 and is used to
bypass the resistors R1 and R2 when the battery is discharging through the load.
MAXIMUM BATTERY CHARGING CURRENT
The maximum recommended charging current of a battery is normally related to
it’s AH (Ampere Hour) capacity, designated as “
C
” (based on 20 A discharge
rate). The maximum charging current for an SLA battery is normally specified as
C/20 ( 0.05C).
Battery backup application using 12 V, 100 AH battery
This battery backup unit has been preset for battery backup application for
a 12 V battery of approximately 100 AH capacity with the maximum
charging current limited to approximately 4A
.
Battery backup application using 24 V, 100 AH battery
When using this unit in the preset condition for 24 V, 100 AH battery backup
application, the charging current will increase approx. two times to 8 A which is
equivalent to approximately C/10 (0.1C). Please check with the battery manufac-
turer if this maximum charging current of C/10 or 0.1C is acceptable for the 100
AH, 24 V battery. If not, the maximum charging current for a 100 AH, 24 V
battery backup operation can be reduced to half or C/20 ( 0.05C) by removing
one of the parallel charging current limiting resistors R1 / R2. Please see details
below under “
Adjustment to reduce the values of the maximum charging
current to half
”.
Battery backup application using batteries with different capacities
Similarly, if different capacities of batteries are used, the maximum charging
current can be achieved by changing the value(s) of R1 / R2 to suit
Equation 1
below. Ensure that the wattage of the selected new resistor(s) is sufficient to
carry the required current flowing through them (If the resistance is
R
Ohm,
current through the resistor is
I
Amps, then the wattage of the resistor
W
should
be > I²
*R
)
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