Siemens SIMATIC S7-410 PLC buffer or back up battery change procedure
Backup battery
The power supply modules of the S7-400 have a battery compartment to accommodate one or two backup batteries (Lithium AA+, 3.6V, 2.3Ah). If two batteries are used and the BATT indication switch is set 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. If BATT1F or BATT2F or BAF LED lits/illuminates then backup battery seems not working properly and we need to go for a replacement. In the event of a power supply failure, this battery buffers the set parameters and the memory contents (RAM) in CPUs and parameterizable modules via the back-plane bus. Furthermore, the backup battery permits restarting of the CPU following a power failure. Both the power supply module and the buffered modules monitor the battery voltage, and indicate when the battery is discharged.
The batteries can form a passivation layer due to longer storage in idle condition in warehouse of shelf. Depassivation takes place when the batteries are inserted in the power supply module.
Example of backup time calculation:
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: 2.3Ah
Maximum backup current (including own requirement at power off) of the power supply: 100microA
Typical backup current of the CPU 417-4:225microA to 40deg 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 = 2.3AH * 0.63/(100+225)microA = (1.4499/325)*1000000 = 4458 h.
This produces a maximum backup time of 185 days.
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