Siemens Simatic S7-400 - Manual (Page 87)

Power Supply Modules
3-7
S7-400, M7-400 Programmable Controllers Module Specifications
A5E00069467-07
Backup Times
The maximum backup time is based on the capacity of the backup batteries used
and the backup current in the rack. The backup current is the sum of all individual
currents of the inserted backed-up modules as well as the requirements of the
power supply module when the power is switched off.
Example for the Calculation of Backup Times
The capacity of the batteries is listed in the technical specifications of the power
supply. The typical and maximum backup current of the backed-up module is listed
in the technical specifications of the module.
The typical backup current of a CPU is an empirically determined value. The
maximum backup current is a worst-case value that is calculated based on the
corresponding manufacturer specifications for the memory blocks.
The following technical specifications produce backup times for a CR with a PS
407 4A and a CPU 417-4 as the only backed-up module:
Capacity of the backup battery: 1.9 Ah
Maximum backup current (including own requirement at power off) of the power
supply:100
µ A
Typical backup current of the CPU 417-4: 75 µ A
A rated capacity of less than 100% is to be assumed when calculating the backup
time because the backup battery is also affected at power on by the regular
depassivation.
A battery capacity of 63% of the rated capacity produces the following values:
Backup time = 1.9 Ah * 0.63 / (100 + 75) µ A = (1.197 / 175) * 1 000 000 = 6840 h
This produces a maximum backup time of 285 days.