Siemens Simatic S7-400 - Manual (Page 86)

Power Supply Modules
3-6
S7-400, M7-400 Programmable Controllers Module Specifications
A5E00069467-07
3.3 Backup Battery (Option)
Introduction
The power supply modules of the S7-400 have a battery compartment for one or
two backup batteries. Use of these batteries is optional.
Function of the Backup Batteries
If backup batteries have been installed, the parameters set and the memory
contents (RAM) will be backed up via the backplane bus in CPUs and
programmable modules if the supply voltage fails. The battery voltage must be
within the tolerance range.
In addition, the backup battery enables you to carry out a restart of the CPU after
power-on.
Both the power supply module and the backed up modules monitor the battery
voltage.
Power Supply Modules With Two Backup Batteries:
Some power supply modules contain a battery compartment for two batteries. If
you use two batteries and set the switch to 2BATT, the power supply module
defines one of the two batteries as the backup battery. This assignment remains in
force until the battery is empty. When the backup battery is completely discharged,
the system switches to the reserve battery which is then in turn used as the backup
battery for the duration of its life. The status “backup battery” is also stored in the
event of a power failure.
Battery Type
Only batteries approved by Siemens must be used (see Appendix C: Spare Parts)
The batteries can form a passivation layer. Depassivation takes place when the
batteries are inserted in the power supply module.
Technical Specifications of the Backup Battery
Backup Battery
Order number 6ES7971-0BA00
Type 1 x lithium AA
Rated voltage 3.6 V
Rated capacity 1.9 Ah