Calculating maximum battery charging current
Max. charging current
Ib = {(Vp – Vd) – Vb} divided by Rs
…….
Equation 1
where,
Vp
= The output voltage of the power supply
Vd
= Forward voltage drop across D1 (0.2 V for < 1 A and 0.4 V > 10 A)
Vb
= Battery voltage at fully discharged condition (Normally 11.4 V for 12 V
and 22.8 V for 24 V)
Rs
= Equivalent series resistance (In this case, two 1 Ohm in parallel = 0.5 Ohm)
Assuming that the voltages of fully discharged 12 V and 24 V batteries to be 11.4
V and 22.8 V respectively, the maximum charging currents will be as follows as
per
Equation 1
:
Charging current in factory pre-set condition
:
In the factory preset condition, resistors R1 and R2, each 1 Ohm, 35 W are
connected in parallel. The charging currents will be as follows:
For 12 V battery backup
= (14 V – 0.2 V – 11.4 V) divided by 0.5 Ohm = 4.8 A,
say 5A
For 24 V battery backup
= (27.6 V – 0.2 V – 22.8 V) divided by 0.5 Ohm = 9.2 A,
say 10 A
Adjustment to reduce the values of the maximum charging current to half
The charging currents can be reduced to half the values by disconnecting either
R1 or R2. R1 and R2 are connected using quick connect terminals. To discon-
nect, remove one of the two terminals of the selected resistor. Now only 1
resistor will be in series and the resistance Rs will be 1 Ohm instead of 0.5 Ohm.
The charging currents will now be as follows:
For 12 V battery backup
= (14 V – 0.2 V – 11.4 V) divided by 1 Ohm = 2.4 A,
say 2.5 A
For 24 V battery backup
= (27.6 V – 0.2 V – 22.8 V) divided by 1 Ohm = 4.6 A,
say 5 A
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