Samlex America SCC-30AB - Manuel (Page 19)

19
3
GENERAL INFORMATION: BATTERIEs
For example, backup energy may be required for say 4 hours or 1 day (24 Hours) or 3
days (72 Hours). In this connection, the following formulae will be applicable:
FORMULA 1 DC Power in Watts (W) DC Volts (V) x DC Current (A)
FORMULA 2 AC Power in Watts (W)
AC Volts (V) x AC Current (A) x Power
Factor (0.8 Typical)
FORMULA 3
DC Power drawn from the
battery by DC load fed
directly from the battery
Power of DC load in Watts (W)
FORMULA 4
DC Power drawn from the
Battery by AC load fed from
DC-AC inverter
1.2 x Power of AC load in Watts (W)
(Assuming typical efficiency of
inverter = 84%)
FORMULA 5
Energy consumption from
the battery in Watt Hour
(Wh)
Power in Watts (W) x Time in Hours (h)
FORMULA 6
Energy consumption from
the battery in Ampere Hour
(Ah)
12V
Battery
Energy consumption in
Watt-Hour (Wh) ÷ 12
24V
Battery
Energy consumption in
Watt-Hour (Wh) ÷ 24
Determining Total Battery Energy Consumption – First step is to determine the total
battery energy consumption in Ampere Hours for running the desired AC and DC
loads during the desired span of backup time:
a) Find out the power rating of each AC and DC device in Watts (W). If Watt rating is not available,
calculate the Watt rating using Formulae 1 or 2.
b) Determine / calculate the power drawn from the battery in Watts (W) by each of the AC and DC
devices. For DC devices, this will be the same as its DC Power rating (Formula 3). For AC devices
powered from DC to AC inverter, use Formula 4 to calculate the power drawn in Watts (W) from
the battery.
c) Calculate the energy consumption in Watt-Hours (Wh) for each load using Formula 5 based on
the number of hours each load is expected to run during the desired span of backup time. Add
all to get the total energy in Watt Hours (Wh).
d) Calculate the total battery energy consumption in Ampere Hours (Ah) for the combined DC and
AC loads using Formula 6.
Determining Actual Ah Capacity of Battery Bank – Actual Ah capacity of the battery
bank is determined based on the following considerations:
e) As pointed out under heading “Reduction in Usable Capacity at Higher Discharge Rates”
on page 13, the Ah capacity of a battery is normally specified at slower C/20 A i.e. 20 Hour
discharge rate. However, in backup applications, batteries get discharged at much higher
discharge
rates. Normally, 3 Hour i.e. C/3 A Discharge Rate is considered for this