Siemens SIMATIC S7-400 Reference Manual page 44

Power supply modules
3.3 Backup battery (option)
Specifications of the backup battery
Backup battery
Order number
Type
Nominal voltage
Nominal capacity
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
(6ES7407-0DA02-0AA0) and a CPU 417-4 (6ES7417-4XT05-0AB0) as the only backed-up
module:
● Capacity of the backup battery: 2,3 Ah
● Maximum backup current (including own requirement at power off) of the power supply:
100 µA
● Typical backup current of the CPU 417-4: 225 µA to 40° C.
A nominal 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 regular depassivation.
A battery capacity of 63% of the nominal capacity produces the following values:
Backup time = 2,3 Ah * 0.63 / (100 + 225) µA = (1,449 / 325) * 1 000 000 = 4458 h
This produces a maximum backup time of 185 days.
44
6ES7971-0BA00
1 x lithium AA
3.6 V
2,3 Ah
S7-400 Automation System Module Data
Reference Manual, Ausgabe 11/2016, A5E00850736-08